Department of Biology and Marine Biology, University of North Carolina Wilmington.
Mol Biol Evol. 2013 Dec;30(12):2688-98. doi: 10.1093/molbev/mst165. Epub 2013 Sep 18.
Marine invertebrate gamete recognition proteins (GRPs) are classic examples of rapid adaptive evolution of reproductive proteins, and hybridizing Mytilus blue mussels allow us to study the evolution of GRPs during speciation following secondary contact. Even with frequent hybridization, positive selection drives divergence of M7 lysin, one of the three Mytilus egg vitelline envelope (VE) lysins. Mytilus trossulus and M. edulis form a broad hybrid zone in the Canadian Maritimes and eastern Maine, isolated by strong (but partial) gamete incompatibility. M7 lysin, however, is an unlikely GRP controlling this gametic incompativility, as earlier studies showed either weak or no positive selection and extensive introgression between the two species. We used reverse transcriptase-polymerase chain reaction and cloned several alleles of M3 lysin, a potent VE lysin encoded by a nonhomologous gene whose evolution has not been studied. McDonald-Kreitman and HKA tests reveal strong positive selection, which PAML branch-site models detect in 19.7% of the codons. Protein structure predictions show that replacements map exclusively to one face of the carbohydrate recognition domain (CRD) of this C-type lectin, with codons under positive selection localizing to CRD regions known to control ligand specificity. Polymorphism/divergence analyses show that selective sweep has purged M. edulis but not M. trossulus of polymorphism, and unique to M3 is an absence of fixed substitutions and broad haplotype sharing between M. edulis and Mediterranean M. galloprovincialis. Taken together, these results suggest that different lysins serve as GRPs in different Mytilus hybrid zones, with M3 likely co-opted to play this role in the western Atlantic.
海洋无脊椎动物配子识别蛋白（GRP）是生殖蛋白快速适应性进化的经典范例，而杂交贻贝允许我们在二次接触后物种形成过程中研究 GRP 的进化。即使频繁杂交，正选择也会推动 Mytilus 卵黄膜（VE）溶菌酶之一 M7 溶菌酶的分化。Mytilus trossulus 和 M. edulis 在加拿大滨海地区和缅因州东部形成了一个广泛的杂交区，由强烈（但不完全）配子不相容性隔离。然而，M7 溶菌酶不太可能是控制这种配子不相容性的 GRP，因为早期研究表明，在这两个物种之间，无论是弱正选择还是没有正选择，以及广泛的基因渗入。我们使用逆转录聚合酶链反应并克隆了 M3 溶菌酶的几个等位基因，M3 溶菌酶是一种由非同源基因编码的强效 VE 溶菌酶，其进化尚未研究。McDonald-Kreitman 和 HKA 检验显示出强烈的正选择，PAML 分支位点模型在 19.7%的密码子中检测到这种选择。蛋白质结构预测表明，替换仅映射到这种 C 型凝集素的碳水化合物识别结构域（CRD）的一个面上，正选择的密码子定位于已知控制配体特异性的 CRD 区域。多态性/分化分析表明，选择清除了 M. edulis 的多态性，但没有清除 M. trossulus 的多态性，而 M3 特有的是缺乏固定替换和 M. edulis 和地中海 M. galloprovincialis 之间广泛的单倍型共享。综上所述，这些结果表明，不同的溶菌酶在不同的贻贝杂交区充当 GRP，而 M3 可能被共同选择在西大西洋发挥这种作用。
