Origin of Species (1996)
T he key to the origin of species is the phenomenon of hybrid sterility. A mule is the hybrid formed by crossing a healthy fertile horse and a healthy fertile ass. Yet the mule is sterile. Theories of this phenomenon are either "genic" or "chromosomal". Until 1996, chromosomal theories had required differences in large segments (e.g. deletions, translocations), which might sometimes be seen on examination with a standard light microscope.
A modified chromosomal theory requiring diffuse differences only in single bases was presented in the Journal of Theoretical Biology (1996). This postulated that (C+G)% is the "accent" of DNA, which, like the accent of human beings (metaphorically speaking), can affect reproductive success.
In chemical terms, (C+G)% differences have a
profound affect on the ability of a duplex DNA molecule to extrude the stem-loop
structures by which homologous chromosomes first recognize each other at meiosis.
by D. R. FORSDYKE
J. Theor. Biol. (1996) 178, 405-417. (With copyright permission from Academic Press.)
2. PRIMARY AND SECONDARY COMPONENTS OF INFORMATION
3. CODON CHOICE AND THE NEUTRALIST SELECTIONIST DEBATE.
4. SEX AND RECOMBINATION REPAIR
5. (C+G)% AND SPECIATION
6. EVIDENCE FOR STEM-LOOP "KISSING" MODEL
7. HOW (C+G)% DIFFERENCES MIGHT IMPEDE RECOMBINATION.
8. THE EARLY EVOLUTION OF INTRONS
9. (C+G)% AND PHYLOGENY
10. MAJOR PRESSURES ON DNA ARE SELECTIVE
11. THE DOMINANCE OF THE GENOME.
Radio can be used as a metaphor for the transmission of information by DNA through time and space. Just as different radio transmitters broadcast at different wavelengths to prevent interference, so different biological species "broadcast" their DNAs at different (G+C)% "wavelengths" to prevent recombination.
It is postulated that species differences in (G+C)% prevent recombination. First, evidence is presented supporting the early Crick-Sobell stem-loop model for genetic recombination, which proposes that the rate-limiting step in recombination is the recognition ("kissing") of complementary sequences in the loops of stem-loop structures extruded from supercoiled DNA. Then, various ways in which differences in (G+C)% might impede complementary loop interactions are outlined.
The strength of the postulate is that it brings together a variety of disparate observations in fields that have not previously been seen as related. Thus explanations are apparent for why most mutations are not selectively neutral (the "neutralist/selectionist" debate), why introns were present in the earliest genes (the "introns-early / introns-late" debate), and the origin of species.
All forms of information share common features (Shannon, 1947). I here consider the transmission of information as radio-waves as a metaphor for the transmission of genetic information through time and space. A variety of observations, including recent work on the distribution of stem-loop potential in genomes (Forsdyke, 1995b-e), are shown consistent with a hypothesis which appears to throw new light on some fundamental questions in biology:
The paper begins by considering the (C+G)% of a DNA molecule as "secondary information" which has been fine-tuned by natural selection to inhibit recombination with DNAs from other biological species (sections 2-5). It is then shown that stem-loop recombination models advanced in the 1970s can be adapted to explain how differences in (C+G)% might impede recombination (sections 6-7). Finally the ways different species balance the various evolutionary pressures on their genomes are explored (sections 8-11).
|At the beginning of the 20th century the first radio transmitter came into operation. A particular wave-band was chosen and messages were broadcast.|
The second radio transmitter to arise did not want to interfere with messages from the first. There were three alternatives:
As the number of transmitters increased the third alternative became the most feasible. Thus different transmitters often broadcast both synchronously and sympatrically.
Operating on a second wavelength did not disadvantage the second transmitter. A message on one wavelength sounded much the same as on another wavelength. Thus the message was essentially wavelength-independent. This component of the radio signal can be regarded as "primary information". Information on wave-length is another component of the signal and can be regarded as "secondary information".
