Concerted evolution of two novel protein families in Caenorhabditis species

Among a large number of homologous gene clusters in C. elegans, two gene families that appear to undergo concerted evolution were studied in detail. Both gene families are nematode specific and encode small secreted proteins of unknown function. For both families in three Caenorhabditis species, concerted groups of genes are characterized by close genomic proximity and by genes in inverted orientation. The rate of protein evolution in one of the two families could be calibrated by comparison with a closely related nonconcerted singleton gene with one-to-one orthologs in all three species. This comparison suggests that protein evolution in concerted gene clusters is two- to sevenfold accelerated. A broader survey of clustered gene families, focused on adjacent inverted gene pairs, identified an additional seven families in which concerted evolution probably occurs. All nine identified families encode relatively small proteins, eight of them encode putative secreted proteins, and most of these have very unusual amino acid composition or sequence. I speculate that these genes encode rapidly evolving antimicrobial peptides.