ALIEN INTROGRESSION IN RICE

Rice (Oryza sativa L.) productivity is affected by several biotic and abiotic stresses. The genetic variability for some of these stresses i s limited in the cultivated rice germplasm. Moreover, changes in insec t biotypes and disease races are a continuing threat to increased rice production. There is thus an urgent need to broaden the rice gene poo l by introgressing genes for such traits from diverse sources. The wil d species of Oryza representing AA, BE, CC, BBCC, CCDD, EE, FF, GG and HHJJ genomes are an important reservoir of useful genes. However, low crossability and limited recombination between chromosomes of cultiva ted and wild species limit the transfer of such genes. At IRRI, a seri es of hybrids and monosomic alien addition lines have been produced th rough embryo rescue following hybridization between rice and several d istantly related species. Cytoplasmic male sterility and genes for res istance to grassy stunt virus and bacterial blight have been transferr ed from A genome wild species into rice. Similarly, genes for resistan ce to brown planthopper, bacterial blight and blast have also been int rogressed across crossability barriers from distanly related species i nto rice. Some of the introgressed genes have been mapped via linkage to molecular markers. One of the genes Xa-21 introgressed from O. long istaminata has been cloned and physically mapped on chromosome 11 of r ice using BAC library and flourescence in-situ hybridization. RFLP ana lysis revealed introgression from 11 of the 12 chromosomes of C genome species into rice. Introgression has also been obtained from other di stant genomes (EE, FF, GG) into rice and in majority of the cases one or two RFLP markers were introgressed. Reciprocal replacement of RFLP alleles of wild species with the alleles of O. saliva indicates alien gene transfer through crossing over. The rapid recovery of recurrent p henotypes in BC2 and BC3 generations from wide crosses is an indicatio n of limited recombination. Further cytogenetic and molecular investig ations are required to determine precisely the mechanism of introgress ion of small chromosome segments from distant genomes in the face of l imited homoeologous chromosome pairing. Future research should focus o n enhancing recombination between homoeologous chromosomes. Introgress ion of QTL from wild species should be attempted to increase the yield potential of rice.