Geographical distribution of genes causing hybrid breakdown in varietal crosses of Asian cultivated rice

Abstract

In a backcrossing program to introduce the wx (glutinous endosperm) gene from a Thai upland rice cultivar, Col.No.15, to a Japanese cultivar, Sasanishiki, of Asian cultivated rice, Oryza sativa L., weak plants were observed in the BC1F1 generation. These weak plants were characterized by poor growth and discoloration at the tillering stage, though they were completely fertile. Hybrid breakdown, which is defined as hybrid weakness and sterility detected in the F2 and later inbred generations of varietal crosses, is controlled by a pair of recessive genes, hwd1 and hwd2, at unlinked loci. Two dominant genes at either the same or different loci, Hwd1/Hwd1 hwd2/hwd2, hwd1/hwd1 Hwd2/Hwd2 or Hwd1/hwd1 Hwd2/hwd2, are needed for normal growth. Using tester lines homozygous for a pair of recessive genes selected in the BC1F3 generation, the genotypes for hybrid breakdown of 100 Asian rice cultivars were determined based on the phenotype of F1 plants. Clinal variation for hybrid breakdown was observed. Cultivars with two dominant alleles at either hwd1 or hwd2 locus, were mainly found in insular Asia (Japan, Philippines and Indonesia), while the frequency of cultivars with four dominant alleles was more common in cultivars from continental Asia. Roles of hybrid breakdown in genetic differentiation of Asian cultivated rice are discussed.