Resistance Gene Diversity and the Eco-Evolutionary Dynamics of Tropical Tree Seedlings

Neighboring plants share microbes and escape species-specific pests and pathogens by seed dispersal. These processes are a central theme in ecology but little-examined from a functional genetic perspective. Conversely, genetic determinants of pathogen resistance are well known in crops that are typically grown in monocultures, but the knowledge has been applied minimally to genetically diverse plants in nature.

In this talk Dr. James Marden will show how scientists are merging these paradigms by testing the hypothesis that, in tropical tree seedlings, sharing resistance (R) alleles with conspecific neighbors increases co-susceptibility and reduces fitness. In naturally-occurring seedlings of taxonomically diverse tree species in Panama, conspecific negative density-dependent mortality (CNDD) was well explained by non-neutral R gene protein coding diversity in the local population, and spatial mixing of variation in gene expression. In an experimental test, seedlings of one focal species had enhanced growth, markedly different defense gene expression, and distinct features of their root microbiomes depending on R gene allele-sharing combinations of seedlings and conspecific soil donors, thereby indicating that host interactions with diverse soil microbes have a spatial genetic component arising from the diversity of R gene alleles in neighboring hosts. Modeling the spatial context-dependence of fitness over time in this population showed reasonable conditions that resulted in stable intermediate frequencies of resistant and susceptible alleles. Ultimately, these results demonstrate strong effects of R gene allelic diversity on microbe-mediated growth and survival of seedlings, causing balancing selection that maintains alleles affecting eco-evolutionary dynamics.

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