DNA can also be considered to have both primary and secondary information components. Table 1 shows four columns of data adapted from a 1952 study by Wyatt on the base composition of DNA of various insect viruses. The first three columns demonstrate the famous "Chargaff ratios" (%A=%T; %G=%C; Chargaff, 1951). The data in these columns are species-invariant [i.e. are usually the same for all species] and led to the introduction of a general model for DNA in 1953 by Watson and Crick.
Their model of DNA as a double helix was immediately successful in explaining many problems in genetics. The "primary information" in DNA, such as information coding for the sequence of a protein, was a characteristic of the sequence of the four bases. Individual DNA strands could act as templates permitting accurate replication of information. One strand could be used as a template to repair damage in the other.
|VIRUS TYPE||Virus host||A/T||G/C||R/Y*||(C+G)%|
|C. P. eurytheme||1.08||1.11||1.09||42.5|
|* R = purine (A or G); Y = pyrimidine (C or T).|
The initial success of the double helix model may have distracted attention from the data in Wyatt's fourth column. This shows that (C+G)% is a species characteristic. The ratio of the bases which engaged in strong Watson-Crick interactions (C and G), to the bases which engage in weak Watson-Crick interactions (A and T), is characteristic of a genome (or large genome sector; Sueoka, 1961; Muto & Osawa, 1987; Bernardi, 1989). In 1952 Wyatt was looking to (C+G)% for clues as to how DNA might carry its "primary information". He imagined that species with similar biological features might encode similar proteins and this might be reflected in similarities in base-ratios. He was forced to conclude that the (C+G)%:
Species with similar biological properties often encode similar proteins with similar primary sequences. It does not necessarily follow, however, that the DNA sequences which encode those proteins are as similar as the proteins. The genetic code, relating 61 nucleotide triplets in DNA to the 20 amino acids in proteins, is a redundant code. For some amino acids there is a choice of six possible codons (synonymous codons).
Early sequencing studies showed that usage of alternative codons is
not random (Grosjean &
Fiers, 1982). In the "genome
Here) Grantham et al.(1980) pointed out that all genes in a genome tend to use
the same subset of codons. The needs of the genome seem to dominate codon
codon subsets of a virus and its host are often different, even though they use the
translation machinery (Grantham et al.
1985). The same principle applies to sectored
vertebrate genomes. Thus, mammalian alpha-globin and beta-globin mRNAs share a common
cytoplasm, yet use different codon subsets. The corresponding genes are in the high and
low (C+G)% genomic sectors (isochores), respectively.
(C+G)% is a major factor determining
codon choice ( Possible explanations of species differences in (C+G)%
are at the heart of the debate between two schools. The "
(C+G)% is a major factor determining codon choice (Nichols et al. 1981; Kagawa et al. 1984). Species with a high (C+G)% tend to use codons rich in C and G, and avoid codons rich in A and T. Because of differences in this "secondary information", different species can transmit the same "primary information" (the sequence of a protein), while maintaining differences in the DNA sequences encoding that information. (For present purposes I ignore small differences in protein sequences which often reflect the exchange of amino acids with similar properties, and thus conserve the properties of the protein.)
Possible explanations of species differences in (C+G)% are at the heart of the debate between two schools. The "neutralists" propose that species-specific mutational biases create mutations which are usually selectively neutral and are fixed in the population by "drift" in finite populations (Kimura,1989). The "selectionists" propose that some selective force drives species to adopt particular C+G percentages (Bernardi & Bernardi, 1986). The neutralists seem currently to be ascendant. They have no difficulty pointing to examples of mutational biases (Cox & Yanofsky, 1976), and seem readily to demolish the arguments of the selectionists.
For example, CG bonds are less readily broken at
high temperatures. The selectionists have argued that high (C+G)%
genomes might have evolved as a response to the selective pressure of temperature. Indeed,
many contemporary organisms which survive at high temperatures do have CG-rich
DNA (Kagawa et al.
