ORIGIN, DISPERSAL, CULTIVATION AND VARIATION OF RICE

There are two cultivated and twenty-one wild species of genus Oryza O. sativa, the Asian cultivated rice is grown ail over the world. The Af rican cultivated rice, O. glaberrima is grown on a small scale in West Africa. The genus Oryza probably originated about 130 million years a go in Gondwanaland and different species got distributed into differen t continents with the breakup of Gondwanaland. The cultivated species originated from a common ancestor with AA genome. Perennial and annual ancestors of O. sativa are O. rufipogon and O. nivara and those of O. glaberrima are O. longistaminata, O. breviligulata and O. glaberrima probably domesticated in Niger river delta. Varieties of O. sativa are classified into six groups on the basis of genetic affinity. Widely k nown indica rices correspond to group I and japonicas to group VI. The so called javanica rices also belong to group VI and are designated a s tropical japonicas in contrast to temperate japonicas grown in tempe rate climate. Indica and japonica rices had a polyphyletic origin. Ind icas were probably domesticated in the foothills of Himalayas in Easte rn India and japonicas somewhere in South China. The indica rices disp ersed throughout the tropics and subtropics from India. The japonica r ices moved northward from South China and became the temperate ecotype . They also moved southward to Southeast Asia and from there to West A frica and Brazil and became tropical ecotype. Rice is now grown betwee n 55 degrees N and 36 degrees S latitudes. It is grown under diverse g rowing conditions such as irrigated, rainfed lowland, rainfed upland a nd floodprone ecosystems. Human selection and adaptation to diverse en vironments has resulted in numerous cultivars. It is estimated that ab out 120000 varieties of rice exist in the world. After the establishme nt of International Rice Research Institute in 1960, rice varietal imp rovement was intensified and high yielding Varieties were developed. T hese varieties are now planted to 70% of world's riceland. Rice produc tion doubled between 1966 and 1990 due to large scale adoption of thes e improved varieties. Rice production must increase by 60% by 2025 to feed the additional rice consumers. New tools of molecular and cellula r biology such as anther culture, molecular marker aided selection and genetic engineering will play increasing role in rice improvement.