Genome exchange and biogeography of chickpea’s nitrogen-fixing bacterial symbionts.    
A. GREENSPAN (1), P. Chang (2), E. Von Wettberg (3), D. Cook (4) (1) University of California, Davis, U.S.A.; (2) University of Southern California, U.S.A.; (3) Florida International University, U.S.A.; (4) University of California, Davis, U.S.A.

Legumes are agriculturally and ecologically important for their ability to form a symbiosis with diverse, nitrogen-fixing soil bacterial—collectively called rhizobia. Different legume crops select distinct rhizobial strains, and vary in the specificity of this selection. We present a population-genomics approach to identifying novel variables and genetic factors that underlie symbiotic selection in the crop chickpea. We conducted whole-genome sequencing of rhizobial strains collected systematically across the full range of chickpea’s historical and modern cultivation, including the natural range of chickpea’s wild ancestors. This has revealed that chickpea’s wild ancestor (C. reticulatum) partners predominantly with Mesorhizobium temperatum. In regions where chickpea has been grown continuously for millennia (Ethiopia and India), we have discovered Mesorhizobium temperatum present in low abundance. However, in these regions, M. temperatum appears to be outcompeted by locally-adapted Mesorhizobium strains that have acquired the genes for symbiosis with chickpea through horizontal transfer. These diverse Mesorhizobium strains vary in nitrogen-fixation and soil-survival phenotypes. Population-level sampling and sequencing informs hypotheses about variables that determine the distribution of these bacteria and their phenotypic diversity. Finished genomes for a subset of strains allow detailed analysis of the global bacterial population dynamics and mechanisms of genetic exchange.   

Abstract Number: P2-26
Session Type: Poster