1984). However, the neutralists point out that some organisms living at
high temperatures have AT-rich DNA (for refs. see
Filipski, 1989, or Click Here
). Some neutralists now tend to
the extreme position that:
Sexual reproduction provides an opportunity for DNA from two individuals to recombine. The new gene combinations which result may be advantageous (Weismann, 1892). However, recombination can disrupt as well as create favorable gene combinations. For this reason, among others, it is difficult to accept the generation of advantageous gene combinations as the main driving force favoring the evolution of sex.
There is currently much debate on the possible advantages of sexual reproduction compared with asexual reproduction. I here accept the postulate, most eloquently argued by the Bernsteins (1991), that recombination evolved primarily to correct DNA damage and mutations. Causes of the latter include DNA damage and replication errors.
Organisms use recombination, among other methods, to maintain the integrity of their DNA. It is unlikely that a DNA molecule in one member of a species will be damaged or mutated at the same site as in the homologous DNA molecule in another (not closely related) member of the same species. One DNA molecule thus has the potential to act as a template for repair of, or to replace a defect in, another. Either a damaged base, or a mismatched base in a heteroduplex, can be dealt with. In the latter case, some mechanism to decide which is the correct strand is helpful.
Organisms of different species may sometimes happen to have similar C+G percentages ("secondary information"; Table 1), but they invariably differ in base order ("primary information"). Thus, to act as a accurate template the DNA molecules must be from the same species. However, it is advantageous for organisms to avoid recombination with other members of their species whose DNA has deviated from the species norm (potential "incipient species"). The DNA of these members is no longer a reliable template for error detection and correction.
Deviant members of a species can thus be seen as exerting a selection pressure for the evolution of mechanisms to monitor deviance from the species norm, and to prevent recombination if that deviance exceeds some unacceptable limit. The genomic deviations responsible for this impairment might precede the appearance of deviations giving rise to incompatibilities at the gene product level.
Ideally the monitoring process would, in some way, first directly summate all the deviant aspects of a genome and then reject a genome which exceeded the limit. Alternatively, the monitoring process could assess some characteristic which would provide an indirect measure of genome deviance (e.g. percentage C+G). In the latter case there should be some explanation why difference species sometimes have very similar C+G percentages. One explanation would be that different genomes may differ locally in (C+G)%, but may maintain an overall similarity in (C+G)%. A more satisfying explanation is given later.
Although a subject of much controversy, species can simply be defined as consisting of organisms which are successful at reproducing sexually with each other. Sexual species are reproductively isolated from other sexual species (Templeton, 1989; Coyne, 1992). Reproductive isolation may be due either to factors which impede fertilization (prezygotic factors: geographical, behavioral, anatomical), or to factors which act after fertilization (post-zygotic factors).
Along lines first set out by
Dobzhansky (1936) and Muller (1939), current hypotheses postulate a major role for
incompatibilities between gene products. These incompatibilities might become greater as
species diverge, and, unless the products happened to influence fertility directly, would
tend to cause hybrid inviability
rather than infertility (Orr, 1995; Forsdyke,
1995a). In contrast, this paper focuses on
possible incompatibilities at the genomic level which would be manifest at meiosis
when parental genomes attempt to recombine, and
would cause hybrid infertility
The main postulates of the paper are that:
The main postulates of the paper are that:
Since to derive a benefit (DNA repair) from recombination an organism must use a homologous DNA molecule from another member of the same species, organisms of different (C+G)% "wavelengths" constitute a selective force driving the emergence of distinct species-specific C+G percentages. By making successful recombination contingent on this "secondary information" in DNA, the integrity of species DNA would be maintained.
