Springer Stevan A, Crespi Bernard J
Department of Biosciences, Behavioural Ecology Research Group, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada.
Evolution. 2007 Apr;61(4):772-83. doi: 10.1111/j.1558-5646.2007.00073.x.
Gamete-recognition proteins often evolve rapidly, but it is not known if their divergence occurs within species and corresponds with the evolution of reproductive isolation, or if divergence typically accumulates between already isolated lineages. We examined the evolution of a candidate gamete-recognition protein in several sympatric and allopatric populations of Mytilus blue mussels, species that hybridize in nature. Within a single species, Mytilus galloprovincialis, we found adaptive divergence of Lysin-M7, a sperm acrosomal protein that dissolves the egg vitelline envelope during fertilization. Mytilus galloprovincialis Lysin-M7 alleles group into two distinct clades (termed G and G(D)), and individual alleles in these clades are separated from each other by at least three and up to eleven amino-acid substitutions. Maximum-likelihood estimates of selective pressure (dN/dS =omega) implicate selection in the divergence between M. galloprovincialis Lysin-M7 clades, and within the G(D) clade. Exact tests of population differentiation indicate that the relative frequency of G and G(D) Lysin-M7 alleles differs significantly among M. galloprovincialis populations. Compared with allopatric Mediterranean samples, Lysin-M7 alleles in the G(D) clade are found at elevated frequency in samples from the East Atlantic and California, areas of secondary contact and hybridization between Mytilus species, and Australia, an area of unknown species composition. Adaptive divergence between the alleles most common in allopatry and those found at elevated frequency in samples from sympatry suggests that selection pressures acting in hybridizing populations, likely following Pleistocene secondary contact with M. edulis in the East Atlantic, drove the divergence of Lysin-M7 in M. galloprovincialis.
配子识别蛋白通常进化迅速，但尚不清楚它们的分化是发生在物种内部并与生殖隔离的进化相对应，还是分化通常在已经隔离的谱系之间积累。我们研究了紫贻贝几个同域和异域种群中一种候选配子识别蛋白的进化，紫贻贝是一种在自然环境中杂交的物种。在单一物种——地中海贻贝中，我们发现了溶素-M7的适应性分化，溶素-M7是一种精子顶体蛋白，在受精过程中溶解卵黄膜。地中海贻贝的溶素-M7等位基因分为两个不同的分支（称为G和G(D)），这些分支中的单个等位基因彼此之间至少有三个到多达十一个氨基酸替换。选择压力的最大似然估计（dN/dS =ω）表明，在地中海贻贝溶素-M7分支之间以及G(D)分支内部存在选择作用。种群分化的精确测试表明，G和G(D)溶素-M7等位基因的相对频率在地中海贻贝种群中存在显著差异。与异域分布的地中海样本相比，G(D)分支中的溶素-M7等位基因在东大西洋和加利福尼亚（紫贻贝物种二次接触和杂交的区域）以及澳大利亚（物种组成未知的区域）的样本中频率升高。异域分布中最常见的等位基因与同域分布样本中频率升高的等位基因之间的适应性分化表明，在杂交种群中起作用的选择压力，可能是在更新世与东大西洋的蓝贻贝二次接触之后，推动了地中海贻贝中溶素-M7的分化。
