Geographical variation of the genes controlling hybrid breakdown and genetic differentiation of the chromosomal regions harboring these genes in Asian cultivated rice, Oryza sativa L.

The geographical variation of two genes, hwd1 and hwd2, controlling hybrid breakdown (Fg weakness) was surveyed in 239 Asian rice cultivars. The culti vars with the genotype Hwd1/Hwd2 Hwd2/Hwd2 were predominant in South Asia a nd Southeast Asia, while those with the genotype Hwd1/Hwd1 hwd2/hwd2 were p redominant in Japan, the Philippines, and Yunnan province, China. Only one cultivar from Laos had the genotype hwd1/hwd1 Hwd2/Hwd2. Preferential assoc iation was found between the presence or absence of hwd2 and the two major cultivar groups (indica and japonica) based on diagnostic RFLP markers. Dom inant homozygotes were frequent in indica rice cultivars, while recessive h omozygotes were frequent in japonica rice cultivars. This association was n ot clear for cultivars from the center of genetic diversity of rice, rangin g from Vietnam to Bangladesh. Polymorphisms were surveyed for RFLP markers on chromosomes 10 and 7, harboring hwd1 and hwd2 loci, respectively. The re gion around hwd2 tended to be differentiated in allelic combination among c ultivars into the indica and japonica rice groups. No such trend was found around the hwd1 region. These facts suggest the hybrid breakdown controlled by this genic system is a rare phenomenon and consequently did not contrib ute to cultivar differentiation of rice.