K. IKEDA, SOBRIZAL, P. L SANCHEZ, H. YASUI and
      A. YOSHIMURA

     Plant Breeding Laboratosy, Faculty of Agriculture, Kyushu University, 
     Fukuoka, 812-8581 Japan

     Hybrid weakness, which is one of the reproductive barriers, has been widely observed both in inter- and intra-specific crosses (Chu and Oka 1972; Kubo and Yoshimura 1998; Fukuoka et al. 1998). We also found that the plants showing hybrid weakness were segregated in the backcross population of a cross between 0. glumaepatula and 0. sativa cv. Taichung 65.

     Two sets of 0. glwnaepatula introgression lines with Taichung 65 cytoplasm and with 0. glumaepatula cytoplasm are being constructed using Taichung 65 as a recurrent parent (Sobrizal et a!. 1999). All of the populations in BC4F1 generation with 0. glumaepatula cytoplasm showed the segregation for hybrid weakness when each population consisted of more than 10 plants. The hybrid weakness was characterized by poor growth stature and small and completely sterile panicles (Fig. 1). But no plant for hybrid weakness was observed in BC4F1 generation with Taichung 65 cytoplasm. In addition, the whole genome genotyping using 100 RFLP markers scattering in 12 chromosomes revealed that all of the normal plants segregated in BC3F1 with 0. glumaepatula cytoplasm always carried alleles of 0. glumaepatula at the locus of C1115 on chromosome 8. This tendency was not observed in BC3F, with Taichung 65 cytoplasm (Sobrizal et al. 1999).

     When a BC4F1 plant with 0. glumaepatula cytoplasm which contained the chromosome segment of 0. glumaepatula around the region of GillS and showed nonnal growth stature was crossed with Taichung 65, 28 normal and 29 weak plants segregated in the BC5FI population. This result fit the monogenic segregation of 1: 1, suggesting that the weakness was governed by one dominant gene. On the other hand, tight linkage between ClilS and the hybrid weakness was detected using a small BC4F1 population (41 plants) with 0. Glumaepatula cytoplasm. No recombinant between the locus for weakness and C1115 was obtained.

     Breeding behavior in the development of two sets of 0. glumaepatula introgression lines as well as linkage analysis demonstrated that the weakness was due to the interaction of 0. glumaepatula cytoplasm and Taichung 65 nuclear genome which resides around the locus of GillS. The allele of 0. glumaepatula is dominant and can restore hybrid weakness of the plants having 0. glumaepatula cytoplasm. Since no gene affecting such hybrid weakness restoration has been reported, this gene was designated as Rhw (hybrid weakness restoration for 0. glumaepatula cytoplasm).