Genome-wide association studies identify new candidate genes and tissues underlying resistance to a natural toxin in drosophilids

Many insects can rapidly evolve resistance to artificial insecticides through changes in toxin target proteins. Over longer timescales, insects also evolve resistance to naturally occurring toxins to exploit new ecological niches, but the underlying mechanisms often remain poorly understood. A classic example is Drosophila sechellia, an extreme specialist for the ripe noni fruit of Morinda citrifolia. Noni is toxic for other insects - including D. sechellia's close relatives D. simulans and D. melanogaster - due to this fruit's high content of octanoic acid (OA). However, the mechanistic bases of OA susceptibility and resistance across species remain unclear. Here, we first show that the species-specific tolerance of OA is independent of these drosophilids' distinct microbiomes. Screening large, genetically-diverse panels of D. melanogaster and D. simulans strains revealed broad variation in OA resistance, with some lines surviving as well as D. sechellia. Resistance to OA does not correlate with resistance of these lines to other insecticides, implying a distinct toxicity mode-of-action. Genome-wide association and transcriptome-to-phenotype analyses identified multiple genes linked to OA resistance, with diverse expression patterns and functions, including epithelial septate junction formation, and lipid transport. Loss-of-function analysis in D. melanogaster confirmed that at least two of these - Bez, a CD36-family fatty acid transporter, and CG13003, a putative extracellular matrix component - positively contribute to OA resistance. Integration of our findings with those from previous complementary genetic approaches supports a model in which OA has no singular target, and that resistance is defined by multigenic and multi-tissue defense mechanisms.