Genetic Differentiation, Transcriptome Variation, and Local Adaptation of Dominant Prairie Grass Androspogon gerardii along the Climate Gradient of the US Midwest: Implications for Climate Change and Restoration

Big bluestem (Andropogon gerardii) is an ecologically dominant forage grass in Central US and widely used in millions of acres of grassland restoration. With wide distribution across a precipitation gradient (400-1200mm yr-1), we expect ecotypic variation in drought tolerance and local adaptation. We investigated phenotypic variation, genetic diversity and transcriptional variation using reciprocal gardens across the Midwest gradient (wet to dry) in Illinois, Manhattan, Hays and Colby KS to predict response to future climate change. Three ecotypes from populations sourced from Central KS, Eastern KS, and Illinois were reciprocally planted in polyculture (with other species). A phenotypic cline in drought tolerance of ecotypes was observed indicating local adaptation of xeric and mesic ecotypes. Because genetic diversity may be critical to adjust to climate change, we assessed genetic diversity and differentiation of the source populations. Seven ecotype-specific loci under diversifying selection were identified and marker frequency related to seasonal precipitation and temperature. Differentiation among populations was due predominantly to different ecological environments of the source populations (IBE) rather than isolation by distance (IBD). We used RNAseq to compare transcriptional variation of Central KS and Illinois ecotypes growing under abiotic stress in xeric Colby KS. The Illinois ecotype mounted a weak transcriptional response, consisting of abiotic stress genes as an inducible, stress strategy. In contrast, the CKS ecotype shows a strong and varied transcriptional response that matches its increased competitiveness under drought. We provide insights into how this dominant grass may withstand climate change and may guide prairie restoration.