Expression of defensin genes across house fly (Musca domestica) life history gives insight into immune system subfunctionalization

Animals encounter diverse microbial communities throughout their lifetime, which exert varying selection pressures. Antimicrobial peptides (AMPs), which lyse or inhibit microbial growth, are a first line of defense against some of these microbes. Here examine how developmental variation in microbial exposure has shaped the expression and amino acid sequences of Defensins (an ancient class of AMPs) in the house fly (Musca domestica). The house fly is a well-suited model for this work because it trophically associates with varying microbial communities throughout its life history and its genome shows largely expanded families of AMPs, including Defensins. We identified two subsets of house fly Defensins expressed during distinct developmental stages (larval/pupal and adult), which form separate monophyletic clades. The adult-expressed Defensins evolve faster than larval/pupal Defensins, consistent with different selection pressures across developmental stages. Our results therefore suggest that varied microbial communities encountered during its life history can shape the evolutionary trajectories of immune genes.