Hybridism and the Germ-Cell

That hybrid sterility (e.g. the mule) was associated with degenerate gonads was long known. At the turn of the 19th century the papers of Guyer, Juel, Cannon, and Montgomery provided a securer cytological foundation for this degeneration and hence for Romanes' "peculiarity" of the reproductive system. Guyer, in his quest for "the real nature of the material basis of heredity" seems to have independently discovered Mendelian segregation of characters and their association with the chromatin of chromosomes. Guyer was cited with approval by Bateson and Saunders in their paper which appeared early in 1902 (Click Here). The time was ripe for theoretical synthesis and things were moving very rapidly; it is of interest to note that the major protogonists dated their publications both by month and year.

D. R. Forsdyke circa 2000

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Spermatogenesis in Hybrid Pigeons (Guyer 1900)

Hybridism and the Germ-Cell (Guyer 1902)

Spermatogenesis of Normal and of Hybrid Pigeons (Guyer 1900)

Chromosomes of the Germ Cells (Montgomery 1901)

Cytological Basis for the Mendelian Laws (Cannon 1902)

Chromosomes in Heredity (Sutton 1903)

The Germ Cell and the Results of Mendel (Guyer 1903)

Spermatogenesis in Hybrid Pigeons

A brief note, by M. F. Guyer.   Science (1900) 21, 248-249

Hybrid pigeons exhibit several abnormalities in spermatogenesis. These are most marked in the sterile hybrids. In such, the first thing to strike the attention is a curious bead-like varicosity about the middle of the spermatozoon head. In the development of such spermatozoa the nucleus does not elongate completely, as it does normally, to form the head; consequently, at one point there remains a sort of vesicle corresponding in position to the original nucleus before the ends pushed out to form the long head.

    Some of the sterile birds showed also a marked degeneration of the germinal cells. In some cases no spermatozoa were matured and many of the cells had degenerated. Often deeply staining masses of protoplasm each with a large central vacuole were present. In both sterile and fertile hybrids there was much variation in cell division. Inequalities in chromatin distribution were common and multipolar spindles, abundant.

    The nuclei of the spermatogonia contained sixteen chromosomes, which is the regular number. In normal primary spermatocytes there are eight large ring chromosomes, each apparently being equivalent to two of the spermatogonial type, but in the hybrids there were often more than eight. Sometimes there were as high as sixteen small rings, in which case a doubling of the chromosomes had evidently not occurred. When sixteen chromosomes were present in the spermatocytes they were usually located on two spindles, eight to a spindle. Frequently both large and small rings were present.

      These peculiarities in chromosome formation may point perhaps to a tendency in the chromatin of each parent species to retain its individuality. If such is the case, then in those cells with two spindles each bearing eight chromosomes, it is evident that after division, some of the new cells will have chromatin from only one of the original parent species and some, from the other. Some of the spermatozoa, therefore, will bear chromatin from only one of these species.

      It is a well-known fact that the offspring of hybrids are extremely variable, a portion of these variations being usually in the form of reversions to one or other of the parent species. The possibility presents itself then, that this reversion may be due to  the persistence of the chromatin of only one species in one or both of the germ cells. Carrying the conception still further, the other variations [of characters] in the offspring of hybrids may be due, perhaps, to the varying proportions of the chromatin of each species in the mature germ cells.

Hybridism and the Germ-Cell

By Michael F. Guyer

Bulletin No. 21 of the University of Cincinnati. Series II, Volume II. November 1902.


THE SPERMATOGONIA
PRIMARY SPERMATOCYTES
SECONDARY SPERMATOCYTES
THE SPERMATID AND ITS TRANSFORMATION
THE GERM-CELLS OF HYBRID PIGEONS
CONCLUSIONS FROM THE STUDY OF HYBRIDS
SUMMARY


The writer prepared a paper on "The Spermatogenesis of Hybrid and of Normal Pigeons" in May, 1900, and placed it with The Journal of Morphology. On account of the temporary suspension of that periodical it is deemed desirable to publish an abridged account of the results of the investigation,* {Footnote: The original paper is the thesis submitted by the writer as a candidate for the degree of Ph.D., at the University of Chicago. It was accepted by the faculty in March, 1900. A few copies of a limited edition of the thesis, published by the author, are available for investigators who are specially interested in the particular problems under discussion. Address the writer at the University of Cincinnati} leaving details to appear in the original paper whenever The Journal shall resume publication. The chief interest of the investigation centers in the peculiarities met with in the transformation of the germ-cells of hybrid pigeons, hence the present paper will emphasize this phase of the subject.

    The germinal cells of the male pigeon are laid down in a great number of delicate convoluted tubules, which wind back and forth throughout the interior of each testis and make up its main bulk.


Figure 1 from Guyer's paper

Figure 1. Section of a tubule showing various stages in the maturation of the spermatozoon. s, Sertoli cell; sg, spermatogonia; scy. 1, primary spermatocyte; scy. 2, secondary spermatocyte; st, spermatid.


Near the periphery of each tubule are the spermatogonia, or parent cells (Fig.1, sg.), which through growth and division give rise to the various generations of germ-cells lying inward toward the lumen of the tubule. The adult spermatozoa are formed through the final transformation of the spermatids, or cells which lie nearest the center of the tubule, the product of the last cell-division. As in many forms, the spermatozoa attach themselves to a supporting cell (Fig. 1, s.) for a period before their complete maturation and ejection from the testis.

     The usual four phases or types of the germinal cells are recognizable, viz.:

  • (1) spermatogonia (Fig. 1, sg.), a more or less regular layer of cells lying next to the wall of the tubule, each cell of which through division gives rise to two new cells. One or both of these may increase in size and become

  • (2) primary spermatocytes (Fig. 1, scy, 1), or remain in the layer and continue as spermatogonia. The primary spermatocytes, after some interesting changes, divide to form

  • (3) the secondary spermatocytes (Fig. 1, scy.2), which divide again shortly and give rise to

  • (4) the spermatids (Fig.1, st.), through the transformation of which the spermatozoa are developed.

The number of chromosomes in each type as seen at the equator of the spindle before division is, in the spermatogonia sixteen loops, in primary spermatocytes eight rings, and in secondary spermatocytes four rings. The nurse cells, or Sertoli cells, to which the spermatozoa become attached at one period of their transformation (Fig. 1, s.), are irregularly disposed among the other cells.

THE SPERMATOGONIA    

The spermatogonia lie in a more or less regular layer along the wall of the tubule. In early stages they are far apart and possess small nuclei, which are oblong with the long axis parallel to the tubule wall. The cell boundaries are at first very indistinct or seemingly absent, and gaps frequently intervene between the individual spermatogonia, so that the latter seem to have been left behind from a preceding set, or to have recently settled in their present position. In later stages the cells are crowded together till they become columnar in shape, while the nuclei increase in size and become very distinct. A condensation of the cytoplasm, or mass of sphere substance (idiozome of Meves), makes its appearance, and gradually increases in size till it becomes a well defined area (Fig. 2, i). 

Figure 2 of Guyer's paper

Near the center of this mass is generally a clear area, in which a minute centrosome (c) is discernible. A nucleolar-like mass is usually visible within the nucleus, but judging from its reactions to various stains, it is nothing more than a clump of the same material that composes the linin. This mass usually takes part, together with the linin network, in forming the achromatic sheath within which each individual chromosome is incased when ready for division. Before division the nucleus passes through an incomplete spirem stage. The spirem breaks up into individual chromosomes, which are visible as irregular threads and loops scattered throughout the nucleus. 

    A filament two or three times as long as the other chromatic bodies is to be seen at times. Before the formation of the spindle for the ensuing cell division, this body is ejected into the sphere substance, where it seems to ultimately break up and become scattered throughout the cytoplasm. The significance of this phenomenon could not be determined. It seems improbable at present that the extruded body can be homologized with the "accessory chromosome" of McClung [Now known as the X chromosome.] {McClung, E. C. A peculiar nuclear element in the male reproductive cells of insects. Zool. Bul. II. 4, 1899}, a curious nuclear element, which he describes as occurring in Xiphidium fasciatum, one of the Locustidae. The "accessory chromosome," according to his account, does not disintegrate and disappear, but retains its individuality, and persists throughout the entire period of spermatogenesis, to take part finally in the formation of the spermatozoon.

    After the formation of the chromosomes, the centrosome, which lies in a clear area of the sphere, divides into two, one of which moves along the outer periphery of the nucleus to the opposite pole.Figure 3 of Guyer's paper The first appearance of the spindle fibers is as radiations which spread around the nucleus from the centrosomes. When the mitotic figure is fully formed, the spindle is short and broad and the chromosomes lie in a confused band at the equator (Fig. 3; a spermatogonium ready for division).

 

The individual chromosomes are loopshaped, with the closed end of the loop toward the center of the spindle. While moving toward the poles after division, not infrequently the free ends of a chromosome fuse to form a small ring. After repeated attempts at counting, it was determined that sixteen chromosomes are present at the equator of the spindle before division.

