6. Identification and mapping of two new loci for hybrid breakdown in cultivated rice

S. FUKUOKA¹, M. C. V. NEWINGHAM², M. ISHTIAQ³, T. NAGAMINE⁴, M. KAWASE¹ and K. OKUNO¹

1) National Institute of Agrobiological Sciences, Kannondai, Tsukuba, Ibaraki, Japan

2) The Philippine Rice Research Institute, Nueva Ecija, Philippines

3) National Agricultural Research Centre, Park Road, Islamabad, Pakistan

4) National Agricultural Research Center for Western Region, Nishi Fukatsu, Fukuyama, Hiroshima, Japan

Hybrid breakdown, which is one of the reproductive barriers in cultivated rice, is found in the progeny of cross between Indica and Japonica rice varieties. The genetic basis of this phenomenon can be elucidated by assuming a set of two complementary loci as postulated by Oka (1957). Under this postulate, the parental genotypes would be A₁ A₁a₂a₂ and a₁a₁A₂A₂, respectively, and segregants with one or zero dominant gene, such as A₁A₁a₂a_2, a₁a₁A₂A₂, and a₁a₁a₂a₂, would express the hybrid weakness. Here we report the chromosome location of novel genes controlling hybrid breakdown in Asian cultivated rice using DNA markers.

Japonica rice variety, Sasanisiki was used as a tester to screen hybrid breakdown among Asian cultivated rice varieties. Weak plants with shorter plant height and fewer tillers occurred in the F₂ of crosses with several varieties from Assam, India. One of the Assam accessions, ARC10303 was used for further genetic analysis. The F₁ plants between Sasanishiki and ARC10303 were normal and the segregation ratio in the F₂ fit the expected ratio, 11 normal and 5 weak by the chi-square test. The B₁F₁ plants segregated into 3 normal and 1 weak when Sasanisihiki was backcrossed to the F₁, whereas the segregation ratio in the B₁F₁ did not fit to 3 normal and 1 weak when ARC10303 was backcrossed to the F₁ (Table 1). Analysis of 84 F₂ plants using 98 DNA markers identified the two chromosomal regions where the segregation of the marker allele associated with normal and weak plant-type are located. One is on chromosome 6 and the other is on chromosome 11, difference based on the expected model without contradiction. The alleles from ARC10303 at the locus on chromosome 6 and those from Sasanishiki at the locus on chromosome 11 produced weak plants. These regions differ from the loci for hybrid weakness in the F₁ or F₂ and later generations previously reported (Fukuoka and Okuno 1997, Fukuta et al. 1998, Ichitani et al. 2000, Kubo and Yoshimura 2002). The chromosomal locations of the two loci designated as hwg1(t) and hwg2(t) were determined by linkage analysis of 104 and 99 B₁F₃ plants, respectively. hwg1(t) is located between marker loci RM7193 and C214 at a distance of 3.4 and 2.4 cM on chromosome 6, while the other hwg2(t) is located between marker loci RM206 and RM254 at a distance of 5.1 and 1.1 cM, on chromosome 11 (Fig. 1). DNA marker analysis confirmed that the deviation of the segregation ratio in one of the B₁F₁ populations was due to distorted segregation that was not observed in the F₂ population.

Table 1. Segregation ratio for plant-type observed

S: Sasanishiki, A: ARC10303

Fig.1. Chromosomal locations of hwg1(t) and hwg2(t).

References

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Fukuta, Y., K. Tamura, I. Ashikawa and T. Yagi, 1998. RFLP mapping of hybrid weakness genes controlling the reproductive barrier in a wide cross between Indica and Japonica rice variety. Plant & Animal Genome VI Conference, p. 142.

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Oka, H.I., 1957. Phylogenetic differentiation of cultivated rice, XV. Complementary lethal genes in rice. Jpan. J. Genet. 32: 83-87.