Thu, Aug 05, 2021:On Demand
Background/Question/Methods
A dilemma in mutualism is that uncooperative or exploitative symbionts could outcompete cooperative symbionts colonizing a host, leading to mutualism breakdown. Hosts often compartmentalize symbionts in specialized physical modules such as fruits, organs, or root nodules, such that hosts can assess symbiont quality and confer resources at the level of the compartment. Symbiont cooperation can be maintained if hosts preferentially confer mutualism resources to more beneficial symbiont compartments via partner choice or sanctions, though this host discrimination is imperfect, susceptible to dishonest symbiont signaling, and compartments are commonly infected with multiple symbiont genotypes, which could reward less beneficial symbionts. Little is known about the rules of engagement in symbiosis: when hosts are faced with multiple symbionts that differ in quality, does host discrimination depend only upon the genotype or quality of an individual symbiont (ie absolute discrimination) or is host discrimination modified by the host’s outside options, that is the presence of other symbionts of different genotypes or phenotypes (ie relative discrimination)? To test the importance of absolute vs relative host discrimination mechanisms, we quantify the impact of a symbiont’s own genotype (direct genetic effects (DGEs)) and the impact of other symbiont genotypes that co-infect a host (indirect genetic effects (IGEs)) on the outcome of host discrimination among and within host compartments. We inoculated the leguminous plant host, Acmispon wrangelianus, with all 28 factorial pairs of eight nitrogen-fixing Mesorhizobium bacterial symbionts that range from unbeneficial to highly beneficial. Mesorhizobium strains were transformed to express fluorescent markers, allowing us to measure symbiont fitness for co-infecting strains on a plant simultaneously.
Results/Conclusions We reveal that host discrimination has both absolute and relative components such that both a focal strain’s genotype (DGEs) and the genotype of other co-infecting strains (IGEs) modulate the amount of mutualism resources a host confers to a symbiont. When we breakdown variation in the relative component, we find main indirect genetic effects (main IGEs) and genotype-by-genotype IGEs (GxG IGEs) are important for explaining variation in symbiont and host fitness. Furthermore, host discrimination preferentially allocates resources to more beneficial strains at both the level of individual modules, as well as among mixed populations of symbiont genotypes within modules. These findings quantify for the first time the importance of both absolute and relative components of host discrimination via partner choice and sanctions, among and within host modules, and demonstrate that both forms of discrimination contribute to the maintenance of cooperation in mutualism.
Results/Conclusions We reveal that host discrimination has both absolute and relative components such that both a focal strain’s genotype (DGEs) and the genotype of other co-infecting strains (IGEs) modulate the amount of mutualism resources a host confers to a symbiont. When we breakdown variation in the relative component, we find main indirect genetic effects (main IGEs) and genotype-by-genotype IGEs (GxG IGEs) are important for explaining variation in symbiont and host fitness. Furthermore, host discrimination preferentially allocates resources to more beneficial strains at both the level of individual modules, as well as among mixed populations of symbiont genotypes within modules. These findings quantify for the first time the importance of both absolute and relative components of host discrimination via partner choice and sanctions, among and within host modules, and demonstrate that both forms of discrimination contribute to the maintenance of cooperation in mutualism.