PRIMARY SPERMATOCYTES

The primary spermatocytes originate from the cells of the last spermatogonial division through a process of growth. The chromatin passes into the resting condition, and an increase in bulk of both the nucleus and the cytoplasm begins. The sphere (i) first appears as an indistinct granular crescenticFigure 4 of Guyer's paper area closely applied to the nucleus, with the horns of the crescent so extended as to inclose more than half of the nuclear surface. As the young spermatocyte grows, the sphere also increases in size, becoming more and more rounded. From an early stage a minute centrosome (c) is visible in the midst of the sphere substance. It is surrounded by a clear area, which becomes more pronounced as the sphere grows older. Thus the developing cell gradually acquires characteristics of size, shape and general appearance, that differ markedly from those of the previous generation.

Synapsis.- Synapsis occurs in the primary spermatocytes, during which there is a marked drifting of the chromatin to the side of the nucleus in contact with the sphere (Fig. 4; primary spermatocyte in synapsis). Some substance from the nucleus apparently passes out into the sphere; it may possibly be concerned in the formation of the extremely coarse-fibered spindle, for almost immediately the centrosome divides and the spindle appears. In the ensuing division of the spermatocyte only eight chromosomes are present, but they are in the form of heavy rings, and are evidently bivalent (Fig. 7).

Figure 5 Figure 6

Figure 5. Primary spermatocyte in the spirem stage.

Figure 6. The formation of ring chromosomes in the primary spermatocyte.  


   During division the eight-ring chromosomes, which are incased in capsules of linin, break transversely, and as they move apart remain connected by threads of the linin casing. These threads constitute the interzonal fibers (Fig. 8, if). An intermediate body is present at the equator of the interzonal fibers and marks out the path of the new cell walls (Fig. 8, ib). The ring type of chromosome seen at this division is formed through the breaking up or rearrangement of the prominent spirem (Fig. 5), which forms immediately after synapsis. The spirem-like appearance inside the nucleus disappears gradually, until by the time the centrosomes reach their positions at opposite poles of the nucleus, the components of the spirem are seen as eight elongated, irregular rings (Fig. 6), which consist of a linin groundwork, in which are imbedded numerous granules and lumps of chromatin. The rings gradually condense into a shorter, heavier type, and the chromatin fuses in such a way that distinct granules are no longer visible. In a few instances rings were found to consist of four more or less spherical, densely staining areas, connected by lighter bands of linin. It is possible that this is comparable to the tetrad formation so frequently observed in maturation phenomena (Fig. 6, tr).
Figure 7 Figure 8

Figure 7. Primary spermatocyte ready for division.

Figure 8. Division of primary spermatocyte nearing completion. if, interzonal fibers; ib, intermediate body.


SECONDARY SPERMATOCYTES

The product of the division just discussed consists of two cells, each of which is considerably smaller than the primary spermatocyte, and which never attains to its volume. These cells are the secondary spermatocytes. They go into a resting stage, which is of very short duration. When the secondary spermatocyte is ready for division, curiously enough only four chromosomes appear (Fig. 9). They are of the same shape and size as those in the division of the primary spermatocyte. In dividing, the chromosomes each break in such a way that a stringing out of the sheaths of the chromosomes gives rise, as in the primary spermatocytes, to a system of interzonal fibers, which, as division proceeds, constrict at the equator to form a large intermediate body. For some time after division the divided chromosomes are seen as four hollow vesicles within the daughter nuclei. They fuse later, ordinarily, into one large, hollow sphere of chromatin near the center of the nucleus (Fig. 10). Numerous fine fragments of chromatin migrate to the nuclear membrane, which has appeared in the meantime, and form a thin shelf of chromatin along its inner surface. The centrosome (Fig. 10, c) persists, and together with the tip of the spindle moves out into the cytoplasm. The tip of the spindle seemingly becomes re-converted into sphere substance.
Figure 9 Figure 10

Figure 9. Secondary spermatocyte ready for division.

Figure 10. Spermatid. c, centrosome lying in the persisting tip of the spindle.


THE SPERMATID AND ITS TRANSFORMATION

The new cell formed from the division of the secondary spermatocyte is the spermatid, and is the cell which will ultimately be transformed into the spermatozoon. An adult spermatozoon as it exists in the vas deferens of the pigeon is shown in Fig. 14. The head is long and narrow, and is intensely stained by nuclear dyes. Favorable preparations show the chromatin arranged in a series of vesicles within the head. Each vesicle of this chain-like series incloses a clear area, which in some preparations appears highly refractive. 

    A remarkable fact is that the number of vesicles is apparently the same as the reduced number of univalent chromosomes should be, namely, eight. In some instances, where only six or seven vesicles were present, it was observed that one or two were unusually large, and hence probably equivalent to two. It will be recalled that in the secondary spermatocyte there were only four chromosomes, but that they were of the bivalent type, or really comparable to eight ordinary chromosomes. There is no positive evidence that the vesicles in the head of the spermatozoon correspond to individual chromosomes, but the striking coincidence in number is at least very suggestive, and it would not be surprising if the fact develops later that after the entrance of the spermatozoon into the egg the vesicles resolved themselves into eight distinct chromosomes.

    At the anterior end of the head is a slender, fine-pointed head-spine. The head posteriorly connects directly with the long cytoplasmic tail. No middle piece is visible. The tail and the head-spine are very difficult to observe accurately, and but little of the details of their structure could be worked out. The only way to gain a satisfactory knowledge of the spermatozoon at all is through a study of its development.

Figure 11 Figure 12

Figure 11. Spermatid. ax, axial filament; c, centrosome; v, vacuole (head-spine).

Figure 12. Elongation of the nucleus of the spermatid to form the head of the spermatozoon. ax, axial filament; c, centrosome; h, head; hs, head-spine.    


    In the transformation of the spermatid to form the spermatozoon the first change to be observed is in the centrosome. It divides, and one of the resulting centrosomes enlarges and becomes ring-shaped (Fig. 11, c). The axial filament of the tail first appears as a thread connecting the two centrosomes, and later continues backward through the ring-like centrosome and out of the cell (Fig. 11, ax). The smaller centrosome, together with material of cytoplasmic origin, finally comes to lie within the nuclear membrane. It may be regarded perhaps as a middle piece, which becomes obscured by a covering of chromatin, and consequently appears to be absent in the adult spermatozoon.

    The long head of the spermatozoon is the transformed nucleus. In the process of elongation only what may be termed the anterior and the posterior ends of the nucleus extend at first, but in a short time the entire nucleus begins to narrow. At the same time the mass of chromatin at the center sprouts out both anteriorly and posteriorly to form a central, threadlike core (Fig. 12). As the process of elongation continues, a narrowing of the sides of the nucleus takes place to some extent, but when one takes into account the enormous elongation that occurs, together with the relatively slight diminution of the transverse diameter, it becomes evident that there must be considerable increase in the volume of the nucleus. The heavy central chromatic filament after a time becomes arranged in a wavy or spiral manner. As the transformation progresses the spiral design, although often very irregular, becomes more perceptible. A splitting of this spiral core finally occurs, and thereafter the chromatin exists as two threads laid down in an irregular double spiral (Fig. 13).

   

Figure 13 Figure 14

Figure 13. Continuation of the process shown in Fig. 12. ch, central core of chromatin which has split to form double spiral.

Figure 14. Mature spermatozoon. h, head; hs, head-spine; t, tail.


    The elongation of the nucleus ceases at about the time the bisection of the central filament has been accomplished, and the nucleus displays itself as an enormously long, sinuous head, which may measure twice the length of the head of the adult spermatozoon. A dense protoplasmic mass encases it and extends backwards along the axial filament. A shrinkage of the nucleus follows, in the course of which the double spiral of chromatin shortens and widens, until the exact relationship of the chromatin of the two filaments can no longer be determined. The final appearance is that of a chain-like series of vesicles, as described for the mature spermatozoon (Fig. 14), the clear area in the center of each vesicle corresponding to the openings between the respective points of intersection of the two spirals of chromatin.

   The head-spine originates from a bubble-like mass of material (Fig. 11, v) which arises in the sphere. This bubble or vacuole moves slowly around the periphery of the nucleus until it lies at the pole opposite the point at which the centrosome, which marks the anterior end of the axial filament, will enter the nucleus.

   

THE GERM-CELLS OF HYBRID PIGEONS

In the pigeon some crosses are fertile, others are not. The sterile hybrids show a greater or less degeneration of the germinal cells. The general rule seems to be that the more divergent the parent forms, the more marked is the degeneration of the germ cells.

    From parents which differ widely in structure or habits there seems to be much greater difficulty in securing female than male offspring [later enunciated as "Haldane’s rule" for the heterogametic sex, which is the female in pigeons. Click Here]. I have been able to obtain but one female offspring of very distinct species for microscopical examination, while, on the other hand, I have had six males.