Thus, throughout evolutionary time each species would have fine-tuned the "wavelength" of its DNA to avoid interference from the DNA of other species. An individual organism from a subpopulation whose (C+G)% had begun to deviate from the species norm (potential incipient species) would usually fail to recombine with individuals from the majority population, and thus would not have the deviation corrected. The organism would also probably fail to recombine with other members from the subpopulation, because of their rarity. The subpopulation would thus be selected against unless rescued by:
Wyatt (1952) noted that the range of C+G percentages in insect viruses was much wider than in other groups of organisms. This observation has been confirmed for viruses with other hosts (Bronson & Anderson, 1994). Viruses seem to have been most susceptible to "CG/AT pressure", which is the postulated evolutionary pressure driving a genome to adopt a particular (C+G)%. In the absence of special mechanisms to prevent coinfection (analogous to prezygotic isolation), the two species of DNA virus with the potential to coexist synchronously and sympatrically within cells of the C. fumiferana (Table 1), would have had every opportunity to recombine. Since they shared the challenges of a common environment ("ecological niche"), it is likely that they would have had some common proteins with similar sequences. If these sequences were similar at the DNA level then, in the absence of some barrier, recombination between the species would be possible (since recombination between DNA molecules is favored by sequence homology). In terms of the hypothesis advanced in this paper, there would thus have been a strong selective pressure on the DNA of these two viral species to arrive at distinct C+G percentages (analogous to post-zygotic isolation). A consequence of this would be that, even though they share a common translation apparatus, each species would have a distinct codon bias.
Returning again to the somewhat imperfect radiowave metaphor, two transmitters which are close to each other (i.e. their transmitting ranges overlap) cannot broadcast simultaneously on the same wavelength. Similarly, two species which are biologically close to each other (i.e. prezygotic isolation may be imperfect), cannot "broadcast" on the same (C+G)% "wavelength" without interfering with each other. Two transmitters whose transmitting ranges do not overlap can broadcast at the same wavelength.
Similarly, two biologic species which are reproductively isolated (through one or more prezygotic isolation factors), can "broadcast" their DNAs at the same (C+G)% "wavelength". If postzygotic isolation precedes prezygotic isolation, then initially reproductive isolation would require different C+G percentages; when prezygotic isolation was achieved, the two C+G percentages could converge, since there would no longer be a selective pressure for (C+G)% divergence. Thus, in modern species the process responsible for the initiation of reproductive isolation could have become disguised by subsequently developing prezygotic isolating factors which would ensure maintenance of reproductive isolation.
How could differences in (C+G)% affect recombination? Many studies have been carried out on the molecular basis of recombination and many models have been advanced taking into account the results of such studies (Szostak et al. 1983; Holliday, 1990). The need to explain differences in (C+G)% has not been seen as requiring an explanation in terms of the models. However, some early "stem-loop" models, which postulate the involvement of stem-loops in single-stranded DNA, lend themselves most favorably to this purpose.
A possible answer to the question will be arrived at here in two steps. First the growing evidence supporting stem-loop recombination models will be summarized. Then it will be shown how interactions between loops might be extremely sensitive to small differences in (C+G)%.
Critical to the reacceptance of early stem-loop recombination models are observations suggesting that the pairing of homologous chromosomes for recombination is not protein-based (i.e. does not require prior formation of a synaptonemal complex). Rather, chromosomal pairing is likely to be the result of a DNA sequence-based homology search (Kleckner et al., 1991; Hawley & Arbel, 1993; Kleckner & Weiner, 1993; Klein, 1994).
In 1971 Crick advanced his "unpairing postulate" to explain how homologous chromosomes in diploid organisms might find each other. This invoked loops of duplex DNA at the tips of which complementary strands would unpair.
FIG. 1. The stem-loop "kissing" model for the initiation of hybridization between two nucleic acid species (Tomizawa, 1984). Single stranded RNA I and its antisense transcript RNA II in plasmid ColE1 first interact transiently at the tips of stem-loops, mainly by conventional GC and AU base pairing. If this pairing is sufficiently stable, the pairing propagates progressively as the stem-loop structures unfold. The rate constant of association between loops is critical in determining the rate of subsequent hybridization. In the ColE1 system, the hybridization is required to regulate the initiation of DNA synthesis. In recombination systems the union would be further consummated by strand exchanges (Sobell, 1972; Wagner & Radman, 1975; Szostak et al., 1983). (This figure is adapted from Annual Reviews of Biochemistry, with the permission of Annual Reviews Inc; Eguchi et al., 1991).
