The Evolution of Fitness After Prolonged Sperm Storage
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A series of recent studies using Drosophila melanogaster suggest that while males may benefit from having access to many partners, female fitness is reduced by extended cohabitation and sexual interaction with males. Yet, even if repeated sexual interactions are harmful to females, limited male exposure will ultimately be detrimental due to sperm-depletion and infertility. Females are therefore expected to balance mating opportunities and sperm storage capacity to maximize lifetime reproductive success. I introduced extended mating deprivation as a selective pressure to experimentally evolve lines of D. melanogaster for characters related to mating and postcopulatory sexual selection. Evolution of the mate-deprived lines over several dozen generations demonstrated that restricted sexual access was indeed a potent selective pressure. I consistently found that when males were removed for an extended time period, female fitness declined substantially, suggesting that mate-deprivation over nine days was harmful. Under these conditions, selected-line males responded to mate-reduced conditions and demonstrated a 13% increase in reproductive success compared to controls. Experimental females had a 15% increase in fertility compared to controls. I investigated a series of developmental characteristics that may have been altered by the selection regime, and while there was some evidence of evolved change, these results were not consistent. Although the data at hand do not substantiate a detailed characterization, both sexes in the experimental populations demonstrated increased fitness after extended mate-deprivation, thus evolutionary change appears to have occurred via selection on one or both relevant male ejaculate characteristics: sperm number and survival, and factors affecting female late-life fertility.