    For all of this material I am indebted to Professor Whitman. From the testis of the offspring of the common ring dove, Turtur risorius, and the white ring dove, Columba alba, a large number of sections were made for microscopical study. These two forms are perfectly fertile when crossed, and the fertility of their offspring seems in no wise diminished. The germ-cells show some of the same phenomena as those of the sterile birds, though in a less marked degree.

    The common brown ring dove when crossed with the white ring dove produces brown offspring. One member of the resulting pair is usually a few shades lighter in color than the other. In the next or third generation there is generally a return to the original colors of the grandparents; one of the young is white, the other brown. There is a marked tendency for the white ones to be female and the brown ones male, this being true at least of the nine pairs killed by the writer. Occasionally, in the third generation both of the young are white or both brown.

    Of the sterile hybrids, whether male or female, the sexual products were abnormal. The abnormalities of male hybrids may be classified conveniently under three heads : (1) Abnormalities in mitosis; (2) abnormalities in the structure of the spermatozoon; (3) degeneration of the germinal cells. Not all hybrids show these various irregularities in the same degree. All three kinds of the phenomena just mentioned are observable in the sterile forms, but the fertile birds differ, for the most part, from normal pigeons only in the slight irregular character of the mitosis.

(1) The abnormalities in mitosis are in the nature of multipolar spindles and asymmetrical division and distribution of the chromosomes (Figs. 15-19). They are more pronounced in sterile birds, but may be met in fertile hybrids also. It is a curious fact that the multipolar spindles are confined largely to the primary spermatocytes, and one is inclined immediately to associate the fact with the pseudo-reduction or formation of bivalent chromosomes, which occurs normally at this stage of spermatogenesis. Figs. 15-18 show some of the different forms of multipolar spindles.

Figure 15 Figure 16 Figure 17 Figure 18 Figure 19

Figures 15-18. Irregularities in division of the primary spermatocytes found in hybrids.

Figure 19. Secondary spermatocyte of a hybrid, showing three normal and one dwarf chromosome.


    The tripolar types are by far the most common. Fig. 15 represents perhaps the most prevalent structure. It was not unusual to observe two spindles in one cell, as shown in Fig.16. When two such spindles exist independently in one cell, they may each have a small number of the large bivalent ring-form chromosomes, or a greater number of small chromosomes, which are apparently univalent. More rarely both large and small chromosomes occupy one or both of the spindles. The facts indicate that a pseudo-reduction has not occurred, or that it is incomplete or abnormal. What has just been said regarding the chromatin arrangement, where two separate spindles occur, is equally applicable to the multipolar forms, only there is generally more variation in the size of the chromosomes. In a tripolar type like Fig. 15 the chromosomes are almost invariably numerous and of small size.

    In a few instances two nuclei were present in the primary spermatocyte, and it seems probable that such cells give rise to the multipolar spindles, bearing an excessive amount of chromatin, which are sometimes seen. Fig. 18 shows a tripolar spindle in a primary spermatocyte, where there is much variation in the size of the chromosomes.

    An asymmetrical distribution of chromatin results, of course, in many cases where the division is by means of multipolar spindles. but in addition to this there is very frequently an unequal division of the chromosomes themselves. This occurs as often where the spindle is single as in any other case. In dividing, perhaps only one quarter of a chromosome will go to one pole and the other three quarters to the opposite pole, or the division may be such that a portion of the chromatin is cut out entirely and left behind in the cytoplasm. Fig. 19 represents a secondary spermatocyte, of which one chromosome is very minute, as if part of its material had been lost in the preceding division.

    In two or three instances one of the large chromosomes of the primary spermatocyte was observed to be made up of four small rings. Whether this indicates an exaggerated demarcation into tetrads it is impossible to affirm, though the fact is a significant one.

(2) Abnormalities in the structure of the spermatozoa are present in sterile hybrids: There is a curious varicosity about the middle of the spermatozoon head in such forms that attract the attention immediately, when the objects are examined under the microscope (Fig. 20; most of the tail is omitted in the drawing). This enlargement seems to be almost universal amongst the spermatozoa, and is sufficient of itself to produce sterility, for such a malformation would prevent its possessor from entering the egg. In a very few instances what appears to be a normal spermatozoon may be observed among the misshapen ones, and it is possible that if these reached a suitable egg, fertility might result. Their chance of meeting with the egg, however, is very slight. Figure 20

A study of the development of these deformed spermatozoa reveals the fact that the bead-like enlargement results from the incomplete development of the nucleus in the formation of the head. The two ends of the nucleus sprout out, as it were, and grow for a short distance, but the remainder of the nuclear wall retains its original form and position. The arrangement of the chromatin is very irregular. A deeply staining mass is visible in the bulb-like swelling, from which thick filaments spring out forward and to the rear.

(3) Degeneration of the germ cells was in progress in the testes of all sterile forms, but was most pronounced in hybrids from birds which were of very divergent species, or hybrids which were themselves descendants of fertile hybrids. There were some cases of such extreme degeneration that only the layer of cells lying along the wall remained in the tubule. Where such a degree of degeneration exists there is of course no approach to the formation of spermatozoa.

    There is often a strong invasion of wandering cells into the tubules, especially where the degenerative activities have become extensive. The interspaces between such tubules are also usually packed with cells which resemble white blood corpuscles. In some preparations it looks really as if the germinal cells themselves lose their walls and characteristic appearance and become leucocytes, though not definite conclusions could be reached regarding this point. Some tubules are occupied almost entirely by cells which have the exact appearance of the large stroma cells present outside the tubules.

    The primary spermatocytes seem to be the cells most susceptible to decay. Frequently the nuclear contents have a watery and disintegrated appearance and the sphere substance is marked by the presence of a large vacuole in its center. In testes where degenerative processes are pronounced a number of cells may run together to form a giant cell, in the center of which is an enormous vacuole surrounded by the nuclei of the original cells. The mass thus formed is very similar in appearance to the giant cells found in many pathological tissues.

    A detailed description of the individual hybrids will not be entered into at present, but will be left to the original paper. The crosses were: (1) Male black tumbler, female brown ring dove; (2) male black tumbler, female brown ring dove; (3) male hybrid from white and brown ring dove, female homer; (4) male wild passenger pigeon, female brown ring dove; (5) male wild passenger pigeon, female brown ring dove; (6) male turtle dove, female Japanese turtle dove. The hybrid from the last named was a female. The ovary and oviduct were rudimentary. Occasional small ova with an incomplete follicle were present. The largest egg had attained to a diameter of only seventy-five micra; (7) numerous crosses of white and brown ring doves.

CONCLUSIONS FROM THE STUDY OF HYBRIDS

It appears that in crosses of very divergent species the degenerative processes in the germ-cells are at a maximum. In closely related forms like the brown and the white ring doves fertility is not diminished, and the testes seem to be normal, except for occasional irregularities in mitosis.

    The formation of multipolar spindles in division and the unequal distribution of chromosomes seen in many instances are among the most interesting phenomena presented. Such irregularities, however, occur to a slight extent in normal birds. A very careful study reveals the fact that they are more prevalent in the ordinary dove-cot pigeons than in pure breeds of doves, which have bred true for many generations.

    That the irregularities of division in hybrids are due to degenerative processes going on in the tubules, which give rise to deleterious chemical substances, is the first thought that presents itself. This seems a very acceptable idea from what we know of the effects of drugs upon cell division, but it does not account for the fact that the primary spermatocytes are the cells attacked in the great majority of cases.

    A hybrid offspring is really a compound of two very different individual plasmas, hence conflicting tendencies must necessarily have been induced within its body. The abnormality in division may be but an attempt of each plasma to assert its individual activity. But why does this effort become apparent only in the primary spermatocytes?

    The answer appears to be simply that there is no necessity for fusion of the components of the chromatin except in the germ-cells at one stage of their maturation. Investigators have found that in maturation of germ-cells a reduction of the ordinary number of chromosomes to one half occurs. Before this actual reduction there is ordinarily a so-called pseudo-reduction, in which the chromosomes fuse in pairs, so that when the cell is ready for division, although only half of the regular number of chromosomes appear, each is really double (bivalent), and equivalent to two of the simple (univalent) type. In spermatogenesis this change generally comes about in the primary spermatocyte. 

