Maximum-likelihood analysis of molecular adaptation in abalone sperm lysinreveals variable selective pressures among lineages and sites

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 pre ssure and allowed the ratio to vary among lineages or sites. Likelihood rat io tests suggested significant variation in selective pressure among lineag es. The variable selective pressure provided an explanation for the previou s observation that the omega ratio is >1 in comparisons of closely related species and <1 in comparisons of distantly related species. Computer simula tions demonstrated that saturation of nonsynonymous substitutions and const raint on lysin structure were unlikely to account for the observed pattern. Lineages linking closely related sympatric species appeared to be under di versifying selection, while lineages separating distantly related species f rom different geographic locations were associated with low evolutionary ra tes. 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 rou te of evolution at individual sites and to provide lysin sequences for futu re laboratory studies.