Studies on agronomic genes in rice.

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The objective of this study is to examine why genetic diversity, especially in agronomic traits, is maintained in crop species. To understand ongoing micro-evolutionary processes, genetic alterations in response to temperature (cold resistance), photoperiod (flowering time) and biotic environments (competition with weeds) were investigated in wild and cultivated rice. The adaptive mechanism in these traits was not well explained by a few major genes, suggesting that epistasis, genotype x environment (GE) interaction and linked genes were involved in addition to genes with a small additive effect. The results showed that the presence of epistasis and GE interaction play a significant role for the maintenance of genetic diversity in rice. Genetic diversity is also affected both by current patterns of micro-evolutionary forces, such as gene flow and selection, and by the phylogenetic history. Genealogies of agronomic genes provided insight into their history. Unexpectedly, "Green Revolution" gene (sd1) preexisted in the wild ancestor, showing that farmers have selected it to obtain a high yield in response to altered practices in agriculture. In contrast, in the case of C, A and wx genes, it was indicated that variants have generated from landraces through natural or artificial selection. These results lead us to consider that each of the genes has a unique history, which might reflect on different patterns of natural and/or artificial selection. It seems that farmers have mined the sd1 allele from the wild population and have maintained it in different landraces, while farmers have selected newly arisen mutations from landraces in the case of C, wx and A genes consciously or unconsciously. Therefore, landraces are also expected to preserve agronomically valuable genes, even if a reduction in nucleotide diversity was caused by the population bottleneck due to domestication.