The evolution of a placenta is predicted to be accompanied by rapid evolution of genes involved in processes that regulate mother-offspring interactions during pregnancy, such as placenta formation, embryonic development, and nutrient transfer to offspring. However, these predictions have only been tested in mammalian species, where only a single instance of placenta evolution has occurred. In this light, the genus Poeciliopsis is a particularly interesting model for placenta evolution, because in this genus a placenta has evolved independently from the mammalian placenta. Here, we present and compare genome assemblies of two species of the livebearing fish genus Poeciliopsis (family Poeciliidae) that differ in their reproductive strategy: Poeciliopsis retropinna which has a well-developed complex placenta and P. turrubarensis which lacks a placenta. We applied different assembly strategies for each species: PacBio sequencing for P. retropinna (622-Mb assembly, scaffold N50 of 21.6 Mb) and 10× Genomics Chromium technology for P. turrubarensis (597-Mb assembly, scaffold N50 of 4.2 Mb). Using the high contiguity of these genome assemblies and near-completeness of gene annotations to our advantage, we searched for gene duplications and performed a genome-wide scan for genes evolving under positive selection. We find rapid evolution in major parts of several molecular pathways involved in parent-offspring interaction in P. retropinna, both in the form of gene duplications as well as positive selection. We conclude that the evolution of the placenta in the genus Poeciliopsis is accompanied by rapid evolution of genes involved in similar genomic pathways as found in mammals.

中文翻译：

---

活鱼 Poeciliopsis revopanna 和 Poeciliopsis turrubarensis 的基因组反映了它们不同的繁殖策略。

据预测，胎盘的进化将伴随着一些基因的快速进化，这些基因参与调节怀孕期间母子相互作用的过程，例如胎盘形成、胚胎发育和向后代的营养转移。然而，这些预测仅在哺乳动物物种中进行了测试，其中只发生了一次胎盘进化。从这个角度来看，Poeciliopsis 属是胎盘进化的一个特别有趣的模型，因为在这个属中，胎盘是独立于哺乳动物胎盘进化的。在这里，我们展示并比较了两种活鱼 Poeciliopsis 属（Poeciliidae 科）的基因组组装，这两个物种的繁殖策略不同：Poeciliopsis Retropinna 具有发育良好的复杂胎盘，而 P. turrubarensis 则缺乏胎盘。我们对每个物种应用了不同的组装策略：PacBio测序用于P.retropinna（622-Mb组装，支架N50为21.6Mb）和10×Genomics Chromium技术用于P.turrubarensis（597-Mb组装，支架N50为4.2Mb）。利用这些基因组组装的高度连续性和基因注释的近乎完整的优势，我们搜索基因重复并对在正选择下进化的基因进行全基因组扫描。我们发现，在 P.retropinna 中，参与亲子相互作用的几个分子途径的主要部分都以基因复制和正选择的形式快速进化。我们得出的结论是，Poeciliopsis 属胎盘的进化伴随着与哺乳动物中发现的类似基因组途径相关的基因的快速进化。