ORIGIN OF ASIAN CULTIVATED RICE AND ITS ECOTYPIC DIFFERENTIATION

Abstract

Oryza nivara is an annual wild species that grows in small seasonal pools mainly in the plateau regions of south and southeast Asia. O. rufipogon Griff. is a perennial wild species adapted to swamps and margins of tanks in the coastal belts and in the lower valleys of rivers of south and southeast Asia. The cultivated rice, O. sativa 1., which shares the same ploidy level (2n = 24) and the genome (AA) with these two wild species, has originated mainly from O. nivara in the same part of the world. O. Sativa has differentiated into ecospecies (aus, aman, japonica and javanica) and, within the ecospecies, into ecotypes. When cultivars of different ecotypes are crossed, the hybrids often express various degrees of sterility. The authors recognise the hill rices of southeast India (that includes the Jeypore tract of Orissa), the japonica-like cultivars of the hilly .treas of southwest China (extending westward upto Nepal) and the hill rices of mainland southeast Asia as the three basic stocks that have evolved directly from O. nivara in their respective regions. The Proto-Australoid people who spoke Austric languages were responsible for the origin of this crop as well as for its initial spread all over south and southeast Asia before the advent of Indian and Chinese civilizations into this region. The aus ecotype was developed directly from the southeast Indian hill rices and the japonica ecotype of northern China from japonica-like types of southwest China. The spread of cultivation of aus types to the lower Gangetic valley and that of japonica- like types to the Brahmaputra valley resulted in the introgression of rufipogon genes into these types and led to evolution of aman and shali ecotypes respectively. The aman ecotype was successful only after iron was introduced for plough and oxen as draught animals in the lower Gangetic valley presumably by the Aryans in the second millenium B.C. Subsequently, the aman types were carried by. Indian colonisers to Indochina and Indonesia during the ninth century A.D. The tjereh ecotype was developed from the aman ecotype in Indonesia. Similarly, migration of hill rices of mainland southeast Asia to Indonesia with introgression of genes of O. rufipogon into them has resulted in the origin of javanica ecotype.