Ecological-Genetic Studies on Rice Germplasm in Tropical Asia (3rd Survey)

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We have carried out the variation survey of the common cultivated rice (Oryza sativa) and its wild progenitor (O. rufipogon) from the viewpoint of genetics and ecology. In 1989, we made study-tours in Bhutan and Bangladesh. In 1989 and 1990, we visited three times our permanent study sites of wild rice which are located in the Central Plains of Thailand.1. Bhutan --- In West Bhutan we made observation of 22 rice fields distributed along three valleys and collected 158 seed samples. The field of the highest altitude we visited (probably the highest rice field in the world) was in 2700 m MSL. Rice fields were heterogeneous in varying degrees within population including variants which showed high seed shattering and semisterility. The origin of such "weedy" type remains unsolved. Character investigation of seed samples revealed that samples collected in higher altitude were mostly Japonica type, while those collected in lower altitudes were mostly Indica type.2. Bangladesh --- Rice fields … More in Bangladesh are distributed mostly in low-lying areas which are deeply flooded in the rainy season. We traveled three districts which are planted to deepwater rice and inhabited by wild rice. 66 cultivated and 30 wild rice samples were collected on 14 sites. Traditional methods of mix-cropping (mixture of two varieties having different growth duration) suggested an interesting problem of ecological genetics. Some of the accessions of cultivars were found to carry Japonica-specific isozyme alleles, although lowland cultivars in tropics have been believed as Indicas. In regards to wild rice, most of wild rice populations were found to be in close proximity to rice fields, and to be introgressed by genes of cultivated type. Fortunately, we found truly wild populations scattered forming small populations in a huge marsh places which were dominated by Datura.3. Population study in Thailand --- Among eight permanent studysites which we are monitoring since 1983, four wild populations became extinct or nearly extinct. They are all annual type. The remaining four are perennial or intermediate perennial-annual type, and persist in relatively stable habitats. Thus, natural populations destruction or urbanization. Less