Department of Biology, Washington University , St. Louis, MO 63130, USA.
Department of Organismic and Evolutionary Biology, Harvard University , Cambridge, MA 02138, USA.
Proc Biol Sci. 2024 Aug;291(2027):20241111. doi: 10.1098/rspb.2024.1111. Epub 2024 Jul 17.
Symbiotic interactions may change depending on third parties like predators or prey. Third-party interactions with prey bacteria are central to the symbiosis between social amoeba hosts and bacterial symbionts. Symbiosis with inedible allows host to carry prey bacteria through the dispersal stage where hosts aggregate and develop into fruiting bodies that disperse spores. Carrying prey bacteria benefits hosts when prey are scarce but harms hosts when prey bacteria are plentiful, possibly because hosts leave some prey bacteria behind while carrying. Thus, understanding benefits and costs in this symbiosis requires measuring how many prey bacteria are eaten, carried and left behind by infected hosts. We found that infection makes hosts leave behind both symbionts and prey bacteria. However, the number of prey bacteria left uneaten was too small to explain why infected hosts produced fewer spores than uninfected hosts. Turning to carried bacteria, we found that hosts carry prey bacteria more often after developing in prey-poor environments than in prey-rich ones. This suggests that carriage is actively modified to ensure hosts have prey in the harshest conditions. Our results show that multi-faceted interactions with third parties shape the evolution of symbioses in complex ways.
共生相互作用可能会因捕食者或猎物等第三方而发生变化。与猎物细菌的第三方相互作用是社会变形虫宿主与细菌共生体之间共生关系的核心。与不可食用的共生关系使宿主能够在分散阶段携带猎物细菌，在这个阶段，宿主聚集并发育成果实体，从而分散孢子。当猎物稀少时，携带猎物细菌对宿主有益，但当猎物细菌丰富时，对宿主有害，可能是因为宿主在携带猎物细菌时会留下一些猎物细菌。因此，要了解这种共生关系的收益和成本，就需要衡量受感染的宿主吃、携带和留下了多少猎物细菌。我们发现，感染会使宿主既留下共生体又留下猎物细菌。然而，未被吃掉的猎物细菌数量太少，无法解释为什么感染宿主产生的孢子比未感染宿主少。我们转而研究携带的细菌，发现宿主在猎物稀少的环境中发育后，携带猎物细菌的频率比在猎物丰富的环境中更高。这表明，携带细菌的行为是主动调整的，以确保宿主在最恶劣的条件下也能获得猎物。我们的研究结果表明，与第三方的多方面相互作用以复杂的方式塑造了共生关系的进化。