  Now, in hybrids it may be supposed that in the ordinary cells of the body the chromosomes from the paternal and the maternal species lie side by side and carry on the customary functions of the cells, but when it comes to an actual fusion of the chromosomes to form the bivalent type necessary for reduction, the incompatibility of the two different plasmas renders the union incomplete or prevents it entirely.
   That the ordinary somatic cells of hybrids, either plants or animals, are under the influence of two distinct tendencies is well shown in the decided mosaic like structures which frequently occur, the classic figure being to liken hybrids to warp and woof. To cite but one example, Macfarlane found that a hybrid of the gooseberry and black currant., instead of being a strictly intermediate type, really possessed, side by side, organs characteristic of each parent; the leaves bore both the shield-shaped, oil-secreting hairs of the currant and the simple hairs of the gooseberry, though each hair was but half the size of the parent type.
    We may infer, then, that in certain hybrid pigeons the univalent chromosomes from each if the parents may lie side by side in the ordinary cells of the body and divide normally, but when it comes to the period of fusion in the germ-cell, they will not unite to form the bivalent type, or else they unite incompletely. The result is that in the primary spermatocyte, instead of one spindle bearing eight bivalent chromosomes, a multipolar spindle, or, not infrequently, two separate spindles, bearing two groups of univalent chromosomes, may appear. In cases where both large and small chromosomes are seen, it is necessary to suppose that a loose union has occurred in some chromosomes. The unequal division of the bivalent chromosomes of many hybrids indicate that such chromosomes have in some way been rendered very unstable.
    If we accept the view that chromatin is a substance capable of varying in qualities in the different regions of the chromosome, then in fertile hybrids, where irregular mitoses occur, the different germ-cells will certainly not be qualitatively similar after division, and one would expect the offspring produced from such cells [i.e. their somatic characters] to be variable. That the chromatin of each parent species often retains its individuality is indicated by the fact observed in many primary spermatocytes where two separate groups of the small or single type of chromosome exists. The division of such a cell into three or four, as the case may be, results in the formation of new cells, some of which will manifestly contain chromatin from only one of the original parent species, and some only from the other. Some of the spermatozoa, then, will bear chromatin from only one of these species. In the offspring from such a cell one would expect a much closer return to whichever one of the parent forms it represented than in the offspring of a "mixed" spermatozoon.
   In discussing irregular divisions, however, it must not be forgotten that many apparently normal divisions of the primary spermatocytes also occur in all hybrids, and constitute by far the predominant kind of division in hybrids from closely related forms. Unequal distributions of chromatin cannot therefore play the most important part in variation or reversion [of characters]. There seems to be no other interpretation, indeed, than that in the many normal mitoses of the bivalent chromosomes which occur, the chromatin of the father and of the mother is set apart so that the ultimate germ-cells are what might be termed "pure" cells; that is, a given egg or sperm-cell contains exclusively, or at least predominantly, qualities from one parent. The offspring from fertile hybrids of the same parentage might then be similar to the mixed type of the original hybrid, or revert to one of the grandparent types, dependent upon the chances of the various cells for union at fertilization. 

    If a spermatozoon and an egg containing characteristics of the same species unite, then the reversion will be to that species; if a sperm-cell containing the characteristics of one species happens to unite with an ovum containing characteristics of the other species, then the offspring will be of the mixed type again. By the law of probability the latter will be the more prevalent occurrence, because there are four combinations possible, and two of the four would result in the production of mixed offspring, while only one combination could result in a return to one of the ancestral species.

   From the fact that in cases of apparently complete return to one parent type, characteristics of the other parent may nevertheless crop out from time to time in succeeding generations, it is evident that all of the germ-cells are not absolutely "pure." The occasional inequalities in the division of individual chromosomes, as already mentioned, may account for this fact. It is probable that the irregular distributions of chromatin, where such occur, have more to do with such succeeding offspring as show variation, and less with those which return to the specific types. In the latter case the chromatin of each species has remained entirely distinct (in marked hybrids), or has normally separated again at the sundering of the bivalent chromosomes (in mild crosses) into the two original plasmas. It is very obvious, of course, that most of the variation seen in the offspring of fertile hybrids is due to the union again of two "pure" germ-cells, each of which represents a different one of the original parent species.

   That irregular divisions can not account entirely for reversions to grandparent types is very evident in the crosses of brown and white doves, where the irregularities are by far too few to equal the percentage of reversions. There is but one ultimate conclusion, then, namely, that the irregularity in division of the primary spermatocytes, which appears in hybrids between very different species is but an index to what occurs in ordinary crosses. In the latter, instead of separate spindles and non-fusion of chromosomes, a true union occurs, but the bivalent chromosomes ultimately divide in such a way that the respective plasmas occupy different cells. There is a separation of the paternal and the maternal chromosomes which had fused during synapsis..

   With regard to the question of the persistence of chromosomes, the evidence is becoming stronger every day that these elements do retain their individuality. Ruckert, for instance, { Ruckert, J. Zur Eireifung bei Copepoden: Qu. Hefte, 1894.} in his study upon the fertilization of cyclops, was able to follow the maternal and paternal chromosomes very distinctly in cleavage. Again, to cite only one or more of the rapidly multiplying examples, Herla and Zoja {Herla, V. Etude des variations de la mitose chez I'ascaride megalocephala: Arch. Biol., XIII., 1893; Zoja. R. Sullo independenza della cromatina paterna e materna nel nucleo delle cellule embrionali: Anat. An. XI., 1895} have shown that in the hybrid fertilization of Ascaris, if the eggs of variety bivalens is fertilized with the spermatozoon of variety univalens, the three chromosomes thus brought together retain their individuality and reappear at each cleavage, at least to the twelve-cell stage. Zoja affirms that the paternal chromosome is of smaller size and is thus distinguishable from the two maternal chromosomes.

   The above interpretations are offered with the hope that they may perhaps lead to some clew concerning the real nature of the material basis of heredity. If the conception proves to be a true one, then it doubtless affords a key, among other problems, to the longstanding one as to why many plants will come true from slips or grafts, but not from seed. The reason may be sought in the pseudo-reduction period of the germ-cell. Plants such as the apple, for example, which do not come true from seed, are practically multi-hybrid. In the germ-cells there will be numerous incompatibilities due to the fact that the plant has been miscellaneously fertilized for a number of generations. In propagation by means of slips, the chromosomes lie side by side and divide in the ordinary way to construct and maintain the new body, so that it is practically a continuation of the old one; but when the time comes for maturation of the germ-cells, the, lack of harmony between the various plasmas represented asserts itself, with the result that bivalent chromosomes are formed, which divide in such a manner as to segregate different sets of ancestral qualities. The resulting combinations in fertilization will give rise to seed many of which may possess dissimilar sets of qualities.

   As to the other abnormalities met with in the spermatogenesis of hybrids, about all that can be said is that the whole phenomena show lack of vigor in the development of the germ-cells, whatever this may mean. The deformed spermatozoa indicate want of sufficient vitality to push the development through to completion. The germ-cells start out apparently to perform their functions normally, but later succumb to the conflicting forces at work within their boundaries.

    As to why the reproductive organs should be more susceptible to changes than other regions of the body, we have no clew. Darwin has pointed out repeatedly the curious parallel between crossing and the change produced by physical conditions. Animals and plants removed front their natural environment are extremely liable to have their reproductive systems affected. Still he recognizes that sterility is incidental and not a necessary concomitant of hybridism. Hybridization in some forms, indeed, increases fertility.

SUMMARY

1. The usual four types of germinal cells are recognizable, viz : 

  • (1) spermatogonia, 

  • (2) primary spermatocytes, 

  • (3) secondary spermatocytes, and 

  • (4) spermatids.

Sertoli or nurse cells are likewise present.

2. The number of chromosomes in the spermatogonia is sixteen, in primary spermatocytes eight, and in secondary spermatocytes four.

3. The spermatogonia vary considerably in appearance at different phases of their activity. A sphere (idizome), within which the centrosome lies, is visible before division.

4. A filament two or three times as long as the other chromatic bodies in the nucleus is cast out into the cytoplasm before the formation of the spindle for division of the spermatogonium.

5. Synapsis occurs in the primary spermatocytes, during which there is a marked drifting of the chromatin to the side of the nucleus in contact with the sphere.

6. At the division of the primary spermatocyte, only eight chromosomes are present, but they are in the form of heavy rings, and are evidently bivalent.

7. In division the chromosomes break transversely, and as they move apart, remain connected by threads of the linin casing which encapsuled the chromosomes. These threads form the interzonal fibers.

8. Intermediate bodies are present at the equator of the interzonal filters, and mark out the path of the ensuing division of the cytoplasm.

9. At the division of the secondary spermatocytes, the four chromosomes which appear are of the same size and shape as those of the preceding division.

10. In the transformation of the spermatid the first perceptible change is in the centrosome. It divides, and one of the resulting centrosomes enlarges and becomes ring-shaped. The axial filament of the tail first appears as a thread connecting the two centrosomes, but later continues backward through the ring-like centrosome and out of the cell.

11. The smaller centrosome, together with material of cytoplasmic origin, finally comes to lie inside of the nuclear membrane. Although a middle piece appears to be absent in the adult spermatozoon, it seems probable that this centrosome within the nucleus may function as a middle piece which has become obscured by a covering of chromatin.

12.The nucleus elongates to form the long head. It contains a central core of chromatin in the form of a spiral filament, which splits later to form a double spiral.

