TRABAJO ESPECIAL RICE ORIGIN, EVOLUTION AND DIVERSITY

SUMMARY This review has the objective to present a detailed bibliographical revision about origin, evolution and diversity of Oryza genus, with emphasis in the cultivated species. The O. sativa species is the one of more economic importance and is widely cultivated throughout the world in tropical and temperate climates (latitude 35° south in Argentina to 50° north in China), while O. glaberrima is only cultivated in the western Africa. These studies suggest that there are two evolutionary patterns of origin and domestication of the cultivated rice, being O. sativa from Asia and O. glaberrima from Africa. The most important consequence of domestication in O. sativa is the reduction of dehiscence, which favors the manual and automatic harvest. Studies show that O. sativa presents higher genetic variability geing up to three sub-species (Indica, Japonicas and Javanicas), with different ecology and morphology; however this tendency was not observed in O. glaberrima. Genomic studies show that O. sativa subsp Japonica has 430 megabases and approximately 50% are repeated sequences. Finally, the narrow genetic base found in liberated varieties of rice in the world in the last years, has resulted in a selection progress plateau in yield and vulnerability to biological factors as a consequence of the lack of variability in both the nuclear and cytoplasmic genetic background. Numerous researchers from many countries have carried out studies of breeding programs using molecular technique to increase genetic diversity in O. sativa, using exotic germoplasm of O. genus, mainly the species O. glaberrima, O. sativa spontanea, O. rufipogon, O. officinalis, belonging to the complex O. sativa and O. officinalis.