Retrospective Genomic Analysis of Sorghum Adaptation to Temperate-Zone Grain Production

Sorghum is a tropical C₄ cereal that was recently adapted to mechanized grain harvest at temperate latitudes using mutations for photoperiod-insensitivity and dwarfism. These traits were previously introgressed from an elite donor into hundreds of diverse sorghum landraces to yield the Sorghum Conversion (SC) lines. These lines, which have been used extensively in grain sorghum breeding, may be useful in producing high biomass yielding, photo-period sensitive hybrids for biofuels through complementary dominance at multiple maturity loci. In order to identify plant height and maturity loci, we used a modified Genotyping-by-sequencing approach to obtain dense marker coverage from 1160 SC lines and their nearly-isogenic exotic progenitors, and to map donor introgressions in each SC line. Genome-wide mapping of introgression frequencies reveals three genomic regions necessary for temperate adaptation across all lines, with multiple targets in each region. Association mapping in the exotic progenitor lines alone, the SC lines alone, and both line types combined, identifies several candidate loci for variation in plant height and maturity. Lastly, we identified numerous additional loci specific to individual subpopulations of sorghum, as determined by principle components analysis (PCA). Our results highlight the utility of retrospective genotyping in breeding populations for studying the genomic basis of complex agronomic traits. The loci identified here could be exploited to improve biomass production in sorghum and related grasses.