Sperm competition in yellow dung flies: No consistent effect of sperm size.

The male competition for fertilization that results from female multiple mating promotes the evolution of increased sperm numbers and can impact sperm morphology, with theory predicting that longer sperm can at times be advantageous during sperm competition. If so, males with longer sperm should sire more offspring than competitors with shorter sperm. Few studies have directly tested this prediction, and findings are inconsistent. Here we assessed whether longer sperm provide a competitive advantage in the yellow dung fly (Scathophaga stercoraria; Diptera: Scathophagidae). Initially, we let brothers with different temperature-mediated mean sperm lengths compete - thus minimizing confounding effects of genetic background - and found no clear advantage of longer sperm. We then used flies from lines subjected to bidirectional selection on phenoloxidase activity that had shown correlated evolutionary responses in sperm and female spermathecal duct lengths. This experiment also yielded no main effect of sperm size on siring success. Instead, there was a trend for a shorter-sperm advantage, but only when competing in females with longer spermathecal ducts. Our data corroborated many previously reported findings (last-male precedence, effects of copula duration and body size), suggesting our failure to find sperm size effects is not inherently due to our experimental protocols. We conclude that longer sperm are not competitively superior in yellow dung flies under most circumstances, and that, consistent with previous work, in this species competitive fertilization success is primarily determined by the relative numbers of sperm competing.

Responses to selection on phenoloxidase activity in yellow dung flies.

Maintaining an immune system is costly. Resource allocation to immunity should therefore trade off against other fitness components. Numerous studies have found phenotypic trade-offs after immune challenge, but few have investigated genetic correlations between immune components and other traits. Furthermore, empirical evidence for the costs of maintaining an innate immune system in the absence of challenges is rare. We examined responses to artificial selection on phenoloxidase (PO) activity, an important part of the insect innate defense against multicellular pathogens, in yellow dung flies, Scathophaga stercoraria (L.). After 15 generations of successful selection on PO activity, we measured reproductive characters: clutch size, egg hatching rates, adult emergence rates, and adult longevity. We found no evidence for negative genetic correlations between PO activity and reproduction. In fact, flies of lines selected for increased PO activity had larger first clutches, and flies of lines selected for decreased PO activity had smaller ones. However, flies from high-PO lines died earlier than did low-PO flies when no food was available; that is, there is a survival cost of running at high PO levels in the absence of challenge. Variation in resource acquisition or use may lead to positive genetic correlations between PO and fertility and fecundity. The negative correlation between PO and longevity under starvation may indicate that variation for resource acquisition is maintained by a cost of acquisition, based on a genotype-environment interaction.