13.The head, during the later stages of development, undergoes a very great contraction, but the spiral arrangement of the chromatin still persists in a modified form.

14.The chromatin appears finally to be arranged within the head in a series of vesicles. A remarkable fact is that the number of vesicles is the same as the reduced number of univalent chromosomes should be, namely, eight.

15.The head-spine originates from a bubble-like mass of material which arises in the sphere of the spermatid.

16. The general plan of spermatogenesis in hybrid pigeons is not essentially different from that of normal pigeons.

17. All hybrid pigeons exhibit multipolar spindles and asymmetrical distributions of the chromatin in cell division. These irregularities are much more infrequent in fertile hybrids.

18. Infertile hybrids show in addition a deformed spermatozoon, and often a marked degeneration of the germinal cells.

19. The irregularities of division are confined for the most part to the primary spermatocytes. Likewise it is in these cells that the formation of bivalent chromosomes occurs normally. In hybrids, it would seem that the conflicting tendencies of the two parental plasmas frequently render the union of the single chromosomes to form the double (bivalent) types impossible or abnormal. 

    There seems to be an attempt o the part of each plasma to assert its individuality. This visible incompatibility of the chromosomes from widely different species serves as an index to a kindred lack of harmony between the plasmas of more nearly related forms, so that even though pseudo-reduction does occur and normal division of the bivalent chromosomes follows, the identity of the individual species is still retained through the segregation of the maternal and paternal chromosomes into separate cells, which may be considered "pure" germ-cells (containing qualities of only one species).

20. Union of two cells containing characteristics of the same species would occasion a reversion to that species. Union of two cells representing each of the two original species would yield an offspring of the mixed type. The latter would predominate because of the greater probability of such union. 

    Besides, through the mixing just indicated, variability may be due also in some cases to the not infrequent inequalities in the division of individual chromosomes, through which varying proportions of the chromatin of each species may appear in certain of the mature germ-cells.

21. Irregular divisions can not of themselves account entirely for reversion and variations, because double spindles and irregularities in the formation of bivalent chromosomes are by far too few to equal the percentage of reversions seen in such mild crosses as [between] the brown and white ring dove. One is forced to the conclusion expressed above, that the double-spindled and multipolar types of cells which occur in hybrids between very divergent forms are but exaggerated images of a tendency which exists in the primary spermatocytes of normal appearance, which are to be found in all hybrids.

22.The above conception may likewise afford a clew to the problem of why certain plants will come true from slips or grafts, but not from seed. The explanation may be sought in the pseudo-reduction period of the germ-cell.

DEPARTMENT OF BIOLOGY. UNIVERSITY OF CINCINNATI

October 1902.

Note. In as much as the present paper is a resume of the writer’s thesis of 1900, it is not deemed advisable to enter into a discussion of any of the more recent papers which have a bearing on the results obtained. Juel’s paper on hybrids of Syringa, which did not appear until later, is perhaps the most significant. His hybrids exhibited abnormal mitoses similar to those that are found abundantly in pronounced pigeon-hybrids. From his facts I would suggest that the same interpretation as set forth in my conclusions holds true, although it can not be extended in the same detail to fertile forms, because his hybrids were sterile.

    Two very brief abstracts of one or two of the more unusual points presented in the writer’s thesis have appeared in Science, but thus detached from the body of the work, it would seem from the tenor of letters which have been received that in some cases a misunderstanding of the writer’s exact position has arisen. It is hoped that the present paper will set forth the main facts in their true perspective.

End of Guyer 1902 paper.

For Guyer's complete 1900 thesis (Click Here)

 

A Cytological Basis for the Mendelian Laws

The International Conference on Plant Breeding and Hybridization opened on September 30th 1902 in New York, USA, with an address by William Bateson (Proceedings, pages 1-9). Later a brief paper by W. A. Cannon of Columbia University was read by D. T. MacDougal (Proceedings, pages 89-92). A fuller account was published in December in the Bulletin of the Torrey Botanical Club.

D. R. Forsdyke 2001

Some Cytological Aspects of Hybrids

W. A. Cannon. Proceedings of International Conference on Plant Breeding and Hybridization. pages 89-92.

Until recently research upon hybrids has been almost entirely confined to the more practical problems, such as are connected with the formation of new or better sorts of plants, and students of pure science have given little attention to the subject. At the present moment, however, much interest is manifested by scientists in this direction, and this awakening interest is largely due to the republication by Correns and De Vries of Gregor Mendel's experiments and results. The more recent studies have a two fold nature. They are either experimental, as the well known work of De Vries, Correns, Webber and Tschernak, or they are cytological; and each phase of the work has been undertaken entirely independent from the other.

    What are the relations of the cytological to the experimental researches of hybrids? This question can hardly be answered at this time, but the possible connection between the two may be pointed out, however.

    Although it is not the purpose of this paper to set forth the conclusions of the experimenters, for illustration and for reference, it will be convenient to briefly present those of Mendel.

    Mendel studied the behavior of hybrid plants, not only for the first generation - that of the immediate cross - but in the following ones as well, and learned

  • (1) that in the first generation the hybrid shows the characters of one parent only, but that

  • (2) the characters which were latent in this generation appear in the later ones, and in such a manner that the plants having the "dormant" and the "recessive" characters bear a certain and definite ratio to one another.

   In this connection it need only be said that while the conclusions of Mendel have been verified by the later researches, they have been found to be limited in their application to a special type of hybrid, namely, the Pisum type, while the greater number of hybrids belong to other types.

    In the Pisum type the plants behave as if the bundles of inheritance which were derived from the parents of the original cross were kept separate, and were delivered as such to the succeeding generation, and, as if in fertilization, these were united in all possible proportions. This has been expressed by the following formulae: A + A, A + a, a + A, a + a

    The result is that a certain percent of the hybrids revert to the characters of the parents of the original cross. Or, in other words, according to the laws of Mendel, we might expect that the chromatin derived from the primitive parents maintained its individuality, and was disposed in such a manner at the time of the maturation mitosis that the resulting sex cells were not hybrid, but pure.

    A portion of this hypothesis has already been demonstrated to be true, namely, in hybrid ascaris, the chromosomes derived from the variety bivalens keep separate from the one which is evidently from the univalens parent. And it will appear presently that an analogous condition may obtain in other hybrids.

    That a hybrid sex cell has chromatin of pure descent - that is, has chromatin from one parent only, as the second law of Mendel apparently demands - is yet to be proved, and may be doubted on a priori grounds, since the wall separating the daughter nuclei is laid down at right angles and not parallel to the spindle of the dividing nucleus. However, if it can be shown that the Pisum type of hybrid, for example, is associated with the general behavior of the chromatin, as just suggested, a very important and forward step in explanation of the nature of this and other types of hybrids shall have been made. On the other hand, it will be quite as important to demonstrate that variation in hybrids may take place, even if he sex cells are formed in a manner identical with those in pure races.

    Now that I have pointed out the possible relation between the experimental and cytological study of hybrids, I hasten to say that, in reality, up to the present moment there has been no such connection, since the cytological work was mainly completed before the experimental was published; and, further, since the forms which have been turned to account cytologically have not been studied experimentally. With this I shall proceed to speak briefly of the cytological work.

    This has comprised studies by Guyer on the spermatogenesis of hybrid pigeons and cannas, by Juel on the spermatogenesis of hybrid Syringa, and by the writer on spermatogenesis of hybrid cotton. In addition, the study on Ascaris, already alluded to, and also the structural study of several hybrid plants by Macfarlane may be included here.

    Concerning all the plants upon which cytological studies have been made it may be said that they are first generation hybrids; that is, they are the offspring of the original cross.

    Guyer reports upon the spermatogenesis of hybrid pigeons, and also cannas, but since the results are analogous and the conclusions identical I need only outline the results of his study of the former.

    I understand that Guyer found in the pigeon normal and abnormal spermatozoa and normal and abnormal maturation mitoses. In the normal male nuclei the first maturation division showed ring-form chromosomes, of uniform size, and of half the somatic number. He found, on the other hand, in the abnormal maturation mitoses that the ring-formed chromosomes might be of two sorts, large and small, or they might all be small; and if the latter, they were the same number as those in the somatic nuclei. In such abnormal nuclei there was a tendency of the chromosomes to form into two groups, and this localization was taken to indicate the maintenance by the chromosomes of their individuality. The general conclusion of the author was, that the variation noted in the maturation mitoses was likely associated with the variation of the hybrids themselves.

    Guyer had, I believe, like results from his study of hybrid cannas, except in the canna he found all gradiations between direct and indirect divisions.

    The writer has studies the maturation divisions in the male nucleus of hybrid cotton, which was supplied to him by Dr. Webber. The results of this study may be summarized as follows: Two sorts of maturation divisions were distinguished, on clearly normal, the other abnormal. Concerning the former it need only be said that the ring-formed chromosomes of the first division were of uniform size and of reduced number. The spindle gave no indication of being double, and was entirely analogous with that in the pure race. The abnormal divisions were all direct, but varied in a manner of which is is unnecessary here to speak further. Transitions between direct and indirect divisions were not observed.

