Abstract
The geographical variation of two genes, hwd1 and hwd2, controlling hybrid breakdown (F2 weakness) was surveyed in 239 Asian rice cultivars. The cultivars with the genotype Hwd1/Hwd1 Hwd2/Hwd2 were predominant in South Asia and Southeast Asia, while those with the genotype Hwd1/Hwd1 hwd2/hwd2 were predominant in Japan, the Philippines, and Yunnan province, China. Only one cultivar from Laos had the genotype hwd1/hwd1 Hwd2/Hwd2. Preferential association was found between the presence or absence of hwd2 and the two major cultivar groups (indica and japonica) based on diagnostic RFLP markers. Dominant homozygotes were frequent in indica rice cultivars, while recessive homozygotes were frequent in japonica rice cultivars. This association was not clear for cultivars from the center of genetic diversity of rice, ranging from Vietnam to Bangladesh. Polymorphisms were surveyed for RFLP markers on chromosomes 10 and 7, harboring hwd1 and hwd2 loci, respectively. The region around hwd2 tended to be differentiated in allelic combination among cultivars 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 contribute to cultivar differentiation of rice.
