Vacquier V D, Swanson W J, Lee Y H
Marine Biology Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla 92093-0202, USA.
J Mol Evol. 1997;44 Suppl 1:S15-22. doi: 10.1007/pl00000049.
Most examples of positive selection inferred from nucleotide sequence data involve hostpathogen interactions. However, positive selection also promotes the divergence of proteins mediating sperm-egg recognition in marine invertebrates. The abalone spermatozoon has a large acrosomal vesicle containing two proteins of 16 kDa and 18 kDa. Lysin, the 16-kDa protein, exhibits species-specificity in dissolving a hole in the egg vitelline envelope through which the sperm swims to reach the egg plasma membrane. The 18-kDa protein coats the sperm acrosomal process and probably mediates fusion of the two gametes. In this review, we compare sequences of both proteins from five species of California abalones. Both proteins show extensive divergence which has been promoted by positive Darwinian selection. The ratios of nonsynonymous to synonymous nucleotide substitutions may be the highest yet discovered for full-length sequences. Although extensive divergence has occurred, there is conservation of the shape and polarity of residues in both proteins. The two acrosomal proteins arose by a gene duplication followed by their extensive divergence. Five hypotheses are presented which attempt to explain the nature of the unknown selective force responsible for the robust positive selection. The positive selection may, in some unknown way, be related to the establishment of prezygotic barriers to reproduction. Because positive selection promotes the divergence of unrelated, species-specific gamete recognition proteins in both abalones and sea urchins, we predict that positive selection may be a general phenomenon in the evolution of gamete recognition systems in marine invertebrates.
从核苷酸序列数据推断出的大多数正选择实例都涉及宿主与病原体的相互作用。然而，正选择也促进了海洋无脊椎动物中介导精卵识别的蛋白质的分化。鲍鱼精子有一个大的顶体泡，其中含有两种分子量分别为16 kDa和18 kDa的蛋白质。16 kDa的蛋白质lysin在卵黄膜上溶解一个洞，精子通过这个洞游动到达卵质膜，表现出物种特异性。18 kDa的蛋白质覆盖在精子顶体突起上，可能介导两个配子的融合。在这篇综述中，我们比较了五种加利福尼亚鲍鱼这两种蛋白质的序列。两种蛋白质都显示出广泛的分化，这是由正达尔文选择促进的。非同义核苷酸替换与同义核苷酸替换的比率可能是全长序列中迄今发现的最高值。尽管发生了广泛的分化，但两种蛋白质中残基的形状和极性仍保持保守。这两种顶体蛋白质是通过基因复制产生的，随后发生了广泛的分化。本文提出了五个假说，试图解释导致强烈正选择的未知选择力的性质。正选择可能以某种未知的方式与建立生殖前的繁殖障碍有关。由于正选择促进了鲍鱼和海胆中不相关的、物种特异性的配子识别蛋白的分化，我们预测正选择可能是海洋无脊椎动物配子识别系统进化中的一种普遍现象。
