Zh. Yang et al., Maximum-likelihood analysis of molecular adaptation in abalone sperm lysinreveals variable selective pressures among lineages and sites, MOL BIOL EV, 17(10), 2000, pp. 1446-1455
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.