Characterization and fine mapping of Double-grain (Dgs) mutant in sorghum [Sorghum bicolor (L.) Moench]

The sorghum inflorescence is consisted of sessile (SS) and pedicellate spikelets (PS). Commonly, only SS could produce seeds and each spikelet produces one single seed. Here, we identified a sorghum mutant, named Double-grain (Dgs), which can produce twin seeds in each pair of glumes. We characterized the developmental process of inflorescence in Dgs and Jinliang 5 (Jin5, a single-seeded variety) using scanning electron microscope (SEM). The results showed that at the stamen and pistil differentiation stage, Dgs could develop two sets of stamens and carpels in one sessile floret, which resulted in twin-seeded phenotype in Dgs. Two F2 mapping populations derived from the cross between Jin5 and Dgs, and BTx622B and Dgs, were constructed, respectively. The genetic analysis showed that Dgs trait was controlled by a single dominant gene. Through bulk segregation analysis with whole-genome sequencing (BSA-seq) and linkage analysis, Dgs locus was delimited into a region of around 210-kb on chromosome 6, between the markers SSR24 and SSR47, which contained 32 putative genes. Further analysis indicated that Sobic.006G249000 or Sobic.006G249100 may be responsible for the twin-seeded phenotype. This result will be useful for map-based cloning of the Dgs gene and for marker-assisted breeding for increased grain number per panicle in sorghum.