    From my study I concluded that the normal divisions lead to fertility, and the abnormal to sterility.

    The remaining purely cytological study is that by Juel on the spermatogeneses of Syringa rothomagensis. Juel found abnormal mitoses only. These were in all stages between direct nuclear divisions and indirect, save only that the chromosomes did not split in the metaphase.

   What is the significance of Juel's results? It is of interest here to note that the plant may be wholly sterile, since Focke observes that he has seen no fruit on it - that is, the abnormalities of spermatogenesis seen by Juel in the hybrid Syringa lead to sterility.

    Thus, Juel's results cause one to ask whether some, if not all, of the abnormal sex nuclei observed by Guyer in pigeon and canna were not functionless, and whether the normal divisions alone do not lead to fertility? Such is apparently the case in the hybrid cotton. If only sex nuclei that undergo typical maturation mitoses form functional reproduction cells, it is clear that the variation, in some hybrids at any rate, must be occasioned by other causes than those of the irregularities in the splitting, distribution and union of the chromatin of the sex cells.

    Here it may be suggested that an experimental study of such a hybrid as shows a tendency toward maintaining the individuality of its chromosomes might be carried on with good results. Such a study should show the relative sterility of the sex cells, and thus it would supplement the results derived solely from the cytological investigations. This has not yet been done, however.

    While the cytological data just given do not harmonize completely, further study may show that they are really not antagonistic, but that the different hybrids studied should either be placed in different hybrid classes, or that the "splitting" may not occur in the first, but later generations.

     Thus the leading results which have been obtained by the cytological study of hybrids may be summarized as follows 

  • (1) The establishment of one cause of sterility among hybrids.

  • (2) The variation of the hybrid may or may not be associated with abnormalities of spermatogeneses; and

  • (3) That in certain cases the chromosomes derived from the original parent tend to preserve their individuality.

    Before closing this brief account of the cytological studies I wish to speak of the work of Macfarlane on the structure of certain hybrids, a work published about ten years ago. After giving in detail the minute structure of several hybrids, this author arrives at his general conclusion concerning the nature of the hybrid, namely, that it is a blending of the characters of the parents, each parent contribution equally to the offspring. The hybrid is thus intermediate in structure between the parents.

    Although it may be at this time premature to suggest a relationship between the results of studies in the structure and those from experiment, nevertheless it is interesting to note that practically all of the hybrids reported by Macfarlane are of the intermediate type of Correns, and perhaps, therefore, such results as Macfarlane obtained might thus have been expected. From this possibility is suggested that hybrids which are not of the intermediate type may have a minute structure which is likewise not intermediate. No work on this phase of the subject has yet been done, however.

    Finally, as has already been stated in the foregoing summary of the recent cytological work upon hybrids, this line of research and the experimental have gone on independently of each other, but it may be reiterated here that in order to better understand the causes of the variations in hybrids and that of the differences in capacity for variability it is highly desirable that the cytological and experimental work go hand in hand, that cytological work be done on forms that give marked experimental results.

A Cytological Basis for the Mendelian Laws

BULLETIN OF THE TORREY BOTANICAL CLUB 29, 657-661.

December 1902

BY WILLIAM AUSTIN CANNON of  NEW YORK BOTANICAL GARDEN.

In the decade following the year 1860, Gregor Mendel, an abbot of the Roman Church, experimented in the garden of his abbey in Brunn with plant hybrids. This experience led him to results and conclusions now believed by students of heredity to he of great importance. These were published in the Verhandung des Naturforschendun Vereins of Brunn and were lost to the view of scientists until their rediscovery about two years ago by de Vries, Correns, and Tschermak. 

    The plants experimented with by Mendel were mainly species of Pisum, Phaseolus, and Hieracium, and, although the results were in a measure contradictory, those founded upon his pea experiments were uniform, and constituted the basis for his conclusions, namely, those expressed by the "Mendelian Laws." The essential conception of Mendel may be briefly stated as follows:

   When one pure form (A) is crossed with another pure form (a) the hybrid of the primary cross shows the (A) characters only. When, however, the hybrid plants of this generation are fertilized among themselves and produce offspring, the (a) characters are first seen, and in a definite proportion to the form bearing the (A) characters. These constitute the hybrids of the second generation. 

    If now the hybrids of the second generation are fertilized in such a manner that plants with (a) characters are crossed with those bearing the same characters, and likewise plants bearing the opposing characters with forms like themselves, the resulting hybrids will behave in a manner characteristic of the respective cross. That is:

1) The plants with (a) characters will be found to transmit those characters only, i. e., they are "fixed"; and

(2) When the plant with (A) characters are fertilized with other plants with the same characters, that is to say, if inbred, two sorts of hybrids will result: one portion will bear only the (A) characters, which may be demonstrated by inbreeding as before, and one portion, apparently also with (A) characters only, will be found to vary just as the hybrids of the primary cross varied, i.e. this portion is really mixed or hybrid.

     The hybrids that bear the (a) characters are known as the "recessives" ; they do not appear in the first generation, and those with the (A) characters are called the "dominants," and they mask in the first generation the recessives. [Section omitted by DRF] Not only do the hybrids vary thus in a regular manner but there is also a definite proportion of recessives (a), and dominants (A), as the table indicates [omitted]. That is, in the second generation one fourth of the offspring is recessive (a), and three fourths apparently dominant (A) only, but really composed of the two sorts, one third of these being dominant (pure A), and two thirds mixed (A(a)). The latter continue in the succeeding generation to vary just as the hybrids of the primary cross varied, i. e., one fourth of their offspring bearing recessive, one fourth bearing dominant characters, and one half being both dominant and recessive.

    The regularity in the variation as just described in the second and later generations is accounted for by supposing that the hybrids of the first generation organize germ cells which are of pure descent, and that these unite in fertilization according to the laws of chance. Taking a specific case by way of illustration, we can imagine the following to take place when the sex cells A(a) of say the second generation meet each other in fecundation.

    The pollen, which is of pure descent, unites with the egg, which also is of pure descent, and the chances of union may be thus expressed: A a; A A ; a a; a A. So that it happens, since the anther forms two sorts of germ cells, and the ovary also two sorts, that in this way one half of the hybrids of say the third generation will be of mixed descent, and one half of pure, the latter being equally recessive and dominant. The results, as calculated by the laws of chance, are thus seen to be precisely the same as what is found empirically to occur.

    Such are the more essential facts and conclusions of the discovery by Mendel, and upon them are based the two so-called "laws" of Mendel, namely, 

  • the law of dominance and that of 

  • the splitting of the hybrid race.

The latter alone concerns us at present.

   We now arrive at the interesting question, Is there a cytological basis for Mendel's law of the splitting of the hybrid race ?

    Bateson has recently suggested the idea that the

"essential part of the discovery (of Mendel --the italics are my [Cannon's] own) is the evidence that the germ cells or gametes produced by cross-bred organisms may in respect of given characters be of the pure parent types and consequently incapable of transmitting the opposite character".

  (The italics are in the original.) This notion has also been expressed by others, or may be implied from their conclusions. Assuming such to be the case, how may we account morphologically for the purity of the sex cells? 

    Do the sex cells, which are thus shown by experiment to be pure, arise by normal maturation mitoses, such as take place in pure races, or are the divisions irregular, abnormal, and peculiar to each hybrid organism? It has, I think, generally been felt by botanists that the variations in the hybrids were, in some manner, connected with that of the formation of the sex cells from which they arose, and this has apparently received cytological support. For instance, both Guyer, from his morphological studies of hybrid pigeons, and Juel, from studies of hybrid Syringa, arrive at the conclusion last given, although this must be implied from Juel's results, as for example, his account of how a Syringa hybrid pollen grain may become pure as respects the chromosomes of its nucleus.

    The pigeon hybrid was a fertile one and the Syringa infertile; the possibility thus comes up of the variation in the hybrid pigeons being caused and brought about by the normal, rather than the irregular maturation mitoses. I have for two years past been studying the spermatogenesis of a fertile cotton hybrid, and I have attained results similar to those of Guyer. In the case of the cotton, however, the abnormal divisions, were so clearly such, that sex cells arising from them would, in all likelihood, not be capable of continuing the race. It, therefore, seemed to me that, at least in the cotton, variation in the hybrid offspring must come about either because the maturation mitoses were such as would induce them, or quite independently of these nuclear divisions, since, in fertile hybrids the mitoses are normal.

    The nuclear divisions, from which the, pollen grains arise, as commonly understood by botanists to take place, would surely not induce the variation in the hybrids after the regular manner demanded by the law of Mendel, and, believing that this variation does not occur independently of these divisions, I venture to suggest a kind of maturation division which would, I believe, account for the variation as above given, and at the same time agree fully with the present day observations on the divisions if not with the conclusions derived therefrom.

