| Hybridization and introgression has played a very important role in the evolution of species and populations. On the one hand, hybrization can break the genetic barrier to generate new species or populations, and make effect on the transportation and location of species. On the other hand, it can generate and maintain genetic diversity within populations, while promoting the adaptive evolution. Hybridization and gene flow also make genetic assimilation, demographic swamping and selective sweep. On the natural conditions, the reproductive isolation can prevent the success of hybrization, which includes prezygotic and postzygotic barrier. The process often results in non-random separation in the genes or traits of hybrid offsprings, which caused by gametic selection or zyotic selection. Linkage disequilibrium plays as a effective tool in detecting natural selection in population genetics study.In this study, wild rice, weedy rice and cultivated rice were used as materials. We carried out the following mainly three studies.Firstly, hybridization-introgression study on natural conditions was observed. Gaozhou was one of the habitate of Oryza rufipogon Griff., which was disrupted by the human activities. In this study,34 pairs of InDel and 34 pairs of SSR markers were used for discriminating 211 individuals from 6 subpopulations. Make PCA and structure analysis to know the introgression between cultivated rice and wild rice. Diversity analysis was used for assessing the effect of the introgression. PCA analysis both based on SSR and InDel markers showed that total wild rice population has divided into mainly three groups. Each of subpopulation C and E clustered into their own group while other subpopulations clustered in one large group (Fst=0.322). Some alleles that occur in cultivars with high frequencies were absent, or extremely rare in some wild subpopulations, but in E subpopulation these alleles had high frequencies. According to the data revealed by MSA, it was indicated that E subpopulation shared most alleles with the cultivars (InDel ps=0.644, SSR ps=0.409), which meant E subpopulation might have introgression with cultivars. According to the data of genetic diversity, it could be estimated that subpopulation E had a highest observed heterozygosity (0.8063), indicated that subpopulation E might hybridize with other population historically. Subpopulation B owned the highest I Index (Shannon Index 1.001) and the total high alleles (7.6471) and effective alleles (3.2588) meant high genetic diversity which deserved more protection value in conservation. This study confirmed hybridization-introgression occurred between wild rice population and cultivated rice.Secondly, to further explain the genetic impact of hybridization-introgression in future generations, a series of hybrid progenies were constructed by artificial hybridization under controlled conditions. Weedy rice and cultivated rice accessions were chosen from different areas and by different indica-japonica types.28 crosses of weedy-weedy rice hybrization had been made, which contained 11 indica-japonica type crosses,15 indica-indica type crosses and 2 japonica-japonica type crosses.25 crosses of weedy-cultivated rice hybrization had been made, which contained 8 indica-japonica type crosses,14 indica-indica type crosses and 2 japonica-japonica type crosses. The results showed that weedy rice and cultivated rice from different sources could make effective crosses. Cross incompatibility were partly due to the genetic background of the parents. All the hybrids seeds from these crosses would be used in the future research.Thirdly, to study the inheritance of alleles descended from different parents and explore the distorted segregation of alleles in their progeny caused by natural selection during the hybridization-introgression process, this study conducted genetic analyses in three F2 populations derived from artificial hybridization between weedy rice accessions. With the aid of microsatellite (SSR) and insertion/deletion (InDel) molecular markers, gene and genotype frequencies of different loci were analyzed in the F2 populations. Chi-square (X2) statistic was used to examine the consistency of observed versus theoretical values of the segregation ratios in the F2 populations for detecting the distorted segregation. Linkage Disequilibrium of paired loci was also analyzed in the F2 populations, with the Chi-square (X2) statistic test to determine the genetic linkage of the concerned loci. Results demonstrated significant segregation distortion of some loci in different F2 populations, of which gametic selection was effective. In addition, some loci showed significant linkage disequilibrium in the F2 populations. The results indicated clear segregation distortion in hybrid progeny of weedy rice, possibly caused by natural selection, which has provided genetic and molecular evidence that the natural selection plays an important role in the adaptive evolution of weedy rice during the hybridization-introgression process.In summary, natural hybridization-introgression events was confirmed between common wild rice and cultivated rice by genetic structure analysis. The effect of hybridization-introgression to future generations was speculated. The results of genetic separation in weedy rice hybrid offspring populations could clarify the genetic impact made by hybridization-introgression. All the studies in this thesis provided some evidence in the adaptive evolution of plant populations by hybridization-introgression. |
