Genetic diversity and relatedness of rice cultivars resistant to straighthead disorder

Authors: AGRAMA, HESHAM - UNIV OF AR RREC; Yan, Wengui

Submitted to: Plant Breeding
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/22/2009
Publication Date: 6/1/2010
Citation: Agrama, H.A., Yan, W. 2010. Genetic diversity and relatedness of rice cultivars resistant to straighthead disorder. Plant Breeding. 129:304-312.

Interpretive Summary: Straighthead, a physiological disorder of rice (Oryza sativa L.), has been reported in numerous rice-growing countries because of its severe impact on grain yield. Genetic diversity, relatedness and structure of parental germplasm are important for breeders to design strategies in breeding programs. Twelve hundred rice accessions that were sampled from the USDA core collection were evaluated for straighthead resistance and genotyped with 72 molecular markers. Forty-two cultivars were resistant with no yield reduction when grown under severe straighthead conditions. The resistant cultivars originated from fifteen countries in ten geographic regions worldwide, with the most coming from China. The accessions had great diversification indicated by fifteen phenotypic characters including a wide range in grain and cooking quality traits. The number of alleles averaged 5.93 ± 0.34, and PIC averaged 0.57 with 62% of the 72 markers being highly informative (PIC>0.5) among the 42 resistant cultivars. Twenty-six resistant cultivars were identified as Indica type, clearly classified in two sub-groups while four other cultivars were classified between the two sub-roups. Ten resistant cultivars were identified as Temperate Japonica type and two cultivars were positioned between Temperate and Tropical Japonica type cultivars. This information on phenotypic diversity and genetic relatedness among the straighthead resistant cultivars will allow breeders worldwide a broad array of resistant parents to transfer straighthead resistance into local cultivars of Indica and Japonica backgrounds.

Technical Abstract: Straighthead, a physiological disorder of rice (Oryza sativa L.), has been reported in numerous rice-growing countries for its severe impact on yield reduction. Genetic diversity, relatedness and structure of parental germplasm are important for breeders to design strategies in breeding programs. We analyzed 1,002 rice accessions that were sampled from the USDA core collection, evaluated for straighthead and genotyped with 72 molecular markers. Forty-two accessions were resistant to straighthead with no yield reduction under severe infestation. The resistant cultivars originated from fifteen countries in ten geographic regions worldwide. The accessions had great diversification indicated by fifteen phenotypic characteristics including all types of grain and cooking quality. The number of alleles averaged 5.9, and the 72 molecular markers were highly informative among the 42 resistant accessions. Twenty-six resistant accessions were identified as indica type and were clearly classified in two groups. Ten resistant cultivars were identified as temperate japonica and the other two were positioned between temperate and tropical japonica rice. Breeders worldwide could use these findings on phenotypic diversification and genetic relatedness among the straighthead resistant accessions in order to make germplasm choices for transferring straighthead resistance to their local cultivars, serving various needs on grain, cooking quality, and genetic classification in their programs.