Genetic basis of octanoic acid resistance inDrosophila sechellia: functional analysis of a fine‐mapped region

Abstract

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 defense compounds, primarily octanoic acid (OA), that are lethal to most other Drosophilids. Although ecological and behavioral adaptations to this toxic fruit are known, the genetic basis for evolutionary changes in OA resistance are 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 neighboring genes in the Osiris family, Osiris 6 (Osi6), Osi7, and Osi8, that lead 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.

Analysis of cytochrome P450 contribution to evolved plant toxin resistance inDrosophila sechellia

“…Because PBO inactivates the function of the MFO system, it is frequently used as a synergist in pesticide formulations. Combination of PBO treatment with exposure to 1.2% OA (concentration determined in previous publication to maximize the ability to see increases and decreases in OA resistance, Andrade Lopez et al ., ) allowed for tests of differences in mortality associated with removal of cytochrome P450 function in the presence of OA. Any observed differences between PBO + OA and OA alone would demonstrate the function of the MFO system in OA resistance.…”

“…Noni fruit produces high levels of medium chain fatty acids including octanoic acid (OA) and hexanoic acid (HA) that represent 58 and 19% of all the volatile compounds produced by M. citrifolia , respectively (Farine et al ., ). Whereas OA and HA are both highly toxic to Drosophila species (Legal et al ., ; Andrade Lopez et al ., ; but see Yassin et al ., ), D. sechellia has evolved resistance to the toxins found in noni fruit.…”

“…Previous research into the genetic basis of OA resistance in D. sechellia adults indicated that there were five genetic loci that contribute to the evolution of this trait (Jones, ). Recent work has identified genes in the Osiris family, that when knocked down in the model species Drosophila melanogaster , influence survival specifically upon OA exposure (Andrade Lopez et al ., ). These candidate genes probably underlie a major effect locus on chromosome 3R that explains approximately 15% of the variation in OA resistance between the susceptible sister species Drosophila simulans and the resistant species D. sechellia (Jones, ; Hungate et al ., ); however, CRISPR/Cas9 allele swaps are still needed to confirm this result.…”

Derived esterase activity inDrosophila sechelliacontributes to evolved octanoic acid resistance

“…For example, a locus on chromosome 3R has been shown to be responsible for the greatest variation in OA resistance between adult D. sechellia and D. simulans and has been narrowed to a 168 kb region containing 18 genes using an introgression based approach (Hungate et al ., ). Utilizing RNA interference in D. melanogaster , this fine‐mapped region was further refined using functional tests of the 18 genes residing in the QTL (Andrade‐Lopez et al ., ). This analysis revealed three genes in the Osiris family, Osiris 6 ( Osi6 ), Osi7 and Osi8 , that led to a decrease in OA resistance when gene expression was ubiquitously knocked down in D. melanogaster (Andrade López et al ., ).…”

“…Utilizing RNA interference in D. melanogaster , this fine‐mapped region was further refined using functional tests of the 18 genes residing in the QTL (Andrade‐Lopez et al ., ). This analysis revealed three genes in the Osiris family, Osiris 6 ( Osi6 ), Osi7 and Osi8 , that led to a decrease in OA resistance when gene expression was ubiquitously knocked down in D. melanogaster (Andrade López et al ., ). A subsequent study revealed knockdown of Osi8 in the larval stage of D. melanogaster also increased susceptibility to OA (Lanno et al ., ).…”