| Rice (Oryza sativa L.) is one of the most important world's cereal crops. Nowadays, however, more and more weeds are occurring in rice fields, particularly the conspecific weedy rice (O. sativa f. spontanea). Weedy rice is one of the most notorious weeds found in rice-planting areas all over the world. It commonly causes reduction of rice yield and affects grain quality. Weedy rice is also called red rice, as most weedy rice has red pericarp. Because of the affinity of weedy and cultivated rice, weedy rice is more difficultly controlled than other weeds in rice fields. On the other hand, as a primary gene pool of cultivated rice, weedy rice may provide a large amount of gene resources for rice breeding.Because of the shortage of water resource and farm labor, cultivation methods of rice are transferring from traditional transplanting to direct seeding in many countries including China. Meanwhile, weed managements turn less intensive. The change of cultivation methods and weed managements accelerate the growth and spread of weedy rice. In recent years, therefore, weedy rice has increasingly affected rice production in many regions of theworld, including northeastern China. Controlling weedy rice is an urgent problem.However, fully understanding the genetic backgrounds and biological characters of weedy rice is the prerequisite for effective control. Our study analyzed the genetic diversity of the 30 populations (909 individuals) of weedy rice from the six regions (Tieling, Sujiatun, Panshan, Haicheng, Donggang, and Zhuanghe) of Liaoning Province in northeastern China using the simple sequence repeat markers (SSR, also referred as microsatellite markers). Moreover, we compared the genetic similarity between weedy rice and its relatives (cultivated and wild rice from different regions). To better understand the genetic diversity of the weedy rice, we selected the six populations (each in one region) to analyze the natural outcrossing rate using simple sequence repeat markers.The results of the analysis of genetic diversity showed that the level of genetic diversity of Liaoning weedy rice was relatively high (He of overll weedy rice was 0.313, and Shannon index I=0.572). The genetic diversity of weedy rice varied considerably across regions and populations. At the region and population level, the weedy rice population from Panshan had the highest level genetic diversity (He=0.393 and I=0.684), while the population from Donggang had the lowest level (He=0.002 and I=0.004). The results of the analysis of outcrossing rate showed that the outcrossing rate of the weedy rice was very low. The highest outcrossing rate [the multilocus outcrossing rate (tm) was 1.3%] was observed in the Panshan population. There was a positive correlation between the level of genetic diversity and the outcrossing rate of weedy rice populations, indicating that the introgression from cultivated to weedy rice may enhance the genetic diversity of weedy rice populations. At the regional level, there was a tendency that the weedy rice populations discovered early had the low level of genetic diversity, which might be explained by the different intensity and time of control of weedy rice in different regions. Additionally, we observed some alleles specific to weedy rice, suggesting that weedy rice may have some specific genes different from cultivated and wild rice, which may provide gene resources for rice breeding.The results of the ananlysi of genetic variation showed that considerable genetic variation of weedy rice from Liaoning existed among regions (Fst—0.346, 35.14% of total variation among regions based on AMOVA), which may be explained by the difference ofenvironments of agro-ecosystems and the introgression from cultivated to weedy rice in every region. The genes of cultivated varieties with different genetic backgrounds introgresse into weedy rice, and then the genetic backgrounds of weedy rice may differ among regions.The results of the analysis of genetic similarity between weedy rice and its relatives showed that weedy rice from Liaoning had the closest genetic relationship with Liaoning cultivars and was distant from cultivars of other regions of China and of Japan and South Korea and from wild rice, indicating that the weedy rice possibly originated from local cultivar in Liaoning, rather than from the hybridization between indica and japonica cultivated rice or between wild and cultivated rice, or from wild rice. The weedy rice may be the derivative of volunteers from hybridization among local cultivated varieties or off-types of cultivated rice with back mutations, through which the volunteers or off-types obtain some feral characters, particular seed shattering and dormancy. This process is continous and dynamic, and might begin occurring long before. However, the traditional rice practice with intensive and meticulous cultivation effectively controlled the spread of weedy rice. In recent years, however, with the popularizing of direct seeding and the extensive weed management, weedy rice has spread rapidly and seriously affected rice production.In order to provide some references for weedy rice control with the shift of cultivation methods, a field experiment was conducted to study the performance of weedy rice and the effect of weedy rice on cultivated rice in transplanted and direct-seeded plots. Our results showed that with the same water and weed management, the performance of weedy rice was relatively poorer in direct-seeded plots than in transplanted plots, while the cultivated rice was more tolerant to weedy rice infestation in direct-seeded plots than in transplanted plots. Therefore, in order to meet the food demand with the shortage of water and farm labor, wide use of the technology of direct seeding may be a good strategy. At the same time, there is an urgent need of effective weed management suitable to direct-seeded rice fields.When the popularizing of direct seeding brings us the challenge of weedy rice control, the risk of transgene eacape from cultivated to weedy rice is not negligible. To provide some references for the evaluation of the risk, we obtained the F1 hybrids between weedyand transgenic rice by hand crossing, and a field experiment was conducted to study the fitness of the F1 hybrids and their parental lines. Our results showed that the F1 hybrids obtained easily by hand crossing. The performance of F1 hybrids was superior to that of their parental lines in terms of the vegetative characters. However, the seed production of F1 hybrids was inferior to that of their parental lines. In the whole, the fitness of F1 hybrids was not better than that of their parental lines, but the F1 hybrids may be the bridge of transgene escape from cultivated to weedy rice.
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