16. Chromosomal location of one of complementary recessive genes controlling
        hybrid breakdown in rice Shuichi Fukuoka and Kazutoshi Okuno
National Institute of Agrobiological Resources, Kannondai, Tsukuba, 305 Japan     A pair of complementary recessive genes control hybrid breakdown in rice (Okuno 1985). Plants with one or no dominant allele showed weakness; lower plant height and fewer tillers than the normal type. The genotype for this trait in rice varieties can be determined by crossing varieties with a tester line which is homozygous for one of complementary recessive genes which control hybrid breakdowon. Varieties having four dominant alleles produce normal F1 plants. Varieties having two dominant alleles produce weak F1 plants. The genotype for these loci was investigated in 100 Asian rice verieties and the varieties were classified into two groups, varieties having four or two dominant alleles (Okuno 1986). A geogrphical dine was observed in the frequency of the genotypes. Varieties with two dominant alleles were mainly found in Japan, Philippines and Indonesia. The frequency of varieties with four dominant alleles was high in South China, Vietnam, Laos, Thailand and Myanmar.
    A variety with two dominant alleles, Siborunauli1 from Nepal was crossed to a tester with four recessive alleles. The tester line was derived from a cross between Thai variety. Col. No.15 and Japanese variety, Sasanishiki (Okuno 1985). Out of 51 F2 plants, 12 showed normal plant type and 39 showed weak plant type. This fitted the expected ratio of 1 normal: 3 weak. DNA was extracted and RFLP analysis was conducted for all F2 plants to determine the location of the complementary recessive genes.
    One RFLP marker, L169 on chromosome 10 (Kurata et al. 1994) segregated in the same ratio (12:39) as normal : weak plant types. This indicates that one of the complementary loci controlling hybrid breakdown is closely linked to L169 on chromosome 10.

References

Kurata, N. et al., 1994. A 300 kilobase interval genetic map of rice including 883 expressed sequences. Nature
        Genet. 8:365-372.
Okuno, K., 1985. Complementary recessive genes controlling hybrid breakdown found in a varietal cross of
        rice. RGN 2:52-54.
Okuno, K., 1986. Geographical distribution of complementary recessive genes controlling hybrid breakdown of
        rice. RGN 3:44-45.