In Bhutan, weedy rice (Oryza sativa L.) was grown together with cultivated rice on terraced paddy fields lower than 2620 m above sea level. Seeds of cultivars and weedy strains were collected at 22 collection sites located from 1000 to 2620 m above sea level. Cultivars with round seeds were frequently found in fields higher than 2250 m, and those with slender seeds in fields lower than 1630 m. All cultivars and weedy strains were divided into indica or japonica types by isozyme (multi-locus) and morpho-physiological (multicharacter) analyses. Japonica cultivars predominated in highland; Indica cultivars predominated in lowland. Plastid type was confirmed by the length polymorphism for the ORF100 region. The japonica cultivars carried non-deletion type ORF100. The indica cultivars carried deletion type ORF100. In contrast, weedy strains showed discrepancy in the combination of the nuclear and cytoplasm types. An intermediate type was found in weedy strains for isozyme genotypes. A recombinant type, which has indica genotypes for isozyme analysis with japonica cytoplasm, and vice versa, was frequently found in weedy strains. These findings suggested that weedy strains would be generated by natural hybridization between indica and japonica. Further, they might fail to recombine their prior genotypes sets for isozyme and cytoplasm. Morphophysiological characters did not show such a tendency. Morpho-physiological analysis of highland plants, in particular, showed indica cultivars and weedy strains with japonica cytoplasm. Hypervariable nuclear microsatellite analysis was then used to compare cultivars and weedy strains. Identical alleles were shared between indica and japonica, and also between cultivars and weedy strains. This suggested that there was gene-flow resulting from natural hybridization.

Weedy rice (Oryza sativa L.) plants found in Okayama, Japan, exhibit various morphophysiological characteristics, and are roughly classified into japonica-type and indica-type. We characterized the weedy strains collected from nine sites with molecular markers in flanking regions of two diagnostic isozyme loci, Acp1 and Sdh1. Haplotype analysis for closely-linked SSR markers flanking Acp1 revealed that 1) japonica-type weedy strains shared the same haplotypes with local varieties, and 2) indica-type weedy strains carrying an indica-specific isozyme allele possessed two haplotypes. One of the two haplotypes was common to a forage variety, 'Moretsu'. PCR analysis revealed that the dwarf weedy strains carried the deletion inside OsGA20ox2 as the semi-dwarf allele (sd1). The deletion was also found in 'Moretsu'. The other haplotype was shared with indica forage variety, 'Tetep', which showed the same sequence haplotype for Sdh1 with the indica-type weedy strains. The results obtained from this study suggested that the weedy strains in Okayama have multiple origins and semi-dwarfness possibly derived from 'Moretsu' or indica modern varieties.

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