      This matter finds an apparently adequate explanation if we accept the results of Ruckert and others (Wilson, The Cell, 257 and 273) based on the study of pure forms of both vertebrates and invertebrates. These results may be stated in brief as follows: The chromosomes derived from the father and the mother unite in synapsis and separate in the metaphase of one of the maturation divisions, and also a single longitudinal division occurs, so that the end is attained that the chromatin is distributed in such a way that two of the cells receive pure paternal, and two cells pure maternal chromosomes, and no cells receive chromosomes from both the father and the mother. In this manner it has been demonstrated that pure races of animals may, and normally do, organize sex cells of pure descent.

    Now since such is shown to be the case in pure races of animals, I suggest that the sex cells of fertile animal hybrids are formed in a similar way, and thus we may have in animals a cytological basis for variation in accord with the Mendelian conception. And I further suggest that this is the case in plants as well. This notion is, I am well aware, squarely opposed to the present conception of the nature of the maturation mitoses in plants, but I submit (1) That the optical effect in the dividing sex nuclei would be the same in either case, and (2) That closer study of the early stages in the spermatogenesis of plants would give a result entirely analogous to the results drawn from analogous morphological studies of animals.

A Study of the Chromosomes of the Germ Cells of Metazoa

Thos. H. MONTGOMERY. University of Pennsylvania, Philadelphia

Trans. Am. Philosophical Society (1901) 20, 154-236.

Montgomery was particularly concerned about the function of the pairing of maternal and paternal genomes, the "synapsis stage". Some quotations follow:

"Thus, each germinal cycle shows the following well-marked stages: conjugation or fertilization, a stage of a number of ovogonic or spermatogonic generations, the synapsis stage coincident with the growth period, and the stage of the two maturation divisions. Each such cycle is succeeded by a similar one, and so on indefinitely for an indefinite number of cycles.

    Now it is unthinkable that a cycle should be without a beginning; it must have been gradually evolved, and some particular stage in it must have been the starting point. What was this first stage?

    An answer is necessary before we can enter into the discussion of the meaning of  the synapsis stage. It appears to me most probable that the stage of conjugation of the germ cells  must be considered the starting point. For from the studies of R. Hertwig and Maupas on Infusoria, it appears probable that conjugation or fertilization is essentially a process of rejuvenation: cells may divide and reproduce for a number of generations asexually, but there comes a period when the cellular vitality diminishes, so that no further reproduction is possible except after rejuvenation by conjugation with another cell. When thus rejuvenated by admixture of substances from the other conjoint, the cell starts upon a new period of generation - the period of conjugation thus being the commencement of the cycle. As we shall see, the synapsis stage is really a delayed part of the process of conjugation, and the growth period is induced by the synapsis of the chromosomes. Having determined the starting point of the germinal cycle, we may now consider the synapsis stage.[Section omitted by DRF]...

    These considerations render it very probable that in the synapsis stage is effected a union of paternal and maternal chromosomes, so that each bivalent chromosome would consist of one univalent paternal chromosome and one univalent maternal chromosome. This conclusion allows us to consider the synapsis stage in an entirely new light, and gives an important significance to this stage.

    The synapsis stage then, which is characterized by the union of chromosomes into bivalent pairs, may be considered the stage of the conjugation of the chromosomes. When the spermatozoon conjugates with the ovum there is a mixture of cytoplasm with cytoplasm, of karyolymph with karyolymph, possibly also an intermixture of other substances; but there is no intermixture of chromatin, for the chromosomes then, as we have seen, remain more separated from one another than at any other stage -- in fact, the paternal chromosomes seem to show a repulsion for the maternal, inasmuch as they are arranged in separate groups.

     But after this beginning stage of the germinal cycle, the repulsion of the paternal for the maternal chromosomes gradually diminishes, is generally no longer recognizable in the last of the spermatogonic and ovogonic divisions, and in the synapsis stage instead of repulsion we find a positive attraction between the paternal and maternal chromosomes.

   The reason for the final union of these chromosomes is obvious; it is evidently to produce a rejuvenation of the chromosomes. From this standpoint the conjugation of the chromosomes in the synapsis stage may be considered the final step in the process of conjugation of the germ cells. It is a process that effects the rejuvenation of the chromosomes; such rejuvenation could not be produced unless chromosomes of different parentage joined together, and there would be no apparent reason for chromosomes of like parentage to unite. At the same time the so-called "reduction in number" of the chromosomes is effected, but this is probably no primal, but rather a necessary result of the conjugation of the chromosomes. [Section omitted]

     In the synapsis stage there is a conjugation of paternal and maternal chromosomes for the purpose of rejuvenation of chromosomes as metabolic centres, and this rejuvenation is exemplified in the great metabolic activity of the growth period. Now R. Hertwig and Maupas have shown for Infusoria  that the two conjoints remain for only a certain period in apposition, and that when the interchange of nuclei necessary for rejuvenation has been accomplished the conjoints separate. Of course, it is not a true analogy to compare conjugating Infusoria (i.e. whole cells) with conjugating chromosomes (i.e. a portion of cells). But still it is very probable that the two chromosomes unite temporarily for the same reason that two Infusoria do, that is, for an interchange of substances; and when the chromosomes have accomplished this interchange there would no longer be an necessity for continued apposition, they they tend to separate from one another."

 

The Chromosomes in Heredity

by Walter S. SUTTON.

Biological Bulletin (1903) 4, 231-251.

 

On pages 24-39 of Biological Bulletin, dated October 17th 1902, Sutton noted Montgomery's "suggestion that maternal chromosomes unite with paternal ones in synapsis" and briefly called "attention to the probability that the association of paternal and maternal chromosomes in pairs and their subsequent separation during the reducing division ... may constitute the physical basis of the Mendelian law of heredity." 

    This was amplified in the 1903 paper which, summarizing the above work and that of Montgomery, Bateson and Saunders, Bovari, McClung, and himself, set out quite clearly the idea of the random assortment of paternal and maternal chromosomes in germ cells where meiosis is normal (no hybrid sterility). Some quotations follow.

D. R. Forsdyke 2001

"It has long been admitted that we must look to the organization of the germ-cells for the ultimate determination of hereditary phenomena. Mendel fully appreciated this fact and even instituted special experiments to determine the nature of that organization. From them he drew the brilliant conclusion that, while in the organism, maternal and paternal potentialities are present in the field of each character, the germ-cells in respect to each character are pure.

    Little was then known of the nature of cell division, and Mendel attempted no comparisons in that direction; but to those who in recent years have revived and extended his results the probability of a relation between cell organization and cell-division has repeatedly occurred. Bateson {Mendel's Principles of Heredity, Cambridge 1902, p. 30} clearly states his impression in this regard in the following words:

"It is impossible to be presented with the fact that in Mendelian cases the cross-bred produces on an average equal numbers of gametes of each kind, that is to say, a symmetrical result, without suspecting that this fact must correspond with some symmetrical figure of distribution of the gametes in the cell divisions by which they are produced." "

 

Random assortment of chromosomes

"The results give no evidence in favor of parental purity of the gametic chromatin as a whole. On the contrary, many points were discovered which strongly indicate that the position of the bivalent chromosomes in the equatorial plate of the reducing division is purely a matter of chance -- that is, that any chromosome pair may lie with maternal or paternal chromatid indifferently toward either pole irrespective of the positions of other pairs, -- and hence that a large number of different combinations of maternal and paternal chromosomes are possible in the mature germ-products of the individual."

Linkage groups

"We have seen reason, in the foregoing considerations, to believe that there is a definite relation between chromosomes and allelomorphs (Bateson's term) or unit characters, but we have not before inquired whether an entire chromosome or only a part of one is to be regarded as the basis of a single allelomorph [allele]

    The answer must unquestionably be in favor of the latter possibility, for otherwise the number of distinct characters possessed by an individual could not exceed the number of chromosomes in the germ products, which is undoubtedly contrary to fact. We must, therefore, assume that some chromosomes at least are related to a number of different allelomorphs. If then, the chromosomes permanently retain their individuality, it follows that all the allelomorphs represented by one chromosome must be inherited together."

Dominance

    "On the other hand, it is not necessary to assume that all [ allelic characters ] must be apparent in the organism, for here the question of dominance enters and it is not yet known that dominance is a function of an entire chromosome. It is conceivable that the chromosome may be divisible into smaller entities (somewhat as Weismann assumes), which represent the allelomorphs and may be dominant or recessive independently."

