Hamilton Phineas T, Leong Jong S, Koop Ben F, Perlman Steve J
Department of Biology, University of Victoria, PO Box 1800, STN CSC, Victoria, British Columbia, Canada, V8W 2Y2.
Mol Ecol. 2014 Mar;23(6):1558-1570. doi: 10.1111/mec.12603. Epub 2013 Dec 28.
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.
遗传共生菌在昆虫中普遍存在，并且会对其宿主的适应性产生重要影响。许多遗传共生菌会保护其宿主抵御寄生虫或其他天敌；然而，大多数共生菌提供保护的方式实际上还不清楚。我们研究了一种最近发现的由共生菌介导的保护作用的防御机制，在这种情况下，细菌共生菌螺原体保护果蝇免受一种毒性很强的线虫寄生虫——霍华德氏奥罗若螨的侵害。通过对螺原体感染强度进行定量PCR和全转录组测序，我们试图区分以下几种防御模式：共生菌-寄生虫竞争、宿主免疫致敏以及螺原体产生毒性因子。我们的研究结果不支持霍华德氏螨和螺原体之间通过剥削性竞争来介导防御的模型，我们也没有找到有力证据支持在螺原体感染期间宿主进行免疫致敏。有趣的是，我们获得了螺原体编码的假定毒素的序列，包括一种新的假定核糖体失活蛋白，其转录本在接触线虫后上调。通过产生毒素进行保护可能是昆虫可遗传防御共生中广泛使用的重要机制。
