Assessment of genetic diversity and genetic structure of wild rice populations in Myanmar.

To evaluate genetic diversity and genetic structure of wild rice (Oryza rufipogon) populations in Myanmar, seven research sites were selected based on various ecological conditions. A large number of samples under natural growth conditions were collected and studied using six simple sequence repeats (SSRs) and two chloroplast DNA markers. A total of 77 alleles were detected from 1559 samples over six SSR loci. The mean number of alleles per population ranged from 3.167 to 8.667, and the mean expected heterozygosity ranged from 0.140 to 0.701. Wild rice populations survived under various environmental conditions and retained different levels of genetic diversity. The large number of samples was effective to confirm the spatial genetic structure of wild rice populations in a relatively small area. Regarding chloroplast DNA polymorphisms, four populations possessed only one pattern, while the other three showed two or five combinations of haplotypes, even within the same population. Additionally, the existence of a new genotype was revealed. Considerable variations in chloroplast DNA exist in the wild rice populations of Myanmar. A high proportion of genetic variation was detected within, rather than among, populations. To ensure maintenance of allelic diversity, it is advisable to preserve many individuals from a large population.

Evaluation of genetic variation among wild rice populations in Cambodia.

A total of 448 samples in five natural populations of wild rice (Oryza rufipogon) were collected in Cambodia. They were examined using 12 SSR and two chloroplast markers to evaluate the degree of variation among populations and the genetic structure within populations. In the two annual populations, the number of plants with homozygous alleles at all 12 SSR loci were high (66.3% and 79.5%), suggesting that these plants propagate mainly through self-pollination. In the three perennial populations, no individuals had all homozygous genotypes, but redundant genotypes resulted from clonal propagation were observed. Percentages of the redundant genotypes were highly varied (3.6%, 29.2% and 86.0%). This may be due to the different stable levels of environmental conditions. As for chloroplast genome, most of the wild plants showed the same chloroplast types as most Indica-type cultivars have. However, plants with different chloroplast types were maintained, even in the same population. In tropical Asian countries, many wild rice populations were observed under similar ecological conditions examined in this study. Therefore, the present results concerning population structure will be important to further elucidate genetic features of wild rice, and will also give strong clues to utilize and conserve wild natural genetic resources.

QTL-based analysis of leaf senescence in an indica/japonica hybrid in rice (Oryza sativa L.).

In order to identify quantitative trait loci (QTLs) for leaf senescence and related traits in rice (Oryza sativa L.), we developed two backcross populations, indica/japonica// japonica and indica/japonica//indica, using IR36 as the indica parent and Nekken-2 as the japonica parent. The QTLs were mapped using a set of simple sequence-repeat markers (SSRs) in the BC(1)F(1) population. Senescence was characterized in these plants by measuring the leaf chlorophyll content 25 days after flowering (DAF), the reduction in chlorophyll content (the difference between the chlorophyll content at flowering and at 25 DAF), and the number of late-discoloring leaves per panicle at 25 DAF in five plants from each BC(1)F(2) line. These plants were moved into a temperature-controlled growth cabinet at the time of flowering and allowed to mature under identical conditions. Eleven QTLs were detected in the two populations. The major of QTLs for senescence were found on the short arm of chromosome 6 and on the long arm of chromosome 9. Of these, one QTL on chromosome 6 and two on chromosome 9 were verified by confirming the effects of the genotypes on the phenotypes of the BC(1)F(3) lines. The japonica parent was found to contribute to late senescence at all but one QTL. Based on a comparison of the effects of heterozygotes and homozygotes on the phenotypic values of each QTL genotype, we concluded that the differential senescence observed in the indica-japonica hybrid was not due to over-dominance; rather, it was the result of partial-dominance genes that were donated from either of the parents.