Investigating the role of Osiris genes in Drosophila sechellia larval resistance to a host plant toxin.

The underlying genetic basis of adaptive phenotypic changes is generally poorly understood, yet a growing number of case studies are beginning to shed light on important questions about the molecular nature and pleiotropy of such changes. We use Drosophila sechellia, a dietary specialist fruit fly that evolved to specialize on a single toxic host plant, Morinda citrifolia, as a model for adaptive phenotypic change and seek to determine the genetic basis of traits associated with host specialization in this species. The fruit of M.citrifolia is toxic to other drosophilids, primarily due to high levels of the defense chemical octanoic acid (OA), yet D.sechellia has evolved resistance to OA. Our prior work identified three Osiris family genes that reside in a fine-mapped QTL for OA resistance: Osiris 6 (Osi6), Osi7, and Osi8, which can alter OA resistance in adult D.melanogaster when knocked down with RNA interference suggesting they may contribute to OA resistance in D.sechellia. Genetic mapping identified overlapping genomic regions involved in larval and adult OA resistance in D.sechellia, yet it remains unknown whether Osiris genes contribute to resistance in both life stages. Furthermore, because multiple genomic regions contribute to OA resistance, we aim to identify other gene(s) involved in this adaptation. Here, we identify candidate larval OA resistance genes using RNA sequencing to measure genome-wide differential gene expression in D.sechellia larvae after exposure to OA and functionally test identified genes for a role in OA resistance. We then test the Osiris genes previously shown to alter adult OA resistance for effects on OA resistance in larvae. We found that Osi8 knockdown decreased OA resistance in D.melanogaster larvae. These data suggest that evolved changes in Osi8 could impact OA resistance in multiple life stages while Osi6 and Osi7 may only impact adult resistance to OA.