Linklater Jon R, Wertheim Bregje, Wigby Stuart, Chapman Tracey
Department of Biology, University College London, Darwin Building, Gower Street, London, WC1E 6BT, United Kingdom.
Evolution. 2007 Aug;61(8):2027-34. doi: 10.1111/j.1558-5646.2007.00157.x.
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.
我们评估了与射精投入相关的性状在具有偏性性别比维持进化史的黑腹果蝇品系中的进化程度。对在雄性偏多（MB）和雌性偏多（FB）的成年性别比下维持的雄性果蝇进行射精投入测量，其中精子竞争水平分别为高和低。理论预测，当精子竞争风险高且交配机会稀少时（如MB种群中的雄性），雄性应增加对其少数几次交配的投入。因此，我们预测与FB雄性相比，MB品系的雄性将（1）在其首次交配机会中表现出增加的投入，并且（2）在多次交配时更快地耗尽其精液。为了研究这些预测，我们测量了来自MB和FB品系各三个重复的雄性果蝇与五只野生型处女雌蝇连续交配的单次交配生产力。与第一个预测相反，没有证据表明MB和FB品系雄性在首次交配时的生产力存在差异。第二个预测得到了支持：MB和FB雄性的配偶随着连续交配生产力逐渐降低，但MB雄性的下降明显比FB雄性更显著。连续五次交配后，来自MB和FB种群的雄性果蝇的附腺和睾丸大小显著减小。然而，MB雄性的附腺比FB雄性的附腺耗尽速度明显更快，但睾丸并非如此。结果表明，雄性生殖性状是根据精子竞争水平进化而来的，并且表明在连续交配中维持生育能力的能力与射精，特别是附腺耗尽的速度有关。
