Department of Evolution Ecology and Organismal Biology, University of California, Riverside, California, 92521, USA.
Current Institution: Schmid College of Science and Technology, Chapman University, Orange, California, USA.
Evolution. 2021 May;75(5):1189-1200. doi: 10.1111/evo.14173. Epub 2021 Feb 8.
Microbial mutualists provide substantial benefits to hosts that feed back to enhance the fitness of the associated microbes. In many systems, beneficial microbes colonize symbiotic organs, specialized host structures that house symbionts and mediate resources exchanged between parties. Mutualisms are characterized by net benefits exchanged among members of different species, however, inequalities in the magnitude of these exchanges could result in evolutionary conflict, destabilizing the mutualism. We investigated joint fitness effects of root nodule formation, the symbiotic organ of legumes that house nitrogen-fixing rhizobia in planta. We quantified host and symbiont fitness parameters dependent on the number of nodules formed using near-isogenic Lotus japonicus and Mesorhizobium loti mutants, respectively. Empirically estimated fitness functions suggest that legume and rhizobia fitness is aligned as the number of nodules formed increases from zero until the host optimum is reached, a point where aligned fitness interests shift to diverging fitness interests between host and symbiont. However, fitness conflict was only inferred when analyzing wild-type hosts along with their mutants dysregulated for control over nodule formation. These data demonstrate that to avoid conflict, hosts must tightly regulate investment into symbiotic organs maximizing their benefit to cost ratio of associating with microbes.
微生物共生体为宿主提供了实质性的好处，这些好处反馈回来增强了相关微生物的适应性。在许多系统中，有益微生物会定植于共生器官中，这些共生器官是专门的宿主结构，容纳共生体并介导双方之间资源的交换。共生关系的特征是不同物种成员之间的净利益交换，然而，这些交换的不平等可能导致进化冲突，使共生关系不稳定。我们研究了根瘤形成的联合适应效应，根瘤是豆科植物的共生器官，在植物体内容纳固氮根瘤菌。我们使用近等基因的日本百合和根瘤菌的突变体，分别量化了依赖于形成的根瘤数量的宿主和共生体适应性参数。经验估计的适应度函数表明，随着根瘤数量从零增加到宿主最佳状态，豆科植物和根瘤菌的适应性是一致的，在这个最佳状态下，宿主和共生体之间的适应性利益从一致转变为分歧。然而，只有当分析野生型宿主及其对根瘤形成调控失调的突变体时，才推断出适应性冲突。这些数据表明，为了避免冲突，宿主必须严格调控共生器官的投资，使它们与微生物关联的收益与成本比最大化。