Unlike RNA, extrusion of stem-loops from duplex DNA requires torsional stress on the DNA duplex (supercoiling; Murchie et al., 1992). Thus, inhibition of supercoiling should impede recombination; indeed, this has been found (Wang et al., 1990). Furthermore, certain enzymes involved in recombination (endonucleases) work only with a supercoiled DNA substrate (Sung et al., 1993). The evidence from a variety of biological systems supporting the involvement of stem-loops in recombination has recently been summarized (Reed et al., 1994).
Tomizawa (1993) has concluded that the major role of the stem in a stem-loop structure is the proper positioning of the loop. This allows the unpaired bases in the loop to pair with those of an appropriately positioned complementary loop projecting from another nucleic acid. This "kissing" is rate-limiting in recombination. Base composition, a genomic rather than a local characteristic, is a major factor affecting the energetics of folding of a DNA segment (Forsdyke, 1995b-e; see also discussion of Fig. 3 in section 8). Small changes in this parameter would affect the looping pattern which a sequence could present for homology search.
Furthermore, it would be more difficult to extrude loops from CG-rich DNA than from AT-rich DNA. Two sequences of different (C+G)% undergoing supercoiling in a common intracellular environment might extrude stem-loops at different times and to different extents. Thus, the pattern of loops presented by regions with quite similar sequences could be different and recombination could be impaired. To recombine, two homologous sequences should be equal both in the local parameter (base order-determined stem-loop potential) and the genomic parameter (base composition-determined stem-loop potential).
It is likely that early biological evolution took place among "replicators" in an "RNA world" (Joyce & Orgel, 1993). At some point, either before or after the evolution of protein-encoding genes, the predominant nucleic acid became DNA. In the early nucleic acid world the ability to shuffle damaged segments so as to create new segment combinations should have been advantageous. Thus, if it could have evolved, it is likely that recombination would have evolved at an early stage.
If the stem-loop model is applicable to this early world, then it can be imagined that primitive replicators which accepted mutations modifying their sequence to enhance the potential for stem-loop formation, would have had a selective advantage. To reap the benefits of efficient recombination a replicator would have had to exchange genetic segments with its own kind of replicator, not with other kinds. Since recombination with foreign replicators would have been disadvantageous, characteristics which prevent such recombination (e. g. a distinctly different C+G percentage) would have been favored by natural selection.
Figure 2 symbolizes two early genome-wide pressures affecting the evolution of nucleic acids. The top row of downward-pointing arrows symbolizes a pressure affecting base-composition which would result in a particular (C+G)% ("CG/AT pressure"). The second row of downward-pointing arrows symbolizes a pressure on the primary sequence to accept mutations favoring an increase in stem-loop potential by changing base order (FORS-D or fold pressure). The two sets of arrows are pointing in the same direction. For present purposes the two pressures are not considered to conflict. FORS-D values can be considered as having been imposed on a CG/AT equilibrium which would have been arrived at independently. Many mutations currently regarded as "neutral" would in fact have been selected during the course of evolutionary adaptation to these pressures.
FIG. 2. Genome-wide and local evolutionary forces acting on a genome. The two upper rows of dispersed downward-pointing arrows symbolize two genome-wide forces which influence the potential to form stem-loop structures.
One way of accommodating the conflict between the pressures is to allow protein-encoding capacity to evolve in dispersed segments separated by regions (introns) where the evolution of stem-loop potential is less constrained.
The upward pointing arrows in Figure 2 are in distinct regions, symbolizing the later-evolving localized pressure for the encoding of specific function. Here there is a conflict. A sequence required to encode a protein might not at the same time be able locally to optimize its folding propensity. The conflict might have been meet in three ways:
If the first two options were not sufficient, then only the third option would have been left. Thus, introns might correspond to parts of a gene where the constraints on the first two options were most severe. Introns would have allowed the interspersing of selectively advantageous stem-loops in coding regions of DNA.
