Indica-Japonica differentiation of rice cultivars viewed from variations in key characters and isozymes, with special reference to landraces from the Himalayan hilly areas.

To assess the extent of differentiation into the Indica and Japonica types of Asian rice in its diversity center, we investigated landraces collected from the hilly areas of Nepal, Sikkim and Assam. We examined variations in four key characters and six isozyme loci known to be diagnostic for classifying Indica and Japonica types, and compared the results with those from a control set of rice cultivars representing the whole of Asia. The hill cultivars showed a high level of genetic diversity in key characters as well as in isozymes. A marked feature found in their character variation was the occurrence of various atypical cultivars that were intermediate between the Indica and Japonica types. With respect to isozymes, however, the hill cultivars could be classified mostly as either Indicas or Japonicas, although the patterns of allelic association were more random than in the control cultivars. Indica-Japonica variation in key characters and in isozymes corresponded well with each other in the controls, but not in the hill cultivars. This means that nonrandom association in characters as well as in genes (gametic disequilibrium) is not fully developed in the diversity center. Populations of hill cultivars were highly polymorphic genetically, but did not show a trend to Indica-Japonica differentiation within their populations. The process of Indica-Japonica differentiation is discussed in view of these observations.