Sperm is the most fundamental male reproductive feature. It serves the fertilization of eggs and evolves under sexual selection. Two components of sperm are of particular interest, their number and their morphology. Mode of fertilization is believed to be a key determinant of sperm length across the animal kingdom. External fertilization, unlike internal, favors small and numerous sperm, since sperm density is thinned out in the environment. Here, we study the evolution of sperm morphology in the genus Daphnia, where fertilization occurs in a receptacle, the brood pouch, where sperm can constantly be flushed out by a water current. Based on microscopic observations of sperm morphologies mapped on a phylogeny with 15 Daphnia and 2 outgroup species, we found that despite the internal fertilization mode, Daphnia have among the smallest sperm recorded, as would be expected with external fertilization. Despite being all relatively small compared to other arthropods, sperm length diverged at least twice, once within each of the Daphnia subgenera Ctenodaphnia and Daphnia. Furthermore, species in the latter subgenus also lost the ability of cell compaction by extracellular encapsulation and have very polymorphic sperm with long, and often numerous, filopodia. We discuss the different strategies that Daphnia evolved to achieve fertilization success in the females’ brood pouch.

中文翻译：

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开放育雏袋内受精甲壳类动物精子形态的演变

精子是男性最基本的生殖特征。它服务于卵子的受精并在性选择下进化。精子的两个组成部分特别令人感兴趣，它们的数量和形态。受精方式被认为是整个动物界精子长度的关键决定因素。与内部受精不同，外部受精有利于小而多的精子，因为精子密度在环境中变薄了。在这里，我们研究了水蚤属中精子形态的进化，受精发生在一个容器，即育雏袋中，精子可以不断地被水流冲走。基于对 15 种水蚤系统发育图的精子形态的显微镜观察和 2 个外群物种，我们发现尽管有内部受精模式，但水蚤的精子是记录的最小的精子之一，正如外部受精所预期的那样。尽管与其他节肢动物相比都相对较小，但精子长度至少有两次分歧，一次在栉水蚤亚属和栉水蚤亚属中。此外，后一个亚属中的物种也失去了细胞外包裹的细胞压实能力，并且具有非常多态的精子，具有长且通常大量的丝状伪足。我们讨论了水蚤为在雌性育雏袋中成功受精而进化的不同策略。