Two genes causing hybrid breakdown were identified. Hybrid breakdown, F_2 sterility, and F_2 weakness were observed in the backcross progeny of a cross between japonica and indica varieties. A set of two genes, hsa1 and hsa2, was responsible for F_2 sterility. The donor parent, IR24, has the recessive sterile alleles teal and the normal alleles hsa2~+. In contrast, the recurrent parent, Asominori, has the dominant normal alleles hsa1~+ and the sterile alleles hsa2. The hybrid progeny with hsalhsalhsa2hsa2 genotype showed high spikelet sterility. Linkage analysis showed that hsa1 and hsa2 were linked to restriction fragment length polymorphism markers G148 on chromosome 12 and G104 on chromosome 8, respectively. Similarly, the F_2 weakness was due toa set of two genes designated as hwe1 and hwe2, which were tightly linked to C443 on chromosome 12 and C955 on chromosome 1, respectively. The genotype of the double recessive homozygous hwelhwelhwe2hwe2 caused hybrid weakness, IR24 has the recessive alleles of hwel, whereas Asominori has the recessive alleles of hwe2. The findings provide a clear evidence of the complementary gene system underlying hybrid breakdown in rice.
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