Genetic Dissection of <i>Anopheles gambiae</i> Gut Epithelial Responses to <i>Serratia marcescens</i>

<div><p>Genetic variation in the mosquito <i>Anopheles gambiae</i> profoundly influences its ability to transmit malaria. Mosquito gut bacteria are shown to influence the outcome of infections with <i>Plasmodium</i> parasites and are also thought to exert a strong drive on genetic variation through natural selection; however, a link between antibacterial effects and genetic variation is yet to emerge. Here, we combined SNP genotyping and expression profiling with phenotypic analyses of candidate genes by RNAi-mediated silencing and 454 pyrosequencing to investigate this intricate biological system. We identified 138 <i>An. gambiae</i> genes to be genetically associated with the outcome of <i>Serratia marcescens</i> infection, including the peptidoglycan recognition receptor <i>PGRPLC</i> that triggers activation of the antibacterial IMD/REL2 pathway and the epidermal growth factor receptor <i>EGFR</i>. Silencing of three genes encoding type III fibronectin domain proteins (<i>FN3Ds</i>) increased the <i>Serratia</i> load and altered the gut microbiota composition in favor of <i>Enterobacteriaceae</i>. These data suggest that natural genetic variation in immune-related genes can shape the bacterial population structure of the mosquito gut with high specificity. Importantly, <i>FN3D2</i> encodes a homolog of the hypervariable pattern recognition receptor Dscam, suggesting that pathogen-specific recognition may involve a broader family of immune factors. Additionally, we showed that silencing the gene encoding the gustatory receptor Gr9 that is also associated with the <i>Serratia</i> infection phenotype drastically increased <i>Serratia</i> levels. The Gr9 antibacterial activity appears to be related to mosquito feeding behavior and to mostly rely on changes of neuropeptide F expression, together suggesting a behavioral immune response following <i>Serratia</i> infection. Our findings reveal that the mosquito response to oral <i>Serratia</i> infection comprises both an epithelial and a behavioral immune component.</p>