Alien introgression in rice.
Genetic variability for tolerance to stress and/or resistance to diseases is limited in cultivated rice germplasm. Wild species of Oryza representing AA, BB, CC, BBCC, CCDD, EE, FF, GG and HHJJ genomes are an important source of useful genes to broaden the gene pool. However, low crossability and limited recombination between chromosomes of cultivated and wild species can limit the transfer of such genes. At IRRI, a series of hybrids and monosomic alien addition lines have been produced through embryo rescue following hybridization between rice and distantly related species. Cytoplasmic male sterility and genes for resistance to rice grassy stunt tenuivirus and bacterial blight [Xanthomonas oryzae pv. oryzae] have been transferred from wild species containing the A genome into rice. Genes for resistance to brown planthopper [Nilaparvata lugens], bacterial blight and rice blast [Magnaporthe grisea] have also been introgressed across crossability barriers from distantly related species into rice. Some of the introgressed genes have been mapped via linkage to molecular markers; Xa-21, introgressed from O. longistaminata and conferring resistance to bacterial blight, was cloned and physically mapped onto chromosome 11 using a bacterial artificial chromosome library and fluorescence in situ hybridization. RFLP analysis of alien introgression showed that in crosses with C genome species, 11 of the 12 chromosomes exhibited introgression, although at low levels. Reciprocal replacement of RFLP alleles of wild species with the alleles of O. sativa indicated alien gene transfer through crossing over. Rapid recovery of recurrent phenotypes in BC2 and BC3 generations from wide crosses was another indicator of limited recombination. Further cytogenetic and molecular investigations are required to determine precisely the mechanism of introgression of small chromosome segments from distant genomes in the light of limited homoeologous chromosome pairing.