Derived esterase activity in Drosophila sechellia contributes to evolved octanoic acid resistance.

The dietary specialist fruit fly Drosophila sechellia has evolved resistance to the secondary defence compounds produced by the fruit of its host plant, Morinda citrifolia. The primary chemicals that contribute to lethality of M. citrifolia are the medium-chain fatty acids octanoic acid (OA) and hexanoic acid. At least five genomic regions contribute to this adaptation in D. sechellia and whereas the fine-mapped major effect locus for OA resistance on chromosome 3R has been thoroughly analysed, the remaining four genomic regions that contribute to toxin resistance remain uncharacterized. To begin to identify the genetic basis of toxin resistance in this species, we removed the function of well-known detoxification gene families to determine whether they contribute to toxin resistance. Previous work found that evolution of cytochrome P450 enzymatic activity or expression is not responsible for the OA resistance in D. sechellia. Here, we tested the role of the two other major detoxification gene families in resistance to Morinda fruit toxins - glutathione-S-transferases and esterases - through the use of the pesticide synergists diethyl maleate and tribufos that inhibit the function of these gene families. This work suggests that one or more esterase(s) contribute to evolved OA resistance in D. sechellia.

Analysis of cytochrome P450 contribution to evolved plant toxin resistance in Drosophila sechellia.

Drosophila sechellia is a dietary specialist species of fruit fly that has evolved resistance to the toxic secondary defence compounds produced by the fruit of its preferred host plant Morinda citrifolia. The genetic basis of adult toxin resistance is the result of evolution at five loci across the genome. Genetic mapping between D. sechellia and Drosophila simulans and subsequent functional studies in Drosophila melanogaster have identified candidate genes potentially underlying one locus involved in toxin resistance but the remainder of the genes involved are unknown. Genes in the mixed function oxidase or cytochrome P450 gene family are frequently utilized in evolved toxin resistance in insects, yet whether they play a role in D. sechellia's resistance to the toxins found in its host plant is unknown. Here we test the role of cytochrome P450 enzymatic activity in evolved resistance to the two primary toxins found in M. citrifolia fruit: octanoic acid and hexanoic acid. We found that although cytochrome P450 enzymatic activity is involved in basal resistance it is not involved in derived toxin resistance in D. sechellia.

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

Drosophila sechellia is a species of fruit fly endemic to the Seychelles islands. Unlike its generalist sister species, D. sechellia has evolved to be a specialist on the host plant Morinda citrifolia. This specialization is interesting because the plant's fruit contains secondary defence compounds, primarily octanoic acid (OA), that are lethal to most other Drosophilids. Although ecological and behavioural adaptations to this toxic fruit are known, the genetic basis for evolutionary changes in OA resistance is not. Prior work showed that a genomic region on chromosome 3R containing 18 genes has the greatest contribution to differences in OA resistance between D. sechellia and D. simulans. To determine which gene(s) in this region might be involved in the evolutionary change in OA resistance, we knocked down expression of each gene in this region in D. melanogaster with RNA interference (RNAi) (i) ubiquitously throughout development, (ii) during only the adult stage and (iii) within specific tissues. We identified three neighbouring genes in the Osiris family, Osiris 6 (Osi6), Osi7 and Osi8, that led to decreased OA resistance when ubiquitously knocked down. Tissue-specific RNAi, however, showed that decreasing expression of Osi6 and Osi7 specifically in the fat body and/or salivary glands increased OA resistance. Gene expression analyses of Osi6 and Osi7 revealed that while standing levels of expression are higher in D. sechellia, Osi6 expression is significantly downregulated in salivary glands in response to OA exposure, suggesting that evolved tissue-specific environmental plasticity of Osi6 expression may be responsible for OA resistance in D. sechellia.