Genetic Diversity and Divergence between Southern Japonica and Northern Japonica Rice Varieties in China

Given the notable ecological and breeding disparities between southern and northern rice regions, delving into the genetic diversity and divergence between southern and northern japonica rice contributes to enhancing the genetic pool for japonica rice breeding. In this study, we analyzed 90 southern and 51 northern japonica rice varieties, focusing on nucleotide diversity (Pi), agronomic trait variations, population structure, genetic divergence, and a neutral test. For genetic diversity, the results demonstrated higher Pi in northern japonica rice varieties (NJRVs) on Chr2, Chr5, Chr6, Chr8, and Chr10, whereas in southern japonica rice varieties (SJRVs) on Chr7 and Chr9. In addition, SJRVs exhibited higher grain width and thickness, whereas NJRVs featured a higher grain aspect ratio, filled grain number, and grain number per panicle. Regarding genetic divergence, geographic differentiation existed between NJRVs and SJRVs, with Chr5 exhibiting numerous higher genetic differentiation windows, including cloned grain shape-controlling genes <i>RGA1</i> and <i>SFD5</i>, stemming from intensified selection pressure on SJRVs. In summary, SJRVs and NJRVs exhibited diversity differences and genetic differentiation. Hence, it was suggested to conduct genetic introgression between NJRVs and SJRVs to broaden the genetic basis of the local japonica rice germplasm. By exploiting their heterotic advantage, new japonica rice cultivars with superior comprehensive traits could be developed.