EJACULATE DEPLETION PATTERNS EVOLVE IN RESPONSE TO EXPERIMENTAL MANIPULATION OF SEX RATIO INDROSOPHILA MELANOGASTER

Abstract

We assessed the extent to which traits related to ejaculate investment have evolved in lines of Drosophila melanogaster that had an evolutionary history of maintenance at biased sex ratios. Measures of ejaculate investment were made in males that had been maintained at male-biased (MB) and female-biased (FB) adult sex ratios, in which levels of sperm competition were high and low, respectively. Theory predicts that when the risk of sperm competition is high and mating opportunities are rare (as they are for males in the MB populations), males should increase investment in their few matings. We therefore predicted that males from the MB lines would (1) exhibit increased investment in their first mating opportunities and (2) deplete their ejaculates at a faster rate when mating multiply, in comparison to FB males. To investigate these predictions we measured the single mating productivity of males from three replicates each of MB and FB lines mated to five wild-type virgin females in succession. In contrast to the first prediction, there was no evidence for differences in productivity between MB and FB line males in their first matings. The second prediction was upheld: mates of MB and FB males suffered increasingly reduced productivity with successive matings, but the decline was significantly more pronounced for MB than for FB males. There was a significant reduction in the size of the accessory glands and testes of males from the MB and FB regimes after five successive matings. However, the accessory glands, but not testes, of MB males became depleted at a significantly faster rate than those of FB males. The results show that male reproductive traits evolved in response to the level of sperm competition and suggest that the ability to maintain fertility over successive matings is associated with the rate of ejaculate, and particularly accessory gland, depletion.

Adaptations to sexual selection and sexual conflict: insights from experimental evolution and artificial selection

“…To test this idea, males from the MB and FB lines were mated to five wild-type virgin females in succession. MB males did indeed lose fertility significantly faster than did males from the FB lines (Linklater et al 2007). The loss of fertility over successive mates was associated more strongly with a reduction in the size of the male accessory glands than with the testes.…”

A neuropeptide circuit that coordinates sperm transfer and copulation duration in Drosophila

“…After three to four successive matings, wildtype males are rendered infertile due to seminal fluid depletion (21,22). Males born to tudor mutant females (son-of-tudor) lack sperm (23,24).…”

Natural variation of the Y chromosome suppresses sex ratio distortion and modulates testis-specific gene expression in Drosophila simulans

“…Notably, protease inhibitors are accessory gland-specific proteins (Figure 4c) that are able to prevent the premature breakdown of the mating plug and increase sperm vitality (Park and Wolfner, 1995;Chapman, 2001), and that might function to mitigate the lower fitness incurred by sex-ratio males (Tao et al, 2007b). Although higher expression of accessory gland proteins (ACPs) might contribute to partly compensate for impaired testis-specific gene expression in males with skewed sexratio (Linklater et al, 2007;Dowling and Simmons, 2012), we cannot formally distinguish between adaptive and non-adaptive mechanisms underlying variation in ACP expression. Similarly, although ejaculate composition might be a mechanism to maintain male fitness in response to the disruption of the testis function in males with highly biased sex ratio, we cannot distinguish direct and indirect effects of Y chromosome on ACP expression.…”