42. Construction of a 300-kb BAC contig containing waxy locus in Japonica rice


1) Faculty of Agriculture, Hokkaido University, Sapporo, 060 Japan
2) National Institute of Agrobiological Resources, Kannon-dai 2-1-2, Ibaraki, 305 Japan

A gene designated as S10 is involved in gamete elimination. It was detected in the backcross line when Wx from PTB10 (Indica) was introduced into the genetic background of T65Hwx (Japonica) (Sano et al. 1994). At the third backcross (BC3), pollen with Wx from the PTB10 was found to be lacking as determined through iodine staining. The elimination of the female gametes carrying PTB10 allele was also observed, since seeds from self pollination showed waxy phenotypes. Based on these results, it was proposed that S10 allele in T65wx eliminated the gametes carrying S10-a allele of PTB10. A recombination frequency between wx and S10 was estimated to be 0.15%. According to this value, S10 is expected to be about 50-kb apart from Wx locus, as lcM in rice genome is assumed to be 300-kb length. We are now attempting an isolation of S10 gene by map based cloning.

A sophisticated cloning method using bacterial artificial chromosome (BAC) provides us a tool to undertake molecular analysis of relatively large size (more than 100kb) of genomic DNA (Zhang et al. 1996: Nakamura et al. 1997). To isolate S10, we constructed 300-kb contig with Wx gene from rice BAC library developed by Nakamura et al. ( 1997). Initially, wx gene was used as a probe, and 7 independent clones were identified (Fig. 1 ). The relative locations of these clones were determined by either the outermost end probes produced by TAIL-PCR or the comparisons of HindIII restriction fragments in each clone. Two secondary probes, the 4104 and the 6189 fragments, were selected from single-copy regions near both the ends of the primary contig. These probes added new fragment in each side of the primary contig. The resultant contig covers a range of about 300-kb including the Wx region. It is large enough to start the effort for the isolation of S10 gene. We are constructing its subclones to make a detailed map for cDNAs prepared from developing anthers in T65wx. Furthermore, we are trying to find recombinants between wx and S10 in a large number of the F2 population from BC3F1 in order to obtain the precise location of S10 gene.


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