Adaptive evolution after duplication of penaeidin antimicrobial peptides

While the average ratio of non-synonymous to synonymous substitutions (ω) for the entire coding region of both active domains is 0.9805, few codon sites showed significantly higher ω (3.73). The likelihood ratio tests that compare models incorporating positive selection (ω > 1) at certain codon sites with models not incorporating positive selection (ω < 1), failed to reject (p = 0) the evidence of positive Darwinian selection. The rapid adaptive evolution of this gene family might be directed by the pathogens and the faster rate of amino acid substitutions in the N-terminal proline-rich and C-terminal cysteine-rich domains could be due to their direct involvement in the protection against pathogens. When the host expose to different habitats/environment an accelerated rate of amino acid substitutions in both the active domains may also be expected.