Assessment of artificial selection in maize (Zea mays L.) and Asian rice (Oryza sativa L.) using QTL data

Maize (Zea mays L.) and Asian rice (Oryza sativa L.), two most important cereals for human nutrition, have undergone strong artificial selection during a long period of time. Currently, a number of genes with stronger signals of selection have been identified through combining genomic and population genetic approach, but research on artificial selection of maize and Asian rice is scarcely done from the perspective of phenotypic difference of a number of agronomic traits. In this study, such an investigation was carried out on the basis of 179 published studies about phenotypic quantitative trait locus (QTL) mapping of Zea and Oryza species via QTL sign test. At the overall level, the proportions of antagonistic QTLs of Zea and Oryza species were 0.2446 and 0.2382 respectively, deviating significantly from neutrality. It indicated that these two genera have undergone similar selection strength during their evolutionary process. A previous study showed that 4 traits undergoing the directional selection during domestication were identified in Asian rice via QTL sign test, and 16 individual traits in Asian rice and 38 ones in maize that newly detected in this study deviated significantly from neutrality as well, demonstrating the dominant influence of artificial selection on them. Moreover, analysis of different categories of cross type including O. sativa × Oryza rufipogon (perennial and annual forms) crosses, maize × teosinte (Zea mays subsp. parviglumis) crosses, O. sativa × O. sativa crosses, and maize × maize crosses showed that their proportions of antagonistic QTLs were 0.1869, 0.1467, 0.2649, and 0.2618 respectively. These results revealed that selection strength of domestication is significantly stronger than that of modern genetic improvement. However, interestingly, the proportion of antagonistic QTLs (0.1591) in maize × maize with long-term selection was very similar to that (0.1467) in the maize × teosinte (Zea mays subsp. parviglumis) crosses. It suggested that some favorable traits could be cultivated within a few decades if we carry out strong selection. In addition, the proportions of antagonistic QTLs of the widely cultivated hybrids of rice (Minghui 63 × Zhenshan 97) and maize (Zheng 58 × Chang 7-2) in China were 0.309 and 0.3472 respectively. It suggested that selection during modern genetic improvement has significantly acted on them.