Evasion of predators contributes to the maintenance of male eyes in sexually dimorphic Euphilomedes ostracods (Crustacea)

Sexual dimorphisms have long drawn the attention of evolutionary biologists. However, we still have much to learn about the evolutionary, genetic, and developmental drivers of sexual dimorphisms. Here, we introduce ostracods of the genus Euphilomedes (Myodocopida, Ostracoda, and Crustacea) as a promising new system in which to investigate why and how sexual dimorphisms evolve. First, we ask whether male-skewed selective pressure from pelagic predators may help explain a dramatic sexual dimorphism in which male Euphilomedes have compound eyes, but females do not. Manipulative experiments demonstrate that blindfolding reduces the survival rate of male Euphilomedes when they are exposed to predatory fish. Blindfolding of the female rudimentary eyes (rudiments) does not, however, similarly influence the survival rate of brooding females. Further, numerical estimates of sighting distances, based on reasonable extrapolations from Euphilomedes’s eye morphology, suggest that the eyes of male Euphilomedes are useful for detecting objects roughly the size of certain pelagic predators, but not conspecifics. We conclude that eyes do not mediate direct interactions between male and female Euphilomedes, but that differences in predation pressure—perhaps associated with different reproductive behaviors—contribute to maintaining the sexually dimorphic eyes of these ostracods. Second, through transcriptome sequencing, we examined potential gene regulatory networks that could underlie sexual dimorphism in Euphilomedes’ eyes. From the transcriptome of juvenile male Euphilomedes’ eyes, we identified phototransduction genes and components of eye-related developmental networks that are well characterized in Drosophila and other species. The presence of suites of eye regulatory genes in our Euphilomedes juvenile male transcriptome will allow us, in future studies, to test how ostracods regulate the development of their sexually dimorphic eyes.

性二态现象（sexual dimorphisms）长期以来一直受到进化生物学家的关注。然而，关于性二态现象的进化、遗传与发育驱动机制，学界仍有诸多亟待探索的内容。在此，我们介绍真腺介虫属（Euphilomedes）介形类（ostracods，隶属于壮肢目Myodocopida、介形纲Ostracoda、甲壳动物门Crustacea）作为极具潜力的全新研究系统，用于探究性二态现象演化的原因与具体路径。首先，我们旨在验证：远洋捕食者施加的雄性偏向性选择压力，是否可以解释该类群中一种显著的性二态现象——雄性真腺介虫拥有复眼，而雌性则不具备复眼。操控性实验结果显示，当雄性真腺介虫暴露于捕食性鱼类环境中时，蒙眼处理会显著降低其存活率；但对雌性的残留眼（rudiments）进行蒙眼操作，并不会对抱卵雌性的存活率产生类似影响。此外，基于真腺介虫眼部形态的合理外推所得到的视距数值估算结果表明，雄性真腺介虫的眼部可用于探测体型与部分远洋捕食者大致相当的物体，但无法识别同种个体。我们由此得出结论：雄性与雌性真腺介虫之间的直接互动并非通过眼部介导；而捕食压力的差异——或许与二者不同的繁殖行为相关——是维持这类介形类性二态复眼的关键因素之一。其次，我们通过转录组测序（transcriptome sequencing），对可能参与真腺介虫眼部性二态形成的基因调控网络进行了探究。从雄性幼体眼部的转录组数据中，我们鉴定出了光转导基因（phototransduction genes）以及与眼部发育相关的调控元件，这类元件在果蝇（Drosophila）及其他物种中已有充分研究。我们在雄性幼体转录组中检测到的整套眼部调控基因，将为未来研究介形类如何调控其性二态复眼的发育提供坚实的研究基础。