Extensive duplication and convergent sequence evolution of an antimicrobial peptide gene

Antimicrobial peptides (AMPs) are host-encoded antibiotics that combat invading pathogens. AMPs are encoded by short genes that tend to evolve rapidly, likely responding to host-pathogen arms races. However recent studies have highlighted roles for AMPs in neurological contexts suggesting functions for these defence molecules beyond infection. Here we characterize the evolution of the Drosophila Baramicin (Bara) AMP gene family. We describe a recent duplication of the immune-induced BaraA locus, and trans-species polymorphisms in BaraA peptides that suggest dynamic selective pressures. We further recover multiple Baramicin paralogs in Drosophila melanogaster and other species, united by their N-terminal IM24 domain. Strikingly, some paralogs are no longer immune-induced. A careful dissection of the Baramicin familys evolutionary history indicates that these non-immune paralogs result from repeated events of duplication and subsequent truncation of the coding sequence, leaving only the IM24 domain as the prominent gene product. Using mutation and targeted gene silencing, we demonstrate that two such genes are adapted for function in neural contexts in D. melanogaster. Using the Baramicin evolutionary history, we reveal unique properties of the different Baramicin domains. In doing so, we provide a case study for how an AMP-encoding gene might play dual roles in both immune and non-immune processes.