Transcriptional Differences of Mesic and Xeric Ecotypes of an Ecologically-Dominant Prairie Grass Andropogon gerardii to Abiotic Stress

Andropogon gerardii (big bluestem) is a dominant forage and potential biofuel grass with wide distribution across a sharp precipitation gradient of the Great Plains (400-1200mm yr-1). Here, we utilize the first de novo reference transcriptome for A. gerardii to compare drought transcript expression differences between xeric and mesic ecotypes. We expected ecotypic variation in drought tolerance and, potentially, local adaptation of the xeric ecotype to drier climates given its better survival and plant performance in reciprocal common gardens with decreased rainfall. Prior evidence supported genetic differentiation and hierarchical structuring of ecotypes, with 8 FST-outlier loci under potential diversifying selection. Plants were sampled (n=4 individuals/ecotype) from seeded common gardens at the margins of the species’ range in western Kansas during severe drought in August 2011 and physiological data were taken on single plants.  454 sequencing of individual plants/per ecotype and de novo transcriptome assembly was complemented with Illumina RNA-seq (n=3 individuals/ecotype). The Tuxedo Suite was used to discover differentially expressed transcripts, revealing 245 significant differences (1.8% of total transcripts) between xeric (167) and mesic (78) ecotypes. To gain a better reference transcriptome, 454 reads were assembled using MIRA, and Illumina RNA-seq reads assembled in Oases/Velvet with multiple k-mers (23-61). The final merged assemblies produced 300K contigs (N50=2.6 kb).  Contigs <200bp were filtered and reads counted across xeric and mesic ecotypes using EdgeR package. Transcriptional variation studies in putative ecotypes of A. gerardii will provide new insights into how this dominant grass may withstand future climate change.