Independent domestication of Asian rice followed by gene flow from japonica to indica.

Results from studies on the domestication process of Asian rice Oryza sativa have been controversial because of its complicated evolutionary history. Previous studies have yielded two alternative hypotheses about the origin(s) of the two major groups of O. sativa: japonica and indica. One study proposes a single common wild ancestor, whereas the other suggests that there were multiple domestication events of different types of wild rice. Here, we provide clear evidence of the independent domestication of japonica and indica obtained via high-throughput sequencing and a large-scale comparative analysis of two wild rice accessions (W1943 and W0106) and two cultivars (a japonica cultivar called "Nipponbare" and an indica cultivar called "Guangluai-4"). The different domestication processes of the two cultivar groups appear to have led to distinct patterns of molecular evolution in protein-coding regions. The intensity of purifying selection was relaxed only in the japonica group, possibly because of a bottleneck effect. Moreover, a genome-wide comparison between Nipponbare, Guangluai-4, and another indica cultivar (93-11) suggests multiple hybridization events between japonica and indica, both before and after the divergence of the indica cultivars. We found that a large amount of genomic DNA, including domestication-related genes, was transferred from japonica to indica, which might have been important in the development of modern rice. Our study provides an overview of the dynamic process of Asian rice domestication, including independent domestication events and subsequent gene flow.