Scent of a killer: How could killer yeast boost its dispersal?

Vector-borne parasites often manipulate hosts to attract uninfected vectors. For example, parasites causing malaria alter host odor to attract mosquitoes. Here we discuss the ecology and evolution of fruit-colonizing yeast in a tripartite symbiosis – the so-called “killer yeast” system. “Killer yeast” consists of Saccharomyces cerevisiae yeast hosting two double stranded RNA viruses (M satellite dsRNAs, L-A dsRNA helper virus). When both dsRNA viruses occur in a yeast cell, the yeast converts to lethal toxin‑producing “killer yeast” phenotype that kills uninfected yeasts. Yeasts on ephemeral fruits attract insect vectors to colonize new habitats. As the viruses have no extracellular stage, they depend on the same insect vectors as yeast for their dispersal. Viruses also benefit from yeast dispersal as this promotes yeast to reproduce sexually, which is how viruses can transmit to uninfected yeast strains. We tested whether insect vectors are more attracted to killer yeasts than to non‑killer yeasts. In our field experiment, we found that killer yeasts were more attractive to Drosophila than non-killer yeasts. This suggests that vectors foraging on yeast are more likely to transmit yeast with a killer phenotype, allowing the viruses to colonize those uninfected yeast strains that engage in sexual reproduction with the killer yeast. Beyond insights into the basic ecology of the killer yeast system, our results suggest that viruses could increase transmission success by manipulating the insect vectors of their host.

媒介传播寄生虫（vector-borne parasites）通常会操控宿主，以吸引未受感染的媒介生物。例如，引发疟疾的寄生虫会改变宿主气味，以此吸引蚊子。本文探讨了定殖于短命果实的酵母在三方共生关系——即所谓的"杀伤性酵母（killer yeast）"系统——中的生态学与演化机制。"杀伤性酵母"由携带两种双链RNA（double stranded RNA, dsRNA）病毒的酿酒酵母（Saccharomyces cerevisiae）构成，这两种病毒分别为M卫星双链RNA（M satellite dsRNAs）与L-A双链RNA辅助病毒（L-A dsRNA helper virus）。当两种双链RNA病毒同时存在于酵母细胞内时，该酵母会转变为可产生致命毒素的杀伤性酵母表型，从而杀死未受感染的酵母菌株。定殖于短命果实上的酵母会吸引昆虫媒介，从而实现跨栖息地的定殖。由于这类病毒不存在胞外阶段，因此它们与酵母依赖相同的昆虫媒介进行传播。病毒还能从酵母的传播中获益，因为这会促进酵母进行有性繁殖，而这正是病毒得以传播至未受感染酵母菌株的途径。本研究探究了昆虫媒介对杀伤性酵母与非杀伤性酵母的偏好性差异。在野外实验中，我们发现果蝇（Drosophila）对杀伤性酵母的偏好性显著高于非杀伤性酵母。这表明以酵母为觅食对象的媒介生物更有可能传播携带杀伤性表型的酵母，进而使病毒得以定殖那些可与杀伤性酵母进行有性生殖的未受感染酵母菌株。除了增进我们对杀伤性酵母系统基础生态学的认知之外，本研究结果还表明，病毒可通过操控其宿主的昆虫媒介来提升自身的传播成功率。