3. Isozyme diversity found among Asian deepwater rices

Deepwater rice is grown in low-lying fields along big rivers which are inundated deep in the rainy season. A number of cultivars with varying degrees of floating ability are found, most of which appear to belong to the Indica type. To estimate genetic diversity among Asian deepwater cultivars with special reference to Indica-Japonica differentiation, 762 varieties originating from seven countries which span almost all deopwater-rice areas in Asia were examined regarding isozyme variations at six loci (Table 1). Each variety was represented by one

Table 1. Gene diversity (1-sum x\i\², x\i\=frequency of an allele) observed at six isozyme loci in deepwater cultivars from different Asian countries

Fig. 1. Distribution of D values discriminating between the Indica and Japonica types on the basis of isozyme alleles in deepwater rices from different Asian countries and in control varieties. The number of varieties examined is the same as given in Table 1.

strains, although a varietal population is often polymorphic. From the data, gene diversity as shown by 1 - sum x\i\² (x\i\ is the frequency of a given allele) was computed for each locus, with the material from each country. In the average gene diversity obtained from the result (H), the deepwater varieties from the Irrawaddy and the Ganges-Brahmaputra deltas (Burma, India and Bangladesh) showed higher values than those from the Mekong and the Chao Phraya deltas (Cambodia, Vietnam and Thailand). This was because in the former group all the six loci were polymorphic, but in the latter group Acp-1 and Cat-1 were nearly monomorphic (Table 1).

When varieties from different countries were all pooled and were compared with ordinary varieties which were used as the control and were classified into the Indica and Japonica types, the deepwater varieties showed greater H value than the control Indicas which had much greater diversity than the Japonicas. This suggests that the Asian deepwater cultivars have a high isozymic diversity.

Based on a bulk of isozyme data for 15 loci, Glaszmann (1987) reported that most of Asian native rice cultivars fell into two major groups corresponding to the Indica and Japonica types classified by diagnostic characters respectively, and four minor groups which represented particular ecotypes. To evaluate Indica- Japonica differentiation in isozymes of the deepwater varieties, an index value, D\Jap\=P\Jap\/(P\Jap\+P\Ind\), was computed for each allele at each locus, where P/Jap and P/Ind are the frequencies of a given allele in the Japonica and Indica control varieties, respectively. The mean index value obtained for a given cultivar (D\Jap\= (1/5) sum \j\ D\Jap.j\, j=1 ... 5 excluding Pox-2) may be taken as a discriminant score showing its likelihood or likeness to the Japonica type in isozymes. Histograms showing the distribution of D-\Jap\ scores in varieties from different countries are given in Fig. 1, together with the distribution of control varieties. The control varieties showed Indica-Japonica differentiation clearly. In deepwater cultivars, those with scores smaller than 0.4 were predominant, which were close to the Indica type. In addition to such Indica-like types, cultivars with scores larger than 0.5 were found in Burma, India, Bangladesh and Indonesia, which varied toward the Japonica type. However, those cultivars having Japonica-like isozyme alleles did not show a KClO\3\ resistance which characterizes the Japonica type. Also the variation in isozyme alleles showed no significant correlation with floating ability tested by Inoue (1987).

Inouye, J., 1987. On floating rice and its ecological traits in Southeast Asia. Southeast Asian Studies 25: 51-61.

Glaszmann, J. C., 1987. Isozymes and classification of Asian rice varieties. Theor. Appl. Genet. 74: 21-30.