Symposium: “Shared genes but not shared genetic variation: legume colonization by two belowground symbionts”
Julia N. Ossler and Katy D. Heath (Mar 2018)
The DOI is https://dx.doi.org/10.1086/695829
Mutualisms are widely established interactions in nature. More often than not, hosts rely simultaneously on multiple symbionts—for example, humans and the microorganisms that make up the human microbiome, or economically important crops (such as soy beans) and their root symbionts. Hosts that rely on multiple symbionts often regulate them through shared genetic pathways. We can think of these genetic pathways as telephone party lines that help hosts regulate the density and exchange of resource interactions with multiple symbionts. However, it is still unclear if these shared pathways constrain the interactions between hosts and their symbionts, and what impacts these pathways could have on the evolution of populations.
To address this question, Julia Ossler and Katy Heath examine the complex multiplayer mutualism among Chamaecrista fasciculata (partridge pea) and its two belowground symbionts (phosphorous acquiring arbuscular mycorrhizal fungi (AMF) and nitrogen fixing rhizobia). In a greenhouse experiment, they examine the response of 75 families of partridge pea to phosphorus fertilizer, specifically examining if changes in symbionts’ colonization on host roots are connected among families due to shared genetic pathways (“telephone party lines”).
Understanding if shared genetic pathways generate limited multiplayer mutualism can help us understand how symbioses (co-)evolve in the context of complex natural communities. Despite the known existence of shared genetic pathways in the mutualism between partridge pea, AMF, and nitrogen fixing rhizobia, the researchers find that hosts retain the ability to interact and evolve independently with each symbiont, speaking to the potentially vast amount of genetic variation that may lie outside of shared genetic pathways, and the important role this plays in natural selection of mutualisms.
Mutualisms between hosts and multiple symbionts can generate diffuse coevolution if genetic covariance exists between host traits governing multiple interactions. Rhizobia and arbuscular mycorrhizal fungi (AMF) both interact with legume hosts, providing complementary nutrients (nitrogen and phosphorous). Molecular approaches have revealed extensive pleiotropy in the plant genetic pathways required for colonization of both symbionts; however, a quantitative genetic approach is required to understand whether this pleiotropy shapes evolution in natural populations. In a greenhouse experiment with 75 families of Chamaecrista fasciculata grown in two phosphorous environments (fertilized and unfertilized), positive covariance between nodule number and plant aboveground biomass within and across environments indicates selection for increased colonization by rhizobia. Genetic variation for host restriction of AMF colonization in response to P suggests that this aspect of context-dependency can evolve in host populations, and that selection in this mutualism varies with P. Despite the existence of gene-level pleiotropy during rhizobium and AMF infection, we find no evidence for genetic covariance in symbiont colonization or its response to phosphorous – suggesting that genetic variation at other, non-pleiotropic loci govern variation in colonization and thus that these traits likely evolve independently in plant populations.