5. A new insight into Asian cultivated rice classification from isozyme studies

The survey of isozyme polymorphism in a sample of 120 varieties has led to the identification of six varietal groups (Glaszmann, in press). The analysis of data for 1688 varieties confirms this structure and permits documenting its correspondence with anterior classifications (Table 1). The distribution of the six groups in Asia is shown in Fig. 1.

The data from 21 polymorphic presumed loci coding for enzymes help quantify the variation within and between the groups (Table 2).

The main conclusion is that the opposition between the Indica and Japonica types, represented by groups I and VI, is (a) the major but (b) not the only component of the varietal differentiation.

(a) Groups I and VI represent 80% of all the varieties. Group I is characterized by the C type morphology. It is distributed in the lowlands of tropical Asia. It still exhibits a high degree of enzymatic polymorphism, which suggests possible further subgoupings. Group VI is characterized by the A and B type morphologies. It is distributed mostly in the temperate regions, in the hilly areas of East and South East Asia and along the Himalayas.

(b) Several groups, groups II to V, can be identified which are not truly intermediate between the major groups I and VI. They represent 15% of all the varieties. They have morphologies close to the C type; on that basis, they have usually been considered as Indica varieties. They are found only in the western part of the rice distribution. Groups II and V spread from Iran to Burma, groups III and IV are restricted to some deepwater areas in Bangladesh. As far as our loci sample is concerned, groups II and III are closer to group I and groups IV and V are closer to group VI.

The characters used for the present classification are monogenic, independent from the environment, free from epistatic relationships and not subjected to conscious human selection. The identifcation of clear-cut groups suggests natural restrictions to free genetic recombination between these groups. The factors for such restrictions have to be further investigated.

Fig. 1. Distribution in Asia of 6 rice varietal groups defined through the analysis of enzyme polymorphism scored at 21 loci. Groups are designated I to VI (see text); Class 0 corresponds to varieties not easily classifiable. A total of 1,688 varieties is represented.

Table 1. Correspondence between the enzymatic classification and anterior classifications of Asian cultivated rice.

Table 2. Probability of identity of two randomly chosen alleles within the groups (Jxx, diagonal), between the groups (Jxy, above diagonal) and genetic distance between the groups (Dxy, Nei, 1975, below diagonal) for 6 enzymatic groups of O. sativa rices, as estimated from 21 polymorphic loci.

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