Divergent Epicuticular Wax and Transcriptomes of Edaphically Differentiated Andropogon Grasses in US Midwest Prairies

Characterizing adaptive genetic diversity across spatially varying environments provides insight into the role of environment in adaptive differentiation, local adaptation and, ultimately, formation of species. In particular, study of edaphically differentiated populations gives fundamental knowledge about the strength of natural selection in evolution. We focus on two grass varieties adapted to contrasting soils and their divergent morphologies, physiologies, and gene expression. Andropogon gerardii (big bluestem) is a widely distributed dominant grass on loamy soils of US Midwest prairies while Andropogon gerardii var. hallii (sand bluestem) is edaphically restricted to sand dunes. We probed the nature of edaphic adaptation, especially the role of epicuticular waxes (ECW) leading to differences in physiology, morphology, and gene expression between these subspecies. We found that sand bluestem: 1) contains more ECW, comprised of ~20% β-diketones in the form of microtubules (all absent in big bluestem), 2) had decreased light absorptance, and 3) lower mimimal stomatal conductance relative to big bluestem, all suggesting greater drought adaptation. Transcriptomes were assembled using miraEST, Velvet and Oases with multiple values of k (21-61), and OasesMerge and the final merged assembly was used to compare gene expression.Paired end Illumina reads (2x100bp) from four individuals of each subspecies allowed comparison of gene expression differences. Using DeSeq2, 2546 transcripts were differentially expressed between the two subspecies, suggesting fundamental genetic differences in part resulting in divergent epicuticular form and function. Taken together, data indicates adaptation of sand bluestem to sandy soils involves differences in transcriptional expression, leading to divergent cuticle chemistry, morphology, and function.