Department of Biology, Emory University, 1510 Clifton Rd., Atlanta, GA 30322, USA; Department of Biology, Davidson College, 209 Ridge Rd., Davidson, NC 28035, USA.
Department of Biology, Emory University, 1510 Clifton Rd., Atlanta, GA 30322, USA.
Curr Biol. 2023 Jul 10;33(13):2830-2838.e4. doi: 10.1016/j.cub.2023.05.062. Epub 2023 Jun 28.
Understanding how horizontally transmitted mutualisms are maintained is a major focus of symbiosis research. Unlike vertical transmission, hosts that rely on horizontal transmission produce symbiont-free offspring that must find and acquire their beneficial microbes from the environment. This transmission strategy is inherently risky since hosts may not obtain the right symbiont every generation. Despite these potential costs, horizontal transmission underlies stable mutualisms involving a large diversity of both plants and animals. One largely unexplored way horizontal transmission is maintained is for hosts to evolve sophisticated mechanisms to consistently find and acquire specific symbionts from the environment. Here, we examine this possibility in the squash bug Anasa tristis, an insect pest that requires bacterial symbionts in the genus Caballeronia for survival and development. We conduct a series of behavioral and transmission experiments that track strain-level transmission in vivo among individuals in real-time. We demonstrate that nymphs can accurately find feces from adult bugs in both the presence and absence of those adults. Once nymphs locate the feces, they deploy feeding behavior that results in nearly perfect symbiont acquisition success. We further demonstrate that nymphs can locate and feed on isolated, cultured symbionts in the absence of feces. Finally, we show this acquisition behavior is highly host specific. Taken together, our data describe not only the evolution of a reliable horizontal transmission strategy, but also a potential mechanism that drives patterns of species-specific microbial communities among closely related, sympatric host species.
理解水平传播的共生关系是如何维持的，是共生关系研究的主要焦点。与垂直传播不同，依赖水平传播的宿主产生无共生体的后代，这些后代必须从环境中寻找和获得有益的微生物。这种传播策略本身存在风险，因为宿主可能无法每一代都获得正确的共生体。尽管存在这些潜在成本，但水平传播是涉及大量动植物的稳定共生关系的基础。一种在很大程度上尚未被探索的维持水平传播的方式是，宿主进化出复杂的机制，从环境中持续找到并获得特定的共生体。在这里，我们研究了南瓜虫 Anasa tristis 中这种可能性，这是一种昆虫害虫，需要细菌共生体属 Caballeronia 才能生存和发育。我们进行了一系列行为和传播实验，实时跟踪个体体内的菌株水平传播。我们证明，若有若无成虫，若虫都可以准确地从成虫粪便中找到粪便。一旦若虫找到粪便，它们就会采取进食行为，几乎可以确保共生体的完全获取。我们进一步证明，若虫可以在没有粪便的情况下定位和喂养分离的、培养的共生体。最后，我们表明这种获取行为具有高度的宿主特异性。总之，我们的数据不仅描述了一种可靠的水平传播策略的进化，还描述了一种潜在的机制，这种机制驱动了密切相关、同域的宿主物种之间特定物种的微生物群落模式。
