Impact Of Introgression From Cultivated Rice On Genetic Differentiation Of Weedy Rice Populations

Hybridization-introgression is one of the most common biological processes in nature and plays an important role in adaptive evolution and genetic differentiation of plant populations. On the one hand, the hybrids can obtain a large number of genetic and phenotypic recombinations by hybridization-introgression, which not only greatly increase and maintain genetic diversity within populations, but also enhance hybrids to adapt to new habitats, and promote the adaptive evolution of plant populations. On the other hand, driven by different ecological factors, plant populations may differentiate in their adaptive evolution and eventually lead to the generation of new species. Given conspecific feral species can frequently acquire new alleles from their coexisting crops via introgression, hybridization-introgression may play more significant role in adaptive evolution and genetic differention of conspecific feral species compared to other plants in nature. However, little is known about this process. As more and more GM crops are released into environment, the transgenic allele with obvious selective advantage may transfer into conspecific feral species by hybridization-introgression and bring a series of serious ecological consequences. In this context, the study on hybridization-introgression between conspecific feral species and their coexisting crops is extremely urgent and important.Weedy rice (Oryza sativa f. spontanea) is a widespread notorious weed with serious hazards.Weedy rice and cultivated rice belong to the same biological species. They can hybridize frequently each other and produce gene introgression. Introgression from cultivated rice plays an important role in producing and maintaining genetic diversity within weedy rice populations, affecting the genetic differentiation and adaptive evolution of weedy rice populations. So, the study revealing the influence of introgression from cultivated rice on adaptive evolution and genetic differentiation of weeds rice populations is significantly important for understanding the origin, evolution and effective governance of weedy rice. In this study, weedy rice and coexisting cultivated rice from northern Italy were used as materials to explore how introgression from cultivated rice effect adaptive evolution and genetic differentiation of weedy rice populations, which is also expected to provide some reference for other conspecific feral species. We carried out the following mainly three studies.(1) A total of 576 maternal plants representing 24 weedy populations were included to estimate genetic diversity and differentiation based on polymorphism of 24 SSR loci. And exploring whether geographical distance among populations is the main factors leading to genetic differentiation.(2) Using the mixed mating model, a total of 5,395 progeny (seedlings) from 299 weedy families (plants) of randomly selected 15 populations were included to analyse their mating system based on six highly polymorphic SSR loci. Multilocus outcrossing rate (tm), single-locus outcrossing rate (ts), the biparental inbreeding rates [(tm-ts)/tm] and their variation among populations were measured.(3) In order to estimate the possible introgression from cultivated rice, paternity (paternal specific alleles) in weedy rice progeny were analyzed. In addition, crop-specific alleles in weedy rice progeny were determined by comparing between the paternity in weedy rice and the alleles actually detected from the coexisting rice varieties for further confirming introgression from cultivated rice. Correlation between single-locus outcrossing rates (ts) and private allele frequencies (Fpa) of the 15 weedy rice populations was analyzed to explore the effect of introgression from cultivated rice on the adaptive evolution and genetic differentiation.The main results are as follows:(1) Relatively high (He=0.48) but variable level of genetic diversity was found in weedy rice populations. Weedy rice demonstrated a^significant population differentiation (Fst=0.26, and detected considerable private alleles in most weedy rice populations), although the differentiation is not associated with the spatial distribution pattern of the populations (Mantel test, R2=0.0038, p= 0.271).(2) A considerably low level of outcrossing rates of weedy rice populations examined (ts=0.7% and tm=1.8%). However, the outcrossing rates varied significantly among populations (ts ranging from 0.1-1.6%, and tm from 0.2-6.7%). The average proportion of the biparental inbreeding to the total outcrossing rate was about 61%. (3) Allelic introgression from cultivated rice to weedy rice was detected. Outcrossing rates were positively correlated (R2=0.34, p=0.02) with the private allele frequencies of the corresponding populations, which indicating outcrossing rate can significantly affect the amount of private alleles of weedy rice populations.In summary, we detected allelic introgression from cultivated rice to weedy rice, and found that outcrossing rate can significantly affect the amount of private alleles of weedy rice populations in this study. Given that outcrossing rate is one of key factors determining gene introgression and private allele is an important measure of genetic differentiation, the result indicated that introgression from cultivated rice affect the genetic differentiation among weedy rice populations. Key words hybridization-introgression, adaptive evolution, genetic differentiation, conspecific feral species, weedy rice, private allele, SSR中图分类号：Q14...