Expression divergence of chemosensory genes between Drosophila sechellia and its sibling species and its implications for host shift [Dsec TW]

Drosophila sechellia relies exclusively on the fruits of Morinda citrifolia, which are toxic to most insects, including its sibling species D. melanogaster and D. simulans. Although several odorant binding protein (Obp) genes and olfactory receptor (Or) genes were suggested to be associated with the D. sechellia host shift, a broad view of how chemosensory genes have contributed to this shift is still lacking. We therefore studied the antennal transcriptomes, the main organ responsible for detecting food resource and oviposition, of D. sechellia and its two sibling species. We wanted to know whether gene expression, particularly chemosensory genes, has diverged between D. sechellia and its two sibling species. Using a very stringent definition of differential gene expression, we found 147 genes (including 11 chemosensory genes) were up-regulated while only 81 genes (including 5 chemosensory genes) were down-regulated in D. sechellia. Interestingly, Obp50a exhibited the highest up-regulation, a ~100 fold increase, and Or85c – previously reported to be a larva-specific gene– showed ~20 fold up-regulation in D. sechellia. Furthermore, Ir84a, proposed to be associated with male courtship behavior, is significantly up-regulated in D. sechellia. We also found expression divergence in most of the receptor gene families between D. sechellia and the two sibling species. Our observations suggest that the host shift of D. sechellia is associated with expression profile divergence in all chemosensory gene families and is achieved mostly by up-regulation of chemosensory genes. Overall design: RNAseq experiments in wild type drosophila antennaes. The strain is D. sechellia (Tuson, #14021-0248-25).

塞氏果蝇（Drosophila sechellia）仅依赖海巴戟天（Morinda citrifolia）的果实为食，该果实对多数昆虫具有毒性，包括其近缘姊妹种黑腹果蝇（D. melanogaster）和拟果蝇（D. simulans）。尽管已有研究提示若干气味结合蛋白（odorant binding protein，Obp）基因与嗅觉受体（olfactory receptor，Or）基因与塞氏果蝇的寄主转移相关，但目前仍缺乏关于化学感受基因如何参与这一寄主转变过程的系统性认知。因此，本研究针对塞氏果蝇及其两个姊妹种的触角——负责感知食物资源与产卵位点的主要器官——的转录组展开分析，旨在明确塞氏果蝇与其姊妹种之间是否存在基因表达分化，尤其是化学感受基因的表达分化。通过采用极为严格的差异基因表达（differential gene expression）判定标准，本研究在塞氏果蝇中发现147个基因（含11个化学感受基因）呈上调表达，仅81个基因（含5个化学感受基因）呈下调表达。值得注意的是，Obp50a的上调幅度最高，表达量提升约100倍；而此前被报道为幼虫特异性基因的Or85c，在塞氏果蝇中也呈现约20倍的上调表达。此外，被推测与雄性求偶行为相关的Ir84a，在塞氏果蝇中也显著上调。本研究还发现，塞氏果蝇与两个姊妹种之间的多数受体基因家族均存在表达分化。本研究结果提示，塞氏果蝇的寄主转移与所有化学感受基因家族的表达谱分化相关，且主要通过上调化学感受基因的表达来实现这一寄主适应性转变。总体实验设计：以野生型果蝇触角为材料开展RNA测序（RNA-seq）实验，所用品系为塞氏果蝇（Tuson，#14021-0248-25）。