9. Isozyme variation in Asian common wild rice, Oryza rufipogon

Hong-Wei Cai1,2 Xiang-Kun Wang2and H. Morishima 1

1 ) National Institute of Genetics. Mishima. 411 Japan

2) Beijing Agricultural University, Beijing. 100094 China
 

         A total of 106 strains of Oryza rufipogon collected from 13 Asian countries were examined for their isozyme variation. Polymorphism at 21 loci was detected using starch and polyacrylamide gel systems.
    In almost all strains examined, we found intra-strain variation at some loci. In the present study, each strain was considered as representing a population. Intra-populational isozyme variation will be reported elsewhere (Cai et al., in preparation).
    The geographical distribution area was divided into four regions, i.e. China (mainland and Taiwan). Indochina (Vietnam, Laos, Thailand and Myanmar). Indian subcontinent (India. Sri Lanka. Bangladesh and Nepal) and Malaya (Malaysia. Indonesia and Philippines). Strains from China were found to carry some alleles at high frequencies which were rarely found in other regions, as shown in Table 1. Of them. Cat-12, Est-13 and Est-120 are known to be specific to Japonica cultivars. Genetic distances between the four regions and average gene diversities within region were estimated based on data of 21 loci (Table 2), genetic distance ranged from 0.085 (Indian subcontinent and Indochina) to 0.254 (China and Malaya).

Table 1. Frequencies of 5 isozyme alleles in 0. rufipogon in tour regions
            of Asia
 

Allele China Indian subcontinent Indochina Malaya
Cat- 12 0.54 0.02 0.00 0.00
Est-120 0.28 0.04 0.04 0.15
Est-131 0.33 0.00 0.00 0.00
Est-91 0.52 0.06 0.01 0.12
Amp-13 0.58 0.00 0.14 0.08

Table 2. Genetic distances between four geographical regions and
            average gene diversity within region
 

China Indian subc. Indochina Malaya
China 0.425a 0.146 0.173 0.254
Indian subc. 0.413a 0.085 0.106
Indochina 0.398a 0.102
Malaya 0.427a
a: Average gene diversity (Nei 1978).     To examine overall variation pattern, data for 21 loci was analyzed by factor analysis. Variation explained by the First three factors was only 24% of total variation (9.7%. 7.8% and 6.4%, respectively) indicating that differentiation within 0. rufipogon is not so distinct at isozyme level. The first factor extracted a variation characterized by the alleles at Cat-1 and Est-13 which are differentially distributed in wild rices of China and other regions and are also diagnostic in classifying Indica and Japonica cultivars. The second factor distinguished a few strains carrying a rare allele Eft-53. The third factor extracted a variation characterized by the alleles at Acp-l and Pox-2 which are also Indica-Japonica diagnostic loci. Fig. 1 shows a scatter diagram of 106 strains plotted by the first and third factor scores. Chinese strains are scattered on the left. This agreed well with the result obtained by Second (1985). Strains from Indian subcontinent tend to be intermediate between China and Indochina/Malaya strains. Perennial and annual types which are the two major ecotypes differentiated within this species are not separated at isozyme variation.
    In order to examine to what extent alleles are nonrandomly associated in Asian wild rice corresponding to Indica-Japonica differentiation, we calculated R2 (squared correlation coefficient of allelic frequencies. Hill and Robertson 1968) between 8 Japonica-indica diagnostic loci (Table 3). Several combinations showed significant nonrandom association. But R2 values (R2=0.022) were generally much lower than those found in 0. sativa (R2=0.382, Sano and Morishima 1992). This indicates that no clear Japonica-indica differentiation occurs in 0. rufipogon as in cultivated rice.

    Table 3. R2 values between 8 loci. statistical significance was tested by corresponding X2

Est-12 Cat-1 Amp-2 Est-2 Esl-13 Pgi-1 Pgi-2 Acp-1 Pox-2
Amp-2 0.012
Est-2 0.127** 0.047**
Est-13 0.122** 0.04** 0.045**
Pgi-1 0.002 0.029* 0.026* 0.009
Pgi-2 0.001 0.006 0.003 0.00 0.004
Acp-l 0.036** 0.00 0.019 0.017 0.001 0.014
Pox-2 0.001 0.00 0.004 0.00 0.023* 0.027* 0.063**
Est-12 0.043** 0.005 0.009 0.004 0.003 0.026* 0.027* 0.007

        From the above results, it may be concluded that 0. rufipogon is a continuum at isozyme level, but tends to be differentiated geographically.

References

Hill, W. G. and A. Robertson. 1968. Linkage disequilibrium in finite populations. Theor. Appl. Genet. 38:
        226-231.
Nei, M., 1978. Estimation of average heterozygosity and genetic distance from a small number of individuals.
        Genetics 89:583-590.
Sano, R. and H. Morishima. 1992. Indica-Japonica differentiation of rice cultivars viewed from variation
        in key characters and isozyme. with special reference to landraces from the Himalayan hilly areas.
        Theor. Appl. Genet. 84:266-274.
Second, G., 1985. Evolutionary relationships in the sativa group of 0ryza based on isozyme data. Genet. Sel.
        Evol. 17:89-114.