27. Mapping of a gene for F2 weakness on chromosome 4 of rice

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.


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