Heritable variation in root bacterial microbiomes alters natural selection on host plants

Heritable variation in microbiome compositions could theoretically evolve in response to selection acting on hosts, but empirical evidence of this is scarce. We tested for effects of allelic variation in Boechera stricta on microbiome composition and downstream consequences for host fitness. In the greenhouse, near-isogenic lines with contrasting phytochemistry alleles differentially conditioned soil microbiota in ways that changed the genotypes' relative fecundity. In the field, root microbiome differences mediated allelic effects on host fitness. Selection gradients on individual taxa and multivariate axes of root microbiome composition suggest that independent components of the bacterial community favor each allele, especially under drought. These results show that microbiota contribute to natural selection on a major-effect phytochemistry gene, empirically "closing the loop" between microbiome heritability and host trait evolution.