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.

动物在其一生当中会接触到多样的微生物群落，这些群落会施加不同的选择压力。抗菌肽（Antimicrobial Peptides, AMPs）可裂解或抑制微生物生长，是抵御部分此类微生物的第一道防线。本研究旨在探究微生物暴露的发育差异如何塑造家蝇（Musca domestica）防御素（Defensins，一类古老的抗菌肽）的表达模式与氨基酸序列。家蝇是开展该研究的理想模型：其一，其整个生活史中均会与不同的微生物群落形成营养关联；其二，其基因组中包括防御素家族在内的抗菌肽家族发生了显著扩张。我们鉴定出家蝇防御素的两个亚家族，它们分别在不同发育阶段（幼虫/蛹期与成虫期）表达，并各自形成独立的单系演化支。成虫期表达的防御素演化速率快于幼虫/蛹期防御素，这与不同发育阶段面临不同选择压力的结论相符。综上，本研究结果表明，家蝇在其生活史中接触的多样微生物群落，能够塑造免疫基因的演化轨迹。