Hellberg M E, Moy G W, Vacquier V D
Department of Biological Sciences, Louisiana State University at Baton Rouge 70803, USA.
Mol Biol Evol. 2000 Mar;17(3):458-66. doi: 10.1093/oxfordjournals.molbev.a026325.
Male-specific proteins have increasingly been reported as targets of positive selection and are of special interest because of the role they may play in the evolution of reproductive isolation. We report the rapid interspecific divergence of cDNA encoding a major acrosomal protein of unknown function (TMAP) of sperm from five species of teguline gastropods. A mitochondrial DNA clock (calibrated by congeneric species divided by the Isthmus of Panama) estimates that these five species diverged 2-10 MYA. Inferred amino acid sequences reveal a propeptide that has diverged rapidly between species. The mature protein has diverged faster still due to high nonsynonymous substitution rates (> 25 nonsynonymous substitutions per site per 10(9) years). cDNA encoding the mature protein (89-100 residues) shows evidence of positive selection (Dn/Ds > 1) for 4 of 10 pairwise species comparisons. cDNA and predicted secondary-structure comparisons suggest that TMAP is neither orthologous nor paralogous to abalone lysin, and thus marks a second, phylogenetically independent, protein subject to strong positive selection in free-spawning marine gastropods. In addition, an internal repeat in one species (Tegula aureotincta) produces a duplicated cleavage site which results in two alternatively processed mature proteins differing by nine amino acid residues. Such alternative processing may provide a mechanism for introducing novel amino acid sequence variation at the amino-termini of proteins. Highly divergent TMAP N-termini from two other tegulines (Tegula regina and Norrisia norrisii) may have originated by such a mechanism.
越来越多的研究报道，雄性特异性蛋白是正选择的目标，由于它们可能在生殖隔离的进化中发挥作用，因此备受关注。我们报道了来自五种笠螺科腹足类动物精子的一种功能未知的主要顶体蛋白（TMAP）的编码cDNA的快速种间分化。通过线粒体DNA时钟（以被巴拿马地峡分隔的同属物种校准）估计，这五个物种在200 - 1000万年前分化。推断的氨基酸序列显示，种间的前肽分化迅速。由于高非同义替换率（每10^9年每个位点> 25个非同义替换），成熟蛋白的分化更快。编码成熟蛋白（89 - 100个残基）的cDNA在10对物种比较中的4对显示出正选择的证据（非同义替换率/同义替换率> 1）。cDNA和预测的二级结构比较表明，TMAP与鲍鱼溶素既不是直系同源也不是旁系同源，因此标志着在自由产卵的海洋腹足类动物中第二个在系统发育上独立且受到强烈正选择的蛋白。此外，一个物种（金色笠螺）中的一个内部重复产生了一个重复的切割位点，导致两种不同加工的成熟蛋白，它们相差9个氨基酸残基。这种可变加工可能为在蛋白的氨基末端引入新的氨基酸序列变异提供一种机制。另外两种笠螺（女王笠螺和诺氏笠螺）高度分化的TMAP氨基末端可能就是通过这种机制产生的。
