Rice Genetics Newsletter, Vol. 18

Rgn18

27.	Mapping of a gene for F₂ weakness on chromosome 4 of rice
	SOBRIZAL, C.TANAKA, H.YASUI and A.YOSHIMURA Plant Breeding Laboratory, Faculty of Agriculture, Graduate School, Kyushu University, Fukuoka, 812-8581 Japan

Hybrid weakness, frequently observed in F₂, BC₁F₁ and subsequent generations of distantly related crosses of rice, are usually controlled by complementary genes (Oka 1957; Okuno 1985; Kubo and Yoshimura 1999). During the development of Oryza glumaepatula (IRGC 105668) introgression lines with O. sativa cv.Taichung 65 genetic background (Sobrizal et al. 1999), we observed that the weak plants also segregated in BC₄F₂ population.Hybrid weakness is characterized by short plant height, few tillers and short panicles with fertile seeds (Fig. 1).

BC₄F₂ population segregated into 86 normal and 20 weak plants. The segregation pattern was confirmed by a progeny test using BC₄F₃ lines: 36 lines fixed for normal, 50 lines segregated and 20 lines fixed for weakness. The ratios fitted the 3:1 (chi²=2.1; P=0.15) and 1:2:1 (chi²=5.2; P=0.07) in BC₄F₂ and BC₄F₃, respectively, indicating that weakness is controlled by a single recessive gene. This gene was designated as hwf1(t) following the report on hwe1 (Kubo and Yoshimura 1999).

To determine the chromosomal location of hwf1(t), RFLP analysis was conducted using BC₄F₂ population. The parental BC₄F₁ plant of BC₄F₂ population retained O. glumaepatula chromosomal segments on chromosomes 4 and 6 when whole genome survey was performed to select the candidate plants of O. glumaepatula introgression lines (Sobrizal et al. 1999). Link-age analysis between hwf1(t) and RFLP markers on chromosomes 4 and 6 revealed that hwf1(t) was located between RFLP markers G3006 and C933 with map distances of 0.5 cM and 1.9 cM, respectively, and co-segregated with C708, C820 and R288 on chromosome 4 (Fig. 2). The plants carrying the O. glumaepatula homozygous alleles at the C708, C820 and R288 loci showed weakness, indicating that O. glumaepatula has a recessive allele at the hwf1(t) locus.

Hybrid weakness is considered to be controlled by a set of complementary genes since both original parents showed normal appearances. The complementary genes are hwf1(t) from O. glumaepatula and another gene derived from Taichung 65. The complementary gene from Taichung 65 genome is now under investigation.

This study was supported by Bio-oriented Technology Research Advancement Institution (BRAIN), Japan.

References

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Kubo, T. and A. Yoshimura, 1999. Linkage analysis of a gene responsible for F₂ weakness in Japonica/Indica cross of rice. RGN 16: 64-65.

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

Okuno, K., 1985. Complementary recessive genes controlling hybrid breakdown found in a varietal cross of rice. RGN 2: 52-54.

Sobrizal, K. Ikeda, P.L. Sanchez, K. Doi, E.R. Angeles, G.S Khush and A. Yoshimura, 1999. Development of Oryza glumaepatula introgression lines in rice, O. sativa L. RGN 16: 107-108.