Immune priming of kissing bugs by their gut bacteria

Obligate microbial symbionts of insects present a conflict for the host – the symbionts are necessary for host development, reproduction, and survival, yet the host must also maintain the ability to fend off microbial pathogens. Many insects sequester their symbionts from their immune system by housing them in specialized organs or intracellularly and some have lost major components of their immune pathways. These changes are thought to accommodate persistent bacterial symbionts in the host. Triatomine kissing bugs (Hemiptera: Reduviidae: Triatominae) harbor extracellular gut bacteria that are essential for their successful development and reproduction as axenic, bacteria free insects fail to develop to adulthood. We asked whether the bacterial symbionts of kissing bugs influenced host immune responses, as the bacteria exist freely in the gut and persist across insect development. We implemented an RNAseq approach comparing the expression of genes in the midguts of axenic and gnotobiotic Rhodnius prolixus harboring a single, known symbiont, Rhodococcus rhodnii. Our transcriptome data revealed several immune-related genes were differentially expressed between the conditions. We then silenced a number of these genes using RNAi and explored how they impacted the abundance of bacteria in the gut. Our results suggested that various bacteria elicited distinct responses from the host immune system, and that symbionts did not trigger a strong immune response. Our results indicate that the gut microbes of R. prolixus, and likely other kissing bug species, play an important role in host immune regulation and protection against pathogens.