Genetic basis of octanoic acid resistance in drosophila sechellia: functional analysis of a fine-mapped region

Abstract

Drosophila sechellia is a species of fruit fly endemic to the Seychelles islands, which are located northeast of Madagascar off the coast of Africa. Unlike its generalist sister species D. simulans and D. mauritiana, and their closest relative D. melanogaster, D. sechellia evolved to specialize on a single plant species, Morinda citrifolia. Specialization on M. citrifolia is surprising because the fruit of the plant contains toxic compounds, primarily octanoic acid (OA), that are lethal to all Drosophila species except D. sechellia. Although the ecological and behavioral adaptations to this toxic fruit are known, the genetic basis for the evolutionary changes in OA resistance is not. Prior work showed that a genomic region on chromosome 3R, containing 18 genes, contributes to OA tolerance. To determine which gene(s) in this region might be involved in the evolution of OA resistance, I knocked-down expression of each gene in D. melanogaster with RNA interference (RNAi) (i) ubiquitously throughout development, (ii) during the adult stage, and (iii) within specific tissues in D. melanogaster. RNAi knockdown flies were tested for resistance to OA using the mixed effects Cox regression model. I found that knock-down of three neighboring genes, Osiris 6, Osiris 7, and Osiris 8, increased OA sensitivity. Tissue specific knockdowns, however, showed that decreasing expression of these genes in the fat body and salivary glands increases OA tolerance. I show that both Osi6/7 are highly expressed during the first 24 hours of development and that exposure to different stressors induces expression in adults. Although Osi6/7 have no coding change, RNA-seq data shows derived lower expression of these genes in D. sechellia; Osi8 has two derived coding changes in D. sechellia. This study sheds light on the genetic basis of ecological adaptation to a toxic host within Drosophila, and insect-host specialization more broadly.