Evidences of introgression from cultivated rice to Oryza rufipogon (Poaceae) populations based on SSR fingerprinting:: implications for wild rice differentiation and conservation

Crop-to-wild introgression may play an important role in evolution of wild species. Asian cultivated rice (Oryza sativa L.) is of a particular concern because of its cross-compatibility with the wild ancestor, O. rufipogon Griff. The distribution of cultivated rice and O. rufipogon populations is extensively sympatric, particularly in Asia where many wild populations are surrounded by rice fields. Consequently, gene flow from cultivated rice may have a potential to alter genetic composition of wild rice populations in close proximity. In this study, we estimated introgression of cultivated rice with O. rufipogon based on analyses of 139 rice varieties (86 indica and 53 japonica ecotypes) and 336 wild individuals from 11 O. rufipogon populations in China. DNA fingerprinting based on 17 selected rice simple sequence repeat (SSR) primer pairs was adopted to measure allelic frequencies in rice varieties and O. rufipogon samples, and to estimate genetic associations between wild and cultivated rice through cluster analysis. We detected consanguinity of cultivated rice in O. rufipogon populations according to the admixture model of the STRUCTURE program. The analyses showedz that four wild rice populations, DX-P1, DX-P2, GZ-P2, and HL-P, contained some rare alleles that were commonly found in the rice varieties examined. In addition, the four wild rice populations that scattered among the rice varieties in the cluster analysis showed a closer affinity to the cultivars than the other wild populations. This finding supports the contention of substantial gene flow from crop to wild species when these species occur close to each other. The introgressive populations had slightly higher genetic diversity than those that were isolated from rice. Crop-to-wild introgression may have accumulative impacts on genetic variations in wild populations, leading to significant differentiation in wild species. Therefore, effective measure should be taken to avoid considerable introgression from cultivated rice, which may influence the effective in-situ conservation of wild rice species.