Data from: 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 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.塞舌尔果蝇（Drosophila sechellia）是一种仅分布于塞舌尔群岛的果蝇物种。与其广食性近缘物种不同，塞舌尔果蝇已演化成为寄主植物诺丽（Morinda citrifolia）的专食性物种。这种专化性颇具研究价值，因为该植物的果实含有次生防御化合物，主要为辛酸（OA），该物质对绝大多数其他果蝇科昆虫具有致死性。尽管已知该果蝇对这种有毒果实的生态与行为适应机制，但针对辛酸抗性演化改变的遗传基础仍未明确。既往研究表明，3R号染色体上一段包含18个基因的基因组区域，是导致塞舌尔果蝇与拟果蝇（Drosophila simulans）之间辛酸抗性差异的最主要遗传位点。为明确该区域中哪些基因参与了辛酸抗性的演化改变，本研究通过RNA干扰（RNAi）技术，在黑腹果蝇（Drosophila melanogaster）中对该区域的每个基因分别进行表达敲低，处理方式包括：(i) 全发育阶段全局敲低；(ii) 仅在成虫阶段敲低；(iii) 在特定组织中敲低。研究发现，奥西里斯基因家族（Osiris family）中的3个相邻基因——奥西里斯6（Osiris 6，Osi6）、Osi7与Osi8——在全局敲低时会导致黑腹果蝇的辛酸抗性下降。但组织特异性RNA干扰实验显示，仅在脂肪体和/或唾液腺中敲低Osi6与Osi7的表达，反而会提升果蝇的辛酸抗性。对Osi6与Osi7的基因表达分析结果显示：尽管塞舌尔果蝇的基础表达水平更高，但在暴露于辛酸环境时，其唾液腺中的Osi6表达会显著下调。这表明，Osi6表达的演化组织特异性环境可塑性，可能是塞舌尔果蝇获得辛酸抗性的关键机制。