Wild species of Oryza: a rich reservoir of genetic variability for rice improvement

The genus Oryza, to which cultivated rice (O. sativa) belongs, has 22 wild species (2n=24, 48) representing 10 genomes. These wild species show tremendous diversity in plant morphology, life cycle, (perennial, annual), growth habit, light requirement (full sun, full shade, partial shade, etc.), including agronomic traits (resistance to biotic and abiotic stresses). Wild species are an important reservoir of useful genetic variability to broaden the gene pool of rice for tolerance of major biotic and abiotic stresses, for cytplasmic diversification, and to introgress yield-enhancing loci/QTLs. Low crossability, increased sterility, reduced recombination, and linkage drag limit the introgression of genes from wild species into rice. Recent advances in tissue culture and molecular markers have facilitated alien introgression in rice. At IRRI, using direct crosses, embryo rescue, anther culture, molecular markers, and flourescence in situ hybridization (FISH), a series of interspecific hybrids, alien introgression lines, monosomic alien addition lines (MAALs), and chromosome segmental substitution lines (CSSLs) have been produced. Genes for resistance to brown planthopper (BPH), bacterial blight (BB), blast, tungro, acid sulfate soils, and iron toxicity have been introgressed from AA, BBCC, CC, CCDD, EE and FF genomes into rice. Some of the introgressed genes (Bph10, Bph18, Xa21, Pi-9) introgressed from wild species have been mapped and used in marker-assisted selection (Xa21, Bph18). Some of the breeding lines with genes introgresses from wild species have been released as varieties. Opportunities exist in wild species germplasm for novel genetic variability for traits such as C4 or C3-C4 intermediates. Once variability for C4 traits is identified in Oryza, it should be possible to exploit the transfer of such traits following wide hybridization techniques already used successfully for the transfer of resistance to biotic and abiotic stresses and thus enhance the photosynthetic efficiency and yield of rice by modifying it from C3 to C4.