Physically interacting sperm-egg proteins have been identified using gene-modified animals in some mammal species. Three proteins are essential for sperm-egg binding: Izumo1 on the sperm surface, and JUNO and CD9 on the egg surface. Most proteins linked to reproductive function evolve rapidly among species by positive selection, and have correlated evolutionary rates to compensate for changes on both the sperm and egg. Up to now, interactions between sperm and egg proteins have not been identified in non-mammalian vertebrates, such as turtles that have interspecific hybrids that can produce surviving F1 generations. To explore the potential physical interactions of sperm-egg proteins in turtle species, the coding region of Izumo1, JUNO, and CD9 homologous genes (named Tu-Izumo1, Tu-JUNO, and Tu-CD9) in six turtle species (Mauremys reevesii, M. mutica, M. sinensis, Cistoclemmys flavomarginata, Platysternon megacephalum and Chrysemys picta bellii) were identified, amplified, and sequenced, and tissue-specific expression was analyzed in M. reevesii. We constructed phylogenetic trees and analyzed the signatures of coevolution between sperm-egg protein pairs using MirrorTree Server and linear regression methods. The results showed that Tu-Izumo1, Tu-JUNO, and Tu-CD9 proteins have correlated evolutionary rates, and that the area where Tu-Izumo1 interacts with Tu-JUNO has only one positive selection site in some turtle species. These results suggest there is a potential interaction between Tu-Izumo1 and Tu-JUNO among turtles that can interbreed, and that a significantly lower positive selection in the interaction region may be one of the reasons why turtle hybrids are so common. Further studies are required to uncover Tu-Izumo1, Tu-JUNO and Tu-CD9 protein biological functions during gamete fusion.

中文翻译：

---

检测海龟精卵融合蛋白中阳性选择的协同进化

已经在某些哺乳动物物种中使用基因修饰的动物鉴定了物理相互作用的精子 - 卵蛋白。三种蛋白质对于精卵结合至关重要：精子表面的 Izumo1，以及卵表面的 JUNO 和 CD9。大多数与生殖功能相关的蛋白质通过正选择在物种之间快速进化，并且具有相关的进化速率以补偿精子和卵子的变化。到目前为止，尚未在非哺乳动物脊椎动物中发现精子和卵蛋白之间的相互作用，例如具有可以产生幸存 F1 代的种间杂种的海龟。为了探索龟类中精子-卵蛋白的潜在物理相互作用，六种龟类（Mauremys reevesii、Mauremys reevesii）的 Izumo1、JUNO 和 CD9 同源基因（命名为 Tu-Izumo1、Tu-JUNO 和 Tu-CD9）的编码区M。mutica、M. sinensis、Cistoclemmys flavomarginata、Platysternon megacephalum 和 Chrysemys picta bellii) 被鉴定、扩增和测序，并在 M. reevesii 中分析组织特异性表达。我们构建了系统发育树，并使用 MirrorTree Server 和线性回归方法分析了精子-卵蛋白对之间共同进化的特征。结果表明，Tu-Izumo1、Tu-JUNO 和Tu-CD9 蛋白具有相关的进化速率，并且Tu-Izumo1 与Tu-JUNO 相互作用的区域在某些龟类中只有一个正选择位点。这些结果表明，在可以杂交的海龟中，Tu-Izumo1 和 Tu-JUNO 之间存在潜在的相互作用，并且相互作用区域中显着较低的正选择可能是海龟杂交如此普遍的原因之一。