Yang Z, Swanson W J, Vacquier V D
Galton Laboratory, Department of Biology, University College London, London, England.
Mol Biol Evol. 2000 Oct;17(10):1446-55. doi: 10.1093/oxfordjournals.molbev.a026245.
Maximum-likelihood models of codon substitution were used to analyze sperm lysin genes of 25 abalone (HALIOTIS:) species to identify lineages and amino acid sites under diversifying selection. The models used the nonsynonymous/synonymous rate ratio (omega = d(N)/d(S)) as an indicator of selective pressure and allowed the ratio to vary among lineages or sites. Likelihood ratio tests suggested significant variation in selective pressure among lineages. The variable selective pressure provided an explanation for the previous observation that the omega ratio is >1 in comparisons of closely related species and <1 in comparisons of distantly related species. Computer simulations demonstrated that saturation of nonsynonymous substitutions and constraint on lysin structure were unlikely to account for the observed pattern. Lineages linking closely related sympatric species appeared to be under diversifying selection, while lineages separating distantly related species from different geographic locations were associated with low evolutionary rates. The selective pressure indicated by the omega ratio was found to vary greatly among amino acid sites in lysin. Sites under potential diversifying selection were identified. Ancestral lysins were inferred to trace the route of evolution at individual sites and to provide lysin sequences for future laboratory studies.
采用密码子替换的最大似然模型，分析了25种鲍鱼（鲍属）的精子溶素基因，以识别处于多样化选择下的谱系和氨基酸位点。这些模型将非同义/同义速率比（ω = d(N)/d(S)）用作选择压力的指标，并允许该比率在谱系或位点之间变化。似然比检验表明，谱系间的选择压力存在显著差异。可变的选择压力解释了先前的观察结果，即在亲缘关系较近的物种比较中ω比率大于1，而在亲缘关系较远的物种比较中ω比率小于1。计算机模拟表明，非同义替换的饱和以及对溶素结构的限制不太可能解释所观察到的模式。连接亲缘关系较近的同域物种的谱系似乎处于多样化选择之下，而将来自不同地理位置的亲缘关系较远的物种分开的谱系则与低进化速率相关。发现ω比率所指示的选择压力在溶素的氨基酸位点之间有很大差异。识别出了可能处于多样化选择下的位点。推断出祖先溶素来追踪各个位点的进化路径，并为未来的实验室研究提供溶素序列。
