Distribution and RFLP mapping of complementary genes causing hybrid breakdown in Asian cultivated rice, Oryza sativa L.

Weak plants were found in the BC1F1 generation in a backcrossing program ai med at introducing the wx gene from a Thai cultivar, Col. No. 15, into a Ja panese cultivar, Sasanishiki, in Asian cultivated rice. These weak plants w ere characterized by poor growth and discoloration at the tillering stage, but they were fertile. Hybrid breakdown, which is defined as hybrid weaknes s and sterility detected in the Fz and later inbred generations from variet al crosses, is controlled by a pair of complementary recessive genes, hwd1 and hwd2, at unlinked loci. Two dominant genes at either the same or differ ent loci, Hwd1/Hwd1 hwd2/hwd2, hwd1/hwd1 Hwd2/Hwd2 or Hwd1/hwd1 Hwd2/hwd2 a re needed for normal growth. Using tester lines homozygous for a pair of co mplementary recessive genes selected in the BC1F3, the genotypes for hybrid breakdown of 100 Asian rice cultivars were tested based on the phenotype o f F-1 plants. Clinal variation for hybrid breakdown was observed. Cultivars with 2 dominant alleles at either hwd1 or hwd2 locus, were mainly found in insular Asia (Japan, Philippines and Indonesia), while the frequency of cu ltivars with 4 dominant alleles was more common in cultivars from continent al Asia. Linkage analysis using RFLP markers mapped over 12 rice chromosome s indicated that hwd1 from Col.No.15 was located between RFLP markers, C701 and R2309, on chromosome 10, and hwd2 of Sasanishiki was tightly linked to 4 RFLP markers on chromosome 7. Role of hybrid breakdown in genetic differ entiation of Asian cultivated rice is discussed.