To estimate genetic diversity of the residual northern populations of Oryza rufipogon, a total of 232 individuals from six populations were analyzed using microsatellites (SSRs). The O. rufipogon populations with different status included three from Dongxiang (Jiangxi Province) and three from Chaling (Hunan Province) in China. The 23 rice SSR primer pairs selected from the RiceGenes Database detected a total of 115 alleles, indicating that all the SSR loci were polymorphic in this study. The total gene diversity was 0.919 in the six O. rufipogon populations, and the Donxiang populations showed higher diversity than the Chaling populations. More significant genetic differentiation and less gene flow were found among the Dongxiang populations than those from Chaling. The two putative introgressed populations showed relatively high genetic variation. One in situ conserved population from Dongxiang had the lowest level of genetic diversity. The re-introduced population from Chaling restored about 90% of the genetic variation, compared with the original source population. It is concluded from these results that a relatively high level of genetic variation resided in the northern O. rufipogon populations and continued efforts of conservation of these populations are needed; and that the conservation of some Chaling and Dongxiang populations has been effective in preventing gene flow from cultivated rice. Introgression of cultivated rice demonstrated significant impacts on genetic variability of the O. rufipogon populations, and should be carefully considered in conserving this wild rice. This study also suggested that re-introduction to its original habitats is an effective approach to restore O. rufipogon populations.

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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.

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SummaryFast development and commercialization of genetically modified plants have aroused concerns of transgene escape and its environmental consequences. A model that can effectively predict pollen-mediated gene flow (PMGF) is essential for assessing and managing risks from transgene escape. A pollen-trap method was used to measure the wind-borne pollen dispersal in cultivated rice and common wild rice, and effects of relative humidity, temperature and wind speed on pollen dispersal were estimated. A PMGF model was constructed based on the pollen dispersal pattern in rice, taking outcrossing rates of recipients and cross-compatibility between rice and its wild relatives into consideration. Published rice gene flow data were used to validate the model. Pollen density decreased in a simple exponential pattern with distances to the rice field. High relative humidity reduced pollen dispersal distances.Model simulation showed an increased PMGF frequency with the increase of pollen source size (the area of a rice field), but this effect levelled off with a large pollensource size. Cross-compatibility is essential when modelling PMGF from rice to its wild relatives. The model fits the data well, including PMGF from rice to its wild relatives. Therefore, it can be used to predict PMGF in rice under diverse conditions (e.g. different outcrossing rates and cross-compatibilities), facilitating the determination of isolation distances to minimize transgene escape. The PMGF model may be extended to other wind-pollinated plant species such as wheat and barley.