Adaptive phenotypic divergence in an annual grass differs across biotic contexts

Abstract

Climate is a powerful force shaping adaptation within species, often creating dramatic phenotypic clines. Yet adaptation to climate does not occur in a vacuum: species interactions filter the fitness consequences of both climatic and phenotypic variation. In other words, the translation of genotype to phenotype may be altered by biotic context, influencing the variation upon which climatic selection can act. We investigate the role of such interactions in changing the phenotypes on which selection acts using ten populations of an annual grass species (teosinte: Zea mays ssp. mexicana) sourced from along an elevational gradient, along with rhizosphere biota sourced from three of those populations. We grow teosinte families in a half-sibling design in separate biota treatments to first test whether the divergence we see among traits in teosinte populations exceeds what we would expect from genetic drift and then whether the source of rhizosphere biota affects the expression of divergent traits. We also assay the influence of these three rhizosphere biotas on contemporary additive genetic variation in teosinte traits across populations. We find that expression of most measured traits in teosinte is altered by rhizosphere biota, as well as the degree of variance and covariance among traits involved in root mass and flowering time. As a number of these traits are also found to underlie adaptive divergence across habitats, our data suggest that biota influence the expression of traits underlying local adaptation. Together, our results suggest that changes in trait expression and covariance elicited by interactor communities in root mass and flowering time may have played a historical role in local adaption of teosinte to environments, and that they would play a contemporary role in responses to changing selection pressures.