Data from: Transcriptional responses in a Drosophila defensive symbiosis

Inherited symbionts are ubiquitous in insects and can have important consequences for the fitness of their hosts. Many inherited symbionts defend their hosts against parasites or other natural enemies; however, the means by which most symbionts confer protection is virtually unknown. We examine the mechanisms of defence in a recently discovered case of symbiont-mediated protection, where the bacterial symbiont Spiroplasma defends the fly Drosophila neotestacea from a virulent nematode parasite, Howardula aoronymphium. Using quantitative PCR of Spiroplasma infection intensities and whole transcriptome sequencing, we attempt to distinguish between the following modes of defence: symbiont–parasite competition, host immune priming and the production of toxic factors by Spiroplasma. Our findings do not support a model of exploitative competition between Howardula and Spiroplasma to mediate defence, nor do we find strong support for host immune priming during Spiroplasma infection. Interestingly, we recovered sequence for putative toxins encoded by Spiroplasma, including a novel putative ribosome-inactivating protein, transcripts of which are up-regulated in response to nematode exposure. Protection via the production of toxins may be a widely used and important mechanism in heritable defensive symbioses in insects.遗传性共生菌在昆虫中广泛存在，且可对宿主的适合度产生重要影响。诸多遗传性共生菌可帮助宿主抵御寄生虫或其他天敌，但绝大多数共生菌实现防御的具体机制仍几乎未被阐明。本研究针对一例新近发现的共生菌介导防御案例展开防御机制解析：其中，细菌共生菌螺原体（Spiroplasma）可保护果蝇属（Drosophila）新测果蝇（Drosophila neotestacea）免受强毒力线虫寄生虫霍华德线虫（Howardula aoronymphium）的侵染。本研究通过定量检测螺原体侵染强度的实时荧光定量PCR（quantitative PCR, qPCR）技术，以及全转录组测序（whole transcriptome sequencing）手段，尝试区分以下三种防御模式：共生菌-寄生虫竞争、宿主免疫致敏（host immune priming），以及螺原体产生毒性因子。研究结果并不支持以霍华德线虫与螺原体之间的资源利用型竞争介导防御的模型，同时也未发现螺原体侵染期间宿主免疫致敏得到显著支持的证据。有趣的是，本研究成功获取了螺原体编码的推定毒素序列，其中包含一种新型推定核糖体失活蛋白（ribosome-inactivating protein）；该蛋白的转录本在暴露于线虫后呈现上调表达特征。通过产生毒素实现防御，可能是昆虫遗传性防御共生体系中一种广泛应用且至关重要的机制。