UMR85 Physiologie de la Reproduction et des Comportements, INRA, Nouzilly, France.
PLoS One. 2012;7(9):e44548. doi: 10.1371/journal.pone.0044548. Epub 2012 Sep 5.
Genes encoding proteins involved in sperm-egg interaction and fertilization exhibit a particularly fast evolution and may participate in prezygotic species isolation [1], [2]. Some of them (ZP3, ADAM1, ADAM2, ACR and CD9) have individually been shown to evolve under positive selection [3], [4], suggesting a role of positive Darwinian selection on sperm-egg interaction. However, the genes involved in this biological function have not been systematically and exhaustively studied with an evolutionary perspective, in particular across vertebrates with internal and external fertilization. Here we show that 33 genes among the 69 that have been experimentally shown to be involved in fertilization in at least one taxon in vertebrates are under positive selection. Moreover, we identified 17 pseudogenes and 39 genes that have at least one duplicate in one species. For 15 genes, we found neither positive selection, nor gene copies or pseudogenes. Genes of teleosts, especially genes involved in sperm-oolemma fusion, appear to be more frequently under positive selection than genes of birds and eutherians. In contrast, pseudogenization, gene loss and gene gain are more frequent in eutherians. Thus, each of the 19 studied vertebrate species exhibits a unique signature characterized by gene gain and loss, as well as position of amino acids under positive selection. Reflecting these clade-specific signatures, teleosts and eutherian mammals are recovered as clades in a parsimony analysis. Interestingly the same analysis places Xenopus apart from teleosts, with which it shares the primitive external fertilization, and locates it along with amniotes (which share internal fertilization), suggesting that external or internal environmental conditions of germ cell interaction may not be the unique factors that drive the evolution of fertilization genes. Our work should improve our understanding of the fertilization process and on the establishment of reproductive barriers, for example by offering new leads for experiments on genes identified as positively selected.
参与精卵相互作用和受精的蛋白质编码基因表现出特别快速的进化，并且可能参与了合子前的物种隔离[1]、[2]。其中一些基因（ZP3、ADAM1、ADAM2、ACR 和 CD9）已被证明在进化过程中受到正选择[3]、[4]，这表明正达尔文选择在精卵相互作用中发挥了作用。然而，涉及这一生物学功能的基因尚未从进化角度进行系统和详尽的研究，特别是在具有内部和外部受精的脊椎动物中。在这里，我们发现，在至少一种脊椎动物的一个分类群中，实验证明参与受精的 69 个基因中，有 33 个基因受到正选择。此外，我们鉴定出 17 个假基因和 39 个在一个物种中至少有一个重复的基因。对于 15 个基因，我们既没有发现正选择，也没有基因副本或假基因。硬骨鱼类的基因，特别是与精子-卵质膜融合相关的基因，似乎比鸟类和真兽类的基因更频繁地受到正选择。相比之下，假基因化、基因丢失和基因获得在真兽类中更为常见。因此，在研究的 19 种脊椎动物中，每一种都表现出独特的特征，包括基因的获得和丢失，以及受到正选择的氨基酸位置。反映这些分支特异性特征，硬骨鱼类和真兽类哺乳动物在简约分析中被恢复为分支。有趣的是，同样的分析将 Xenopus 与硬骨鱼类区分开来，Xenopus 与硬骨鱼类具有原始的外部受精，并且与羊膜动物（具有内部受精）一起定位，这表明精卵相互作用的外部或内部环境条件可能不是驱动受精基因进化的唯一因素。我们的工作应该有助于提高我们对受精过程的理解，并有助于建立生殖隔离，例如为鉴定为正选择的基因的实验提供新的线索。
