Genetic Resource of RiceCultivated rices fall into two groups similar in appearance but distinguish by their different areas of origin and by the sterility of hybrids between them, indicating significant genetic separation. They are Oryza sativa, Asian rice, and Oryza glaberrima, African rice. Both are presumed to have descended from a common ancestor, perhaps O. perennis, a wild perennial species widely distributed throughout the tropics. The evolution of the two groups proceeded independently with the similar steps in each case; first, the evolution from the presumptive common ancestor, of two wild perennial, one Asian the other Africa. Each in turn gave rise to a wild annual species from which the cultivated annuals arose. Chromosome studies show that the species in the two pathways share the same genome AA, which appears to have undergone some modifications in the O. glaberrima pathway. These differences are associated with, though not necessarily responsible for, the genetic isolation between the two groups. Uncertainties exist about the precise identity of ancestral species, partly because of the absence of major genomic differentiation, which provided strong evidence of origin in other species groups such as wheat and cotton. All cultivated rices are diploids.
O. glaberrima occurs in sub-Saharan Africa, in flood plain habitats where it is sown before the floods arrive and harvested after they have receded. Upland or ‘dryland’ varieties are also grown. Although it is hardier than O. sativa, it nevertheless has been replaced by O. sativa whenever condition permit the latter’s cultivation. The greatest diversity of O. glaberrima is in West Africa.The starting point for the differentiation of the Asian rices is perennial wild grass O. rufipogon, widely distributed in south and south-east Asia, south China, and South America, from which the annual wild species O. nivara arose. O. nivara has a wide distribution range in India, south-east Asia and Oceania. Domestication of nivara involved selection for non-brittle ears, larger plants and grain, and shorter dormancy and it probably occurred, perhaps in several independent cases, within the large area north east India, northern Bangladesh, Burma, Thailand, Laos, Vietnam and south east China. From this diffuse area origin, cultivate rice was rapidly and widely dispersed into new regions and habitats by farmers who doubtless could see the immense benefits to be gained from the addition of a cereal crop to a farming culture which up to that time had lacked one.
Genetic Resource of Rice
