Detection of novel loci involved in non-seed-shattering behaviour of an indica rice cultivar, Oryza sativa IR36.

Asian rice (Oryza sativa) was domesticated from O. rufipogon, and reduced seed-shattering behaviour was selected to increase yields. Two seed-shattering loci, qSH3 and sh4, are involved in reducing seed shattering in both japonica and indica rice cultivars, while qSH1 and qCSS3 are likely specific to japonica cultivars. In indica cultivars, qSH3 and sh4 fail to explain the degree of seed shattering, as an introgression line (IL) of O. rufipogon W630 carrying domesticated alleles at qSH3 and sh4 still showed seed shattering. Here we analysed differences in seed-shattering degree between the IL and the indica cultivar IR36. The values for grain detachment in the segregating population between the IL and IR36 were continuous. Based on QTL-seq analysis using the BCF population between the IL and IR36, we detected two novel loci, qCSS2 and qCSS7 (QTLs for the Control of Seed Shattering in rice on chromosomes 2 and 7), which contributed to the reduced seed shattering in IR36. We further investigated the genetic interaction of qCSS2 and qCSS7 under the presence of qSH3 and sh4 mutations in O. rufipogon W630 and found that IL carrying IR36 chromosomal segments covering all four loci are required to explain seed-shattering degree in IR36. Since qCSS2 and qCSS7 were not detected in previous studies on seed shattering in japonica, their control may be specific to indica cultivars. Therefore, they are important to understanding the history of rice domestication as well as to adjusting the seed-shattering degree of indica cultivars to maximise their yield.