Genetic Analysis And Gene Mapping Of A Rice Round Kernel 4 (rk4) Mutant

Rice is a leading cereal crop and staple food for over half of the world's population. Increasing grain yield and improving quality of rice have been the hotspot objectives of breeders. Grain shape is one of the most important agronomic traits of rice that directly determine the rice yield and quality. Therefore, identification and cloning of the grain shape-related genes plays an important role in increasing grain yield and improving the rice quality.Round kernel 4 mutant (rk4) was obtained from a japonica variety Zhonghua 11 by radiation of 60Co-γ. The significant features of rk4 mutant are short and round seeds, reduced plant height and long heading date. After investigating the traits of grain, we found that the round kernel mutant is shorter in grain length, longer in grain width and grain thickness and slightly reduced in grain weight than the wild-type, respectively. Scanning electron microscope reveals that the round kernel mutant is bigger than the wild-type in width, so the grain width and thickness were increased, and the grain length was reduced. We also found that the AC content in rk4 is lower than that in the wild-type which implying that this mutant is closely related with rice quality.The rk4 mutant, as maternal parent, was crossed to Xianhui 8006 (an indica variety) and made to obtain F1 and F2 populations. The observation of the progenies showed that all the F1 plants were normal grain shape, but the grain shape in the all F2 populations was segregated into round grain and normal grain types. Three hundreds and sixty-six plants with normal grain shape and one hundred and nineteen plants with round grain shape were found in the F2 population, and the segregation was fitted for the ratio of 3:1 (Х2=0.056<Х20.05,1=3.84). This result suggests that the mutant rk4 was controlled by a single recessive gene. The F2 population was used for molecular mapping of this recessive gene. Using SSR and STS markers, the rk4 gene was preliminarily located on the rice chromosome 5 between markers, S5-18 and RM17962, with the genetic distance of 5.04cM and 0.84cM, respectively. In order to fine map rk4, 77 new STS markers were developed between makers S5-18 and RM17962. There are 29 new STS markers displaying polymorphism between the two parents. Linkage analysis using these STS markers and 521 recessive individuals from F3 population, rk4 was finally mapped between LSTS5-60 and LSTS5-77 markers, with genetic distance of 0.096cM and 0.57cM, respectively. A physical map covering the rk4 locus was constructed. The target gene and the two makers can be considered as located on the BAC clone AC087425, and the locus of rk4 was delimited to a 46 kb region on the clone. This result is very useful for cloning of the rk4 gene.