Evidence supporting this is presented elsewhere (Forsdyke, 1995b-e). As an example, Figure 3 (upper) shows FORS-D plots for the human troponin-c gene, which may have been under positive Darwinian evolutionary selection (Ohta, 1994). Negative FORS-D values are associated with certain exons. For exons which are not associated with negative FORS-D values, it can be assumed that it was possible to accommodate FORS-D pressure by the use of synonymous codons and conservative amino acids. Negative FORS-D values in parts of the first intron and 5' flank suggests functions for these regions, perhaps regulatory.
|Figure 3 (lower) shows that profiles for the folding of the natural sequence (FONS values) and the mean value for randomized sequences (FORS-M values), closely follow each other. This implies that the genome characteristic which controls the FORS-M value (base composition), is a major factor influencing the energetics of stem-loop formation (Forsdyke, 1995b-e). Once introns are removed, the cDNA product (not shown) has generally lower FORS-D values (average 2.3±1.9 kcal/mol) than the corresponding genomic segment shown in Figure 3 (average 4.4±0.7 kcal/mol).|
FIG. 3. Comparison of fold energy minimization
values for windows in a human DNA segment containing the troponin-C gene, with arithmetical
mean values for multiple versions of each window repeatedly shuffled to randomize base
order before determining the fold energy minimization value (Forsdyke, 1995b-e).
Comparison of fold energy minimization values for windows in a human DNA segment containing the troponin-C gene, with arithmetical mean values for multiple versions of each window repeatedly shuffled to randomize base order before determining the fold energy minimization value (Forsdyke, 1995b-e).
The first 4400 nt of the 4567 nt DNA sequence from GenBank file HUMTROC was divided into 85 windows of 200 nt, which overlapped each other by 150 nt. Secondary structure energy minimization values, determined using the program RNAFOLD (Zuker, 1989), were obtained for each window in the natural sequence (FONS values). Each 200 nt sequence was then subjected to 10 independent randomizations, and energy minimization values for each of the 10 randomized versions (FORS values) were determined using RNAFOLD.
In A (lower) the mean fold value for each set of 10 randomized sequences (FORS-M value) is plotted with the corresponding FONS value.
In A (upper) the differences between the FORS-M values and the corresponding FONS values are plotted (FORS-D values).
Each data-point is at the middle of its 200 nt window. Exon positions are shown as open boxes in A (lower). Vertical dashed lines in A indicate, from left to right, the beginning of exon 1, the beginning of the protein-encoding region, the end of the protein coding region, and the end of exon 6.
Gilbert & Glynias (1993) postulate a correlation between exons and functional domains of proteins as part of their introns-early model. Failure to find the correlation has recently lead Stoltzfus and coworkers (1994) to embrace the introns-late viewpoint. The "introns-early" viewpoint presented here does not require a correlation of intron boundaries with some feature of protein structure.
This is expected, since introns can occur both in protein-coding and in non-coding parts of a gene (Hawkins, 1988). Indeed, some genes encode "mRNAs" with no protein product, yet the genes have introns, just like protein-encoding genes (Brannan et al. 1990; Brockdorff et al. 1992). The viewpoint presented here suggests why introns were established, but does not explain their great variation in length. It is possible that this variation shifts the register between genes, and makes "kissing" interactions more difficult.
9. (C+G)% AND PHYLOGENY
Similarities in (C+G)% between species are sometimes indicative of a close evolutionary (phylogenetic) relationship (Sueoka, 1961; Muto & Osawa, 1987). However, the arrows in Figure 2 indicate a potential conflict between CG/AT pressure and the later-evolving information to encode proteins (protein pressure). Figure 4 shows how CG/AT pressure, acting on a whole genome or large genome segment (Bernardi, 1989), might act independently of local protein pressures on the same genome or genome segment.
