Halloway Abdel H, Heath Katy D, McNickle Gordon G
Department of Plant Biology, University of Illinois at Urbana-Champaign, 505 S. Goodwin Avenue (M/C 116), Urbana, IL 61801, USA.
Department of Botany and Plant Pathology, Purdue University, 915 W. State Street, West Lafayette, IN 47907, USA.
AoB Plants. 2022 Mar 10;14(2):plac010. doi: 10.1093/aobpla/plac010. eCollection 2022 Apr.
Due to their non-motile nature, plants rely heavily on mutualistic interactions to obtain resources and carry out services. One key mutualism is the plant-microbial mutualism in which a plant trades away carbon to a microbial partner for nutrients like nitrogen and phosphorous. Plants show much variation in the use of this partnership from the individual level to entire lineages depending upon ecological, evolutionary and environmental context. We sought to determine how this context dependency could result in the promotion, exclusion or coexistence of the microbial mutualism by asking if and when the partnership provided a competitive advantage to the plant. To that end, we created a 2 × 2 evolutionary game in which plants could either be a mutualist and pair with a microbe or be a non-mutualist and forgo the partnership. Our model includes both frequency dependence and density dependence, which gives us the eco-evolutionary dynamics of mutualism evolution. As in all models, mutualism only evolved if it could offer a competitive advantage and its net benefit was positive. However, surprisingly the model reveals the possibility of coexistence between mutualist and non-mutualist genotypes due to competition between mutualists over the microbially obtained nutrient. Specifically, frequency dependence of host strategies can make the microbial symbiont less beneficial if the microbially derived resources are shared, a phenomenon that increasingly reduces the frequency of mutualism as the density of competitors increases. In essence, ecological competition can act as a hindrance to mutualism evolution. We go on to discuss basic experiments that can be done to test and falsify our hypotheses.
由于植物不具备移动能力，它们在很大程度上依赖互利共生关系来获取资源并完成各项功能。一种关键的互利共生关系是植物与微生物的共生，在这种关系中，植物将碳提供给微生物伙伴，以换取氮和磷等养分。从个体层面到整个谱系，植物在利用这种共生关系时表现出很大的差异，这取决于生态、进化和环境背景。我们试图通过询问这种共生关系是否以及何时能为植物提供竞争优势，来确定这种背景依赖性如何导致微生物共生关系的促进、排斥或共存。为此，我们创建了一个2×2的进化博弈模型，其中植物既可以成为共生者与微生物配对，也可以成为非共生者而放弃这种共生关系。我们的模型同时包含频率依赖性和密度依赖性，这使我们能够了解共生关系进化的生态进化动态。与所有模型一样，只有当共生关系能够提供竞争优势且其净收益为正时，共生关系才会进化。然而，令人惊讶的是，该模型揭示了共生基因型和非共生基因型之间共存的可能性，这是由于共生者之间对微生物获取的养分存在竞争。具体而言，如果微生物衍生的资源是共享的，宿主策略的频率依赖性会使微生物共生体的益处减少，随着竞争者密度的增加，这种现象会越来越多地降低共生关系的频率。本质上，生态竞争可能成为共生关系进化的障碍。我们接着讨论了可以进行的基础实验，以检验和证伪我们的假设。