Blending inheritance

    "In treating of this class Bateson clearly states the possibility that the case may be one entirely "apart from those to which Mendel's principles apply", but goes on the show how it may possibly be brought into relation with true Mendelian cases. He says in part: 

"It must be recognized that in, for example, the stature of a civilized race of man, a typically continuous character, there must certainly be on any hypothesis more than one pair of possible allelomorphs. There may be many such pairs, but we have no certainty that the number of such pairs and consequently of the different kinds of gametes are altogether unlimited, even in regard to stature. If there were even so few as, say, four or five pairs of possible allelomorphs, the various homo- and heterozygous combinations might, in seriation, give so near an approach to a continuous curve that the purity of the elements would be unsuspected, ..."

 

Synapsis

"In the normal process synapsis must be accounted for by the assumption of an affinity existing between maternal and paternal homologs, and conversely reduction is the disappearance of that affinity or its neutralization by some greater force."  ...

Sutton's discussion of Guyer's paper 

    "The interesting and important communication of Guyer on 'Hybridism and the Germ-Cell' is received too late for consideration in the body of this paper. ... Irregular division of chromosomes would be likely to produce marked variation, but as Guyer himself observes, these irregularities increase with the degree of infertility. It seems natural to conclude, therefore, that they are not only pathological, but perhaps in part the cause of the infertile condition".

Two major issues emerge from the above papers. Seeming to be viewed as the most important was the relationship of cytological observations of meiosis to the degree of relatedness of the parents (i.e. the cytological basis of hybrid sterility). The issue of whether the chromosomes were the material carriers of hereditary information, the issue for which the papers were most cited throughout the 20th century, received less attention. This was perhaps because there was already little doubt in the minds of the authors that this was so. 

    For example, the Dutch botanist Hugo de Vries in his Intracellular Pangenesis (1889) had already speculated that an observed character of an organism could be "determined by a single hereditary character or a small group of them." These determining "hereditary characters" were "units, each of which is built up of numerous chemical molecules" and were "arranged in rows on the chromatin threads of the nucleus." 

Donald Forsdyke circa 2002

The fine work of Patrick Bungener and Marino Buscaglia (2003) reinforced the above conclusions ("Early Connection between Cytology and Mendelism: Michael F. Guyer's Contribution" Hist. Phil. Life Sci. 25, 27-50). A paper they cited suggested an answer to the question:  How did Guyer himself respond to the rediscovery of the work of Mendel? 

The Germ Cell and the Results of Mendel. 

Dr. MICHAEL F. GUYER 

The Cincinnati lancet-clinic 1903, 53, 490-491  

Mendel and other investigators have found that when two pure forms are crossed, a given character of one parent as a rule predominates in the hybrid offspring, and masks the corresponding character of the other parent. For example, if in plants the seed of one parent is round and that of the other long, then the seed of the hybrid will all be round or all long, depending upon which parental character predominates.

    Furthermore, in respect to a given character, as, for example, the shape of a seed, if the hybrids are fertilized among themselves, one-fourth of the resulting offspring will show again the particular ancestral character which was masked ("recessive"), one-fourth the ascendent character ("dominant"), which obscured its fellow, and the remaining half, though apparently of the same ascendent type, will really continue to be hybrid or mixed. 

    Both the particular isolated dominant character in question and its complementary recessive persist and remain pure in succeeding generations, provided the respective forms which bear them are not again cross-bred, but the mixed type will break up each time into three groups similar in proportion and kind to those derived from the original hybrids.

To account for the behavior of the two sets of parental characters it was supposed that any particular quality from one line of ancestry does not occur in the same germ-cell with a corresponding quality from the other line of ancestry. In such an event, since there are two parents and consequently two sets of qualities or characters, half of the germ cells will contain a given quality from one line of ancestry and the other half will contain the corresponding character from the opposite line. 

    If we designate these parallel qualities as A and B respectively, then half the germs cells of both the male and the female hybrid will contain the A character and half the B character. Consequently, when two germ cells unite in fertilization, according to the laws of chance, the qualities may be brought together as AA, AB, BA or BB. AB and BA are of the mixed type, and will occur in one-half of the offspring; AA will be represented exclusively in one-fourth and BB in the remaining fourth. That is, with respect to this quality, one fourth have reverted to one grandparent, one-fourth to the other, and half continue hybrid.

The above conclusions were derived from the study of the characters of hybrids, and not from a study of the germ cells. The question naturally arises as to whether there is any evidence from the study of the germ cells themselves to bear out this theory of the separation and isolation of the corresponding qualities of each parent.

    In studying the germ cells of hybrid pigeons some years ago (1897-1900){Footnote: Guyer, M. F. "Spermatogenesis in Normal and Hybrid Pigeons." Dissertation, University of Chicago, 1900; also, Bulletin 22, University of Cincinnati.}, the writer observed certain phenomena which led him to much the same opinion regarding the separation of qualities in germ cells as that expressed by Mendel, although at the time the results of Mendel were wholly unknown to him.

     In the light of the Mendelian law, the facts discovered at that time become doubly significant, and seem to point precisely to such a state of affairs in the germ cells as the law would necessitate, although the general conclusions originally drawn from the facts by the present writer will have to be qualified somewhat to bring them within the pale of this law, if it proves to be universal.

    The important fact is that in the germ cells of hybrid pigeons an actual separation of paternal and maternal germ plasm apparently occurs. When germ cells are to be matured, before the real reduction, there is in most forms a so-called false reduction, in which the chromosomes fuse in pairs so that there appears to be only half the normal number present, though in reality each is double (bivalent) and equivalent to two of the simple (univalent) type.

     The doubling of chromosomes which normally occurs at such times is frequently incomplete, or lacking, in hybrids. This is especially true if the hybrids are from widely separated species. Instead of a normal spindle bearing the usual number of bivalent chromosomes, multipolar spindles, or two separate spindles may appear, thus apparently permitting the two kinds of parental chromatin to remain apart

     In the most extreme cases a complete separation may occur subsequently, the entire chromatin of one parent occupying one cell, that of the other a different cell. Such visible separations, however, only occur extensively in sterile hybrids from markedly different parent species. Fertile hybrids from closely related forms, for the most part, display spindles normal in appearance. We may suppose, however, that there is the same tendency for the respective parental characters to separate, though the incompatibilities of the plasmas are not sufficient to prevent the formation of bivalent chromosomes and normal spindles. 

    It does not necessarily follow that in the ensuing division all of the characters of one parent will be set apart in a separate cell, as was the case in the abnormal mitoses. If such a separation were actually to occur, then the offspring which returns to a grandparent type must revert in not only one given character, but in all characters; that is, the reversion would be complete. The Mendelian law, however, confines itself to a given character, and if any other character is chosen, although it will follow the same law, it does so without any reference to the first character, so that offspring may be pure with respect to a particular character, yet also possess other characters of a mixed nature, or even pure characters of the other parent.

     In the case of these milder fertile crosses, then, where reversions follow the Mendelian law, the germinal incompatibilities must be narrowed down to the qualities themselves rather than confined to the respective germ plasms as a whole. These qualities must separate and each take up its abode in a different germ cell irrespective of whether the other qualities of that particular germ cell are of a different parentage or not. The cases in which the entire plasmas are segregated are then probably but magnified images of what occurs among the specific qualities of the milder crosses. 

     The interesting possibility arises that if fertile hybrids can be secured from widely different species the plasmas of which must be more incompatible than those of nearly related forms, such hybrids will give rise to offspring in which there is reversion, not only of one character, but of many or all characters in the same individual, due to a more thorough segregation of the parental germ plasm as a whole.

     In other words, the farther apart the parent species are, the more complete will be the return in any given offspring which shows reversion. To the mind of the writer, the nearness or the remoteness of the relationship between the two ancestral plasmas will be found to be an important factor in determining the amount (number of characters) of reversion in any given offspring.

End of article.

Comment: It is seen here that Guyer emphasizes not only the degree of parental relatedness per se, but also that relatedness interferes with the random assortment of chromosomes so that all the characters of one parent are potentially coinheritable as one linkage group. For Guyer, this implies that, at least in germ-line cells, each parental haplotype somehow retains its unity until a commitment to unimpaired meiotic segregation has been made. Chromosomes are only let off their haplotypic leash when that commitment has been made. He did not spell out in his thesis that in normal gametogenesis parental chromosomes might randomly assort, so that each gamete might contain a mixture of parental chromosomes.

    To remind yourself about the number of chromosomes in meiosis Click Here. Note that Guyer found that there were 16 chromosomes, 8 of paternal origin and 8 of maternal origin. From the reduction division (primary spermatocytes) 8 chromosomes should emerge. Indeed, when the chromosomes pair there are 8 multivalent chromosomes. These 8 pairs should divide to produce 8 univalent chromosomes in the secondary spermatocytes. But Guyer observes 4! Nevertheless, successful spermatozoa end up delivering 8 chromosomes to the zygote, to restore the diploid complement (16). How should we interpret the appearance of the secondary spermatocytes?

Donald Forsdyke. May 2004

Next: For Bateson and Saunders (1902) "Two Levels of Genetic Information(Click Here)

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This web-page was created circa 2000 and was last edited 01 May 2004 by D. R. Forsdyke