Accelerated evolution of Trimeresurus flavoviridis venom gland phospholipase A₂ isozymes

Six Trimeresurus flavoviridis (Habu snake) venom gland phospholipase A₂ (PLA2) isozyme genes were found to consist of four exons and three introns and to encode proteins of 138 amino acid residues, including the signal sequence of 16 amino acid residues. Comparison of the nucleotide sequences showed that the introns are much more homologous than the protein-coding regions of exons except for the signal peptide-coding region of the first exon. The numbers of nucleotide substitutions per site (AN) for introns are approximately one-fourth of the numbers of nucleotide substitutions per synonymous site (K_S) for the protein-coding regions, indicating that the introns are unusually conserved. The absence of an apparent functional role for the introns suggests that the protein-coding regions, except for the signal peptide-coding domains, have evolved at greater substitution rates than introns. The fact that the numbers of nucleotide substitutions per nonsynonymous site (K_A) are close to or larger than K_S values for relevant pairs of genes revealed that Darwinian-type accelerated substitutions have occurred in the protein-coding regions or exons. This is compatible with the presence of PLA2 species with diverse physiological activities in the venom.