Evolution of the avian β-defensin and cathelicidin genes

β-defensins and cathelicidins are two families of cationic antimicrobial peptides (AMPs) with a broad range of antimicrobial activities that are key components of the innate immune system. Due to their important roles in host defense against rapidly evolving pathogens, the two gene families provide an ideal system for studying adaptive gene evolution. In this study we performed phylogenetic and selection analyses on β-defensins and cathelicidins from 53 avian species representing 32 orders to examine the evolutionary dynamics of these peptides in birds.

Avian β-defensins are found in a gene cluster consisting of 13 subfamiles. Nine of these are conserved as one to one orthologs in all birds, while the others (AvBD1, AvBD3, AvBD7 and AvBD14) are more subject to gene duplication or pseudogenisation events in specific avian lineages. Avian cathelicidins are found in a gene cluster consisting of three subfamilies with species-specific duplications and gene loss. Evidence suggested that the propiece and mature peptide domains of avian cathelicidins are possibly co-evolving in such a way that the cationicity of the mature peptide is partially neutralised by the negative charge of the propiece prior to peptide secretion (further evidence obtained by repeating the analyses on primate cathelicidins). Negative selection (overall mean dN < dS) was detected in most of the gene domains examined, conserving certain amino acid residues that may be functionally crucial for the avian β-defensins and cathelicidins, while episodic positive selection was also involved in driving the diversification of specific codon sites of certain AMPs in avian evolutionary history. These findings have greatly improved our understanding of the molecular evolution of avian AMPs and will be useful to understand their role in the avian innate immune response. Additionally, the large dataset of β-defensin and cathelicidin peptides may also provide a valuable resource for translational research and development of novel antimicrobial agents in the future.