Evolutionary selective trends of insect/mosquito antimicrobial defensin peptides containing cysteine-stabilized α/β motifs

Abstract

Insect defensins containing cysteine-stabilized α/β motifs (Cs-α/β defensin) are cationic, inducible antibacterial peptides involved in humoral defence against pathogens. To examine trends in molecular evolution of these antimicrobial peptides, sequences similar to the well-characterized Cs-α/β defensin peptide of Anopheles gambiae, using six cysteine residues as landmarks, were retrieved from genomic and protein databases. These sequences were derived from different orders of insects. Genes of insect Cs-α/β defensin appear to constitute a multigene family in which the copy number varies between insect species. Phylogenetic analysis of these sequences revealed two main lineages, one group comprising mainly lepidopteran insects and a second, comprising Hemiptera, Coleoptera, Diptera and Hymenoptera insects. Moreover, the topology of the phylogram indicated dipteran Cs-α/β defensins are diverse, suggesting diversity in immune mechanisms in this order of insects. Overall evolutionary analysis indicated marked diversification and expansion of mature defensin isoforms within the species of mosquitoes relative to non-mosquito defensins, implying the presence of finely tuned immune responses to counter pathogens. The observed higher synonymous substitution rate relative to the nonsynonymous rate in almost all the regions of Cs-α/β defensin of mosquitoes suggests that these peptides are predominately under purifying selection. The maximum-likelihood models of codon substitution indicated selective pressure at different amino acid sites in mosquito mature Cs-α/β defensins is differ and are undergoing adaptive evolution in comparison to non-mosquito Cs-α/β defensins, for which such selection was inconspicuous; this suggests the acquisition of selective advantage of the Cs-α/β defensins in the former group. Finally, this study represents the most detailed report on the evolutionary strategies of Cs-α/β defensins of mosquitoes in particular and insects in general, and indicates that insect Cs-α/β defensins have evolved by duplication followed by divergence, to produce a diverse set of paralogues.