5. Spontaneous occurrence of low amylose genes and geographical distribution of amylose content in Asian rice

Changes in endosperm and embryo characters are frequently found among induced mutants. Glutinous endosperm (wx), low amylose (du), high amylose (ae) are known as mutant genes. Presumably, they may occur spontaneously in the field. This paper concerns genetic variations in amylose content observed among native cultivars from various localities of Asia.

Material used for amylose content determination was seeds harvested in a greenhouse. Out of 872 accessions examined, 202 were from India, 65 from Nepal, 161 from Burma and the northeastern states of India, 86 from Bangladesh, 121 from Yunnan Province of China, and 237 from Malaysia. A Japanese cultivar, Nipponbare' was used as the control. Amylose content was measured by the autoanalyzer using a grain of pearled rice.

The pattern of geographical distribution of rice cultivars with varying contents of amylose is shown in Fig. 1. The data can be summarized as follows:

1) Only strains with high amylose contents were found in India and Bangladesh. Cultivars from this area showed less of variation than those from other areas. No glutinous variety was found.

2) Nepalese strains were clearly divisible into two groups, having either high amylose (24-30%) or low amylose (16-22%). The former group showed a range similar to that of the strains from India and Bangladesh. The latter group showed a range similar to that of Japonica cultivars (16-20%). A few glutinous strains were found in Nepal.

3) Malaysian cultivars showed a wide variation in amylose content, but the majority belonged to the high amylose group.

4) Strains from Burma and northeastern India showed an almost continuous array of integrades from high to zero amylose, suggesting the presence of rice genic variations in this area. Cultivars from Yunnan showed the same trend. Some of them with a low amylose content (8-15%) looked like the du mutant. Many glutinous cultivars were also found in this area.

The data indicate that the center of gene diversity for amylose content is present in the hilly area of Southeast Asia, as was shown for the esterase isozymes (Nakagahra 1978). It seems that gene diversity decreases with increasing distance from the center. Similar ranges of amylose content are found among indigenous landraces and induced mutants. There should be a series of genes controlling amylose content. Genetic experiments are needed to compare such spontaneous and induced genic changes.

Fig. 1. Geographical distribution of cultivars for the amylose content of rice endosperm in Asia.

Nakagahra, M., 1978. The differentiation, classification and center of diversity of cultivated rice by isozyme analysis. Trop. Agr. Res. Ser. 11: 77- 82.