Data from: Sperm competition in yellow dung flies: no consistent effect of sperm size

Study abstract: 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 (<em>Scathophaga stercoraria</em>; 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 utilized 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. <br> <br> The experimental designs, data collection and variables are described in the original paper by Laugen et al. (2022, Journal of Evolutionary Biology) <br> <br>

研究摘要：雌性多次交配引发的雄性受精竞争，会推动精子数量增多的演化进程，并可对精子形态产生影响。现有理论预测，在精子竞争中，更长的精子有时可具备竞争优势。若该理论成立，则拥有更长精子的雄性相比精子更短的竞争者，能够产下更多后代。目前直接验证这一预测的研究寥寥无几，且所得结果并不一致。

本研究以黄粪蝇（<em>Scathophaga stercoraria</em>；双翅目：粪蝇科）为实验对象，探究更长精子能否为雄性带来受精竞争优势。初始实验中，我们让通过温度调控获得不同平均精子长度的同胞兄弟进行竞争——以此尽可能降低遗传背景带来的混淆效应——结果未发现更长精子具备明显的竞争优势。随后，我们使用了经酚氧化酶活性双向选择培育的黄粪蝇品系，该品系的精子与雌性受精囊管长度均呈现出相关演化响应。该实验同样未发现精子大小对父权成功率存在主效应。相反，仅当与拥有更长受精囊管的雌性进行交配竞争时，才呈现出精子更短的雄性具备竞争优势的趋势。

本研究的数据验证了诸多此前已报道的研究结果（最后雄性优势、交配时长与体型的效应等），这表明我们未能检测到精子大小的效应，并非源于实验方案本身的问题。综上，我们认为在多数情境下，更长的精子并不会为黄粪蝇雄性带来受精竞争优势；且与过往研究结果一致，该物种的受精竞争成功率主要由参与竞争的精子相对数量所决定。

实验设计、数据采集与变量设置详见Laugen等人2022年发表于《Journal of Evolutionary Biology》的原论文。