Genetic analysis and fine mapping of the RK4 gene for round kernel in rice (Oryza sativa L.)

Grain shape of rice is an important trait for both yield and quality. A rice rk4 (round kernel) mutant was obtained from the japonica variety Zhonghua 11 by radiation of 60Co-γ. The grain width of the mutant was increased and the length was decreased. Simultaneously, the 1000-grain weight was slightly reduced, therefore the grain shape of the mutant tended to be small and round. In this study, genetic analysis and gene mapping of the mutant gene were carried out using the F2 and F3 populations derived from the mutant and the indica variety Xianhui 8006. The results suggested that the round kernel was controlled by a single recessive allele (rk4) which was located on chromosome 5. The RK4 gene was further mapped between the molecular markers LSTS5-77 and LSTS5-60 with 0.57 and 0.096 cM, respectively. A BAC clone was found to span the RK4 locus, and the RK4 gene was placed in a 46 kb region that contains six annotated genes according to the available sequence annotation database. This result will help us to isolate the RK4 gene and reveal the molecular mechanism of the round kernel in rice.