Genetic Differentiation, Transcriptome Variation, and Local Adaptation of an Ecologically Dominant Prairie and Bioenergy Grass Andropogon gerardii (big bluestem) Occurring Along the Precipitation Gradient in Midwest U.S. Grasslands

The U.S. central grassland is one of the most productive rangelands accounting for 6 billon dollars annually in productivity in KS alone. Within the Central grasslands, big bluestem (Andropogon gerardii) is a dominant forage and biofuel grass widely used in grassland restoration. With wide distribution across a precipitation gradient (400-1200mm/yr KS to IL), we expect ecotypic variation in drought tolerance and local adaptation. We investigated phenotypic variation, genetic diversity and transcriptional variation using reciprocal gardens in Carbondale, Illinois, Manhattan and Hays KS and Colby, KS. Three ecotypes with seeds collected from populations in Central KS, Eastern KS, and Illinois were reciprocally planted in polyculture communities (with competitors). We measured phenotypic variation across ecotypes and sites. We assessed genetic diversity and population differentiation of 12 founding source populations. A phenotypic cline in drought tolerance was observed. Establishment and cover showed significant ecotype, site, and interaction effects. The Central KS ecotype had greater cover in Hays and Colby relative to other ecotypes, indicating local adaptation. Eight loci under diversifying selection were identified and related to climate. We compared transcriptional variation of Central KS and Illinois ecotypes growing under abiotic stress in Colby KS. The Illinois ecotype mounted a weaker transcriptional response, primarily consisting of genes involved in abiotic stress as an inducible, short-term stress strategy. In contrast, the Central KS ecotype shows a more varied transcriptional response that matched its increased competitiveness under drought. We provide insights into how this dominant grass may withstand environmental change and may guide prairie restoration.