The percentage identities of DNA and protein for various genes
in two organisms which have diverged from a common ancestor, are plotted against each
other. Exons of genes encoding a highly conserved protein (e.g. histone) might show
identity at the protein level, but only 90% identity at the nucleotide level due to the
utilization of synonymous codons. The downward pointing arrow symbolizes this effect of CG/AT
pressure which tends to drive the (C+G)%
of the two species away
from each other (to prevent recombination).
The environmental factor in this case is other close species, from which prezygotic isolation is of only limited effectiveness, and with which recombination must also be avoided. The conflicting directions of CG/AT pressure are in equilibrium in Figure 4 in the case of exons encoding proteins which show 75% identity between the two species.
Thus, CG/AT pressure acts both to drive two species apart and together (away from other species). A phylogenetic relationship between the C+G percentages of the two species may still be evident. Data supporting this interpretation of percentage identity differences have recently been presented by Wolfe and Sharp (1993; although they do not offer this interpretation). They compared various homologous genes of mice and rats, and found that the cross-over point, when nucleotide identity equals amino acid identity, occurs at 93%.
FIG. 4. Hypothetical relationship between degrees of nucleotide and amino acid identity for exons of two species which share a recent common ancestor. The diagonal dashed line indicates the relationship expected if corresponding nucleotide and amino acid sequences have changed equally during evolution.
Exons of conserved proteins have small differences in amino acids (e.g. 100% identity between the two species), but CG/AT pressure (symbolized by the downward-pointing arrow) generates larger differences in nucleotides. This results in a sufficient difference in C+G percentage between the two species to inhibit recombination.
Exons of poorly conserved proteins have large differences in amino acids (e.g. only 30% identity between the two species), but CG/AT pressure (symbolized by the upward-pointing arrow), working to prevent recombination with other species in the environment, conserves nucleotide identities. Thus, phylogenetic relationships between C+G percentages of the two species may still be evident. The vertical dashed line indicates the cross-over point (75% in this case) corresponding to exons of genes in which nucleotide and amino acid percentage identities are equal.
However, whereas phylogenetically-unrelated species may sometimes have similar C+G percentages, phylogenetically-related species may differ markedly in (C+G)%. This is particularly evident in the case of the genomes of viruses which have the potential to share a common host cell where frequent opportunities for recombination might arise (e.g. C. fumiferana; Table 1). There is a 22% difference between the (C+G)% of two herpesviruses which are biologically very similar (Schachtel et al. 1991), and C+G percentage differences between biologically similar retroviruses are even greater (Bronson & Anderson, 1994). The latter authors do not relate this difference to the prevention of recombination, but suggest that it has arisen to allow viruses to occupy different "ecological niches" within a cell.
Major pressures affecting the evolution of DNA
are CG/AT pressure, and fold (FORS-D) pressure, as defined above. Two
other important pressures are protein pressure and translation pressure. Natural selection
results in the differential reproductive success of individuals with the functionally most
effective proteins, thus favoring genotypes which encode those proteins. This
inflexibility of amino acid sequence (protein pressure) places constraints on the use of :
Thus, in some circumstances, protein pressure might be manifest
as a positive correlation between non-synonymous and synonymous substitution rates in a
Thus, in some circumstances, protein pressure might be manifest as a positive correlation between non-synonymous and synonymous substitution rates in a gene (Li and Grauer 1991).
FIG. 5. Summary of pressures influencing the evolution of codons. The four major pressures are shown in boxes. For details please see text.
Salser in 1978, when analyzing the first globin mRNA sequences, challenged the current (and continuing) protein-centered view of sequence conservation during biological evolution:
Salser went on to show that base
substitutions predominate in mRNA loop regions, which are least likely to be involved in
maintaining the configuration of the folded RNA structure. This observation has since been
shown to apply to other RNAs (for refs. see Forsdyke,
This work was supported by the Medical Research Council of Canada.
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Last edited 17 May 2001 by D. R. Forsdyke