Genetic diversity as gene source is very important for crop breeding. To utilize genetic diversity efficiently for rice breeding, (Kojima et al. 2005) developed the core collection (WRC) in rice which included ca. 90% of RFLP alleles and ca. 80% of phenotypic variation in the 332 accessions selected randomly. However, it is very important to reveal genetic characteristics of WRC lines, especially reproductive barriers such as hybrid sterility, hybrid weakness and hybrid breakdown. Further, presence of gametophyte genes which are also one of the reproductive barriers (Harushima et al. 2001, 2002) should be examined in WRC lines, because frequency of interest phenotype is reduced by linkage with the gametophyte gene.

Harushima et al. (2001) reported that highly effective factor for segregation distortion of RFLP allele in F₂s of remote crosses was located on the short arm of chromosome 3. We tried to reveal the genetic characteristics for segregation distortion on the short arm of chromosome 3 in WRC lines, using T-65 pyl-stb with pyl gene (Tsugane et al. 2006). In this study, 53 lines out of 68 WRC lines (Kojima et al. 2005) were crossed to T-65 pyl-stb as a female and spikelet fertilities in F₁ plants of the crosses were examined. Segregation of pyl plants in F₂ of each cross was examined at seedling stage. Fig. 1 shows the relationship between spikelet fertility in F₁ plants and segregation frequency of pyl plants in F₂. Most of F₁ plants showed 50 % or higher spikelet fertility, except for two cross combination with extremely low fertility.

Segregation frequencies of pyl plants in 53 F₂ populations varied from 5% to 45% as shown in Fig. 1. Out of 53 F2 populations 29 F2s showed normal segregation as indicated in closed dotted line in Fig. 1. On the other hand, 10 and 14 F2s showed significantly increased and decreased segregation frequency, respectively. Since segregation of pyl plants fitted to 3:1 segregation ratio well in F2 of T-65 pyl-stb x T-65, segregation of pyl gene was normal. These results suggested that WRC lines carry factor(s) for segregation distortion of pyl gene located on the short arm of chromosome 3. Although crosses using indica type showed wide range of segregation frequency of pyl plants, 5 out of 10 crosses using tropical japonica type showed significantly increased segregation type. Harushima et al. (2001) reported that the most effective male-gametophytic factor was located on the short arm of chromosome 3, the segregation distortions of pyl plants were possibly caused by the same factor as Harushima et al. (2001) reported.

Although it is not clear that one locus or multi loci for male gametophytic factor is responsible for varied segregation frequencies of pyl plants with cross combinations, it was concluded that each WRC line carries diversified segregation distortion gene(s) on the short arm of chromosome 3.

References

Harushima Y., M. Nakagahra, M. Yano, T. Sasaki and N. Kurata, 2001. A genome-wide survey of reproductive barriers in an intraspecific hybrid. Genet. 159: 883-892.

Harushima Y., M. Nakagahra, M. Yano, T. Sasaki and N. Kurata, 2002. Diverse variation of reproductive barriers in three intraspecific rice crosses. Genet. 160: 313-22.

Kojima Y., K. Ebana, S. Fukuoka, T. Nagamine and M. Kawase, 2005. Development of an RFLP-based Rice Diversity Research Set of Germplasm. Breeding Sci. 55: 431-440.

Tsugane K., M. Maekawa, K. Takagi, H. Takahara, Q. Qian, C. H. Eun and S. Iida, 2006. An active DNA transposon nDart causing leaf variegation and mutable dwarfism and its related elements in rice. The Plant J. 45: 46-57.