Adaptive Evolution Of Weedy Rice Revealed By Outcrossing Rate, Genetic Structure And Seed Germination

Weedy rice (Oryza sativa L. f. spontanea) is one of the most notorious weeds in paddy fields worldwide. It has caused severe yield loss and quality problems to cultivated rice(Oryza sativa L.). Weedy rice has evolved quickly to adapt different agriculture environments. Weedy rice is a conspecific species with cultivated rice, gene flow between weedy rice and cultivated rice occurs which may influence the evolution and promote the adaptation of weedy rice to various rice ecosystems. Knowledge on the relationships among outcrossing rate, genetic diversity, genetic structure and adaptation of weedy rice will shed meaningful light on understanding the evolution of weedy rice and also be useful for the management of weedy rice.This study focused on the evolutionary consequence of outcrossing and crop introgression in weedy rice. I used several techniques, e.g. molecular marker analysis, mixed mating model analysis, and seed dormancy experiment, to explore the outcrossing rate, genetic structure, seed dormancy and germination response to temperature. Based on these studies, I discussed the adaptive evolution of weedy rice.The main results are as follows:(1) Outcrossing rate of weedy rice was very low, with 4.1% multilocus outcrossing rate and 2.9% single-locus outcrossing rate on average. Outcrossing rate varied significantly (0.4-11.7%) among the weedy rice populations. These findings suggest that introgression from cultivated rice to weedy rice can happen.(2) Relatively high but variable level of genetic diversity was found in weedy rice populations. Weedy rice demonstrated a significant population differentiation (Fst= 0.52) which was also correlated to the spatial distribution pattern (Mantel test, r2= 0.734, P< 0.001). Observed heterozygosity was significantly correlated to outcrossing rate among the populations (r2= 0.783, P< 0.001). Allelic introgression from cultivated rice to weedy rice was detected. Genetic structures of cultivated and weedy rice showed some degrees of correlation (Mantel test, r2= 0.253, P= 0.004), indicating that crop introgression affects the genetic differentiation of weedy rice.(3) Weak or no seed dormancy of weedy rice in temperate regions was detected. As a contrast, weedy rice from tropical regions showed strong dormancy under the same experimental conditions. Considerable percentage (>30%) of non-dormant weedy rice seed survived through winter, indicating weedy rice seed can regulate germination depending on mechanisms other than seed dormancy.(4) Seed germination response varied among weedy rice populations of different latitudes. Critical germination temperature of weedy rice populations was correlated to the latitude of the population (r2=0.672, P< 0.001) and also correlated to the 10-year local habitat temperature (Student t-test,t= 0.272, d.f.= 20, P<0.05). These findings demonstrate that non-dormant weedy rice in temperate China has a temperature-based germination differentiation to adapt to different environments.In conclusion, the results obtained here suggest that outcrossing can promote the amount of heterozygosity in the populations which may influence genetic variability and evolutionary potential of weedy rice. Introgression from distinct rice varieties influences genetic differentiation and may further affect the adaptation of weedy rice. Focusing on seed adaptation as a case, it was found that non-dormant weedy rice had evolved temperature-dependent germination to adapt to different environment instead of evolving dormancy. These findings suggest that outcrossing and crop introgression may influence the adaptive evolution of weedy rice. These results deepen our understanding of the evolution of crop conspecific weed. Implications of these findings on the management of weedy rice were discussed.