Origin, dispersal, cultivation and variation of rice.
An account is given of the evolution and current cultivation of the genus Oryza. The 2 cultivated species are O. sativa and African cultivated rice, O. glaberrima, which is grown on a small scale in West Africa. The genus Oryza probably originated about 130 million years ago in Gondwanaland and species were distributed amongst different continents when Gondwanaland split up. Cultivated species originate from a common ancestor with an AA genome. Perennial and annual ancestors of O. sativa are O. rufipogon and O. nivara and those of O. glaberrima are O. longistaminata and O. breviligulata [O. barthii]. O. glaberrima was probably domesticated in the Niger river delta. Varieties of O. sativa are classified into 6 groups on the basis of genetic affinity. Widely known indica rices correspond to group I and japonica rices to group VI. The javanica rices also belong to group VI and are designated as tropical japonica in contrast to temperate japonica. Indica and japonica rices have a polyphyletic origin. Indica rices were probably domesticated in the foothills of the Himalayas in eastern India and dispersed throughout the tropics and subtropics whilst japonica rices were domesticated in South China and moved northwards resulting in temperate ecotypes; they also spread southward to Southeast Asia and from there to West Africa and Brazil and became tropical ecotypes. A summary of current rice cultivation, breeding and the work of the International Rice Research Institute is also presented.