Genetic diversity in rice (Oryza sativa) and estimation of outcrossing rate using morphological markers

[摘要] English: Rice is an important food crop with two domesticated species namely Oryza glaberrima Steud. also known as African rice and O. sativa L. also known as Asian rice. Rice is a self-pollinating plant that shows some level of outcrossing under field conditions. Understanding the levels of gene flow is important for managing the conservation and maintenance of germplasm for gene banks and plant breeding programmes. It is equally important for the production of quality rice seed. There is also the concern of gene flow between genetically modified rice and other rice species; wild relatives and weedy (red) rice. This study used agro-morphological and microsatellite markers to evaluate the diversity in a collection of 36 accessions consisting of intraspecific (O. sativa and O. glaberrima) and interspecific (O. sativa x O. glaberrima) hybrid genotypes.The Shannon-Weaver diversity index was low measured by agro-morphological markers. The highest level of diversity was detected in the Indica accessions and the lowest diversity in the O. glaberrima accessions. Japonica accessions had brown or green apiculus and were awnless. O. glaberrima and landrace accessions had purple apiculus and some were awned. The Japonica group comprised mostly of improved varieties with a white pericarp whilst O. glaberrima accessions and landraces had a red pericarp. Selected interspecific hybrids combined traits of O. sativa and O. glaberrima to varying degrees. Microsatellite markers showed a total of 321 alleles with an average of 11.9 alleles per locus and an average major allele frequency of 0.29 per locus. The average gene diversity value was 0.81 and polymorphism information content was 0.80 per locus. Diversity indices for interspecific hybrids were intermediate between O. sativa and O. glaberrima, but closer to the O. sativa Japonica group. Two populations were revealed which corresponded to the O. sativa Indica group and O. sativa Japonica group. Interspecific hybrid accessions were dispersed between the two groups. Microsatellite data detected higher diversity between accessions in comparison to agro-morphological data. There was correlation between the different matrices, indicating that they all reflected similar patterns in the structure of diversity of the collection.Agro-morphological markers of red kernel colour and leaf pilosity of the pollen donor was also used to estimate outcrossing rates in four genotypes of rice consisting of improved cultivars of O. sativa Japonica group (one accession) and interspecific hybrids (three accessions). The dominant markers of red kernel colour and leaf pilosity were effective in estimating outcrossing in rice. There was an average outcrossing rate of 0.7% + 0.51, with a potential outcrossing rate of 2.45% + 0.86. Outcrossing rates decreased with increase in distance. It ranged from 2.45% at 0.2 m from the donor to 0.05% at 25 m from the donor. Gene flow was influenced by type of genotype, the distance of recipient from the pollen donor and flowering synchronisation. The study suggests that natural outcrossing could be partly responsible for the appearance of non-uniform and off-type plants in rice fields in sub-Saharan Africa. The study recommends a minimum distance of 20 m between genotypes to minimise outcrossing to acceptable levels.

[发布机构] University of the Free State