Multivariate divergence in wild microbes: no evidence for evolution along a genetic line of least resistance

Abstract

Fungal endophytes are ubiquitous residents within plant tissues. While taxonomic characterization of endophyte communities is common, we know little about how even closely related individuals differ functionally because fungal taxonomy is often a poor predictor of variation in ecological traits, including those related to the metabolism of plant-derived substrates. Here, we tested whether two fungal endophyte lineages from the same family (Mollisiaceae) sampled from the same host species (Vaccinium angustifolium) differ phenotypically by comparing metabolic trait means and correlations related to growth across a panel of plant phenolics, carbohydrates, and tissue extracts. We found that lineages harbor significant, selectable metabolic variation. While one lineage grew faster across nearly all substrates, lineages differed in their mean response to specific substrates. For example, lineages responded differently to phenolics and carbohydrates, most dramatically for caffeic acid which abolished differences between lineages. However, most correlations between substrates within lineages were positive, suggesting that metabolic evolution is generally unconstrained by trade-offs among the examined traits. Our findings illustrate that characterizing multivariate phenotypes- measuring multiple metabolic traits and the correlations between them- provides key insight into the process of endophyte evolution and reveals that both fungal-and plant-specific determinants shape opportunities for ecological differentiation within plant microbiomes.