45. Frequency and distribution of intragenic recombination at 
      the wx locus in rice

T. Inukai, A. SAKO, KH1N-THIDAR and Y. Sano

Faculty of Agriculture, Hokkaido University, Sapporo, 060-8589 Japan

     It is well known that the frequency of recombination is not constant at the chromosome level in eukaryotes. It was also reported in rice that the genetic distance did not correlate with the physical distance at the chromosome level (Umehara eta!. 1995). Additionally, the recombination frequencies greatly varied between and within loci. For instance, the frequency of intragenic recombination at the bz locus in maize was >100 times higher than the genome’s average (Dooner and Martinez-Ferez 1997). On the other hand, the distribution of recombination within loci was random at the bz and wx loci (Dooner and Martinez-Ferez 1997; Okagaki and Weil 1997), but non-random at the a), b and R loci (reviewed in Puchta and Hohn 1996) in maize. We investigated the frequency and distribution of intragenic recombination at the wx locus in rice.

     We previously reported the recombination map of wx locus using a spontaneous mutant and v-rays or EMS-induced mutants (Sako et al. 1997). Because it was reported that insertion/deletion mutations and single base-pair heterologies could influence the frequency and distribution of intragenic recombination (Dooner and Martinez-Perez 1997), only the EMS-induced mutants derived from the same cultivar and possessing single base-pair substitutions were used for this study. To investigate the frequency and distribution of intragenic recombination, we compared the genetic distances with the physical distances between mutation points of those EMS-induced mutants.

     In order to detect sequence changes in the mutants, about 4 kb DNA segments including the wx coding region were amplified by PCR, cloned and sequenced. From comparisons with sequence of the original cultivar, the location of mutation point in each mutant was estimated as shown in Fig.1. In M8A (M-b in the previous report) the single basepair substitution in exon 13 resulted in nonsense codon. M8E (M-c), M8F and M8J possessed the single base-pair substitutions giving rise to amino acid changes in exon 3, 5 and 2, respectively. The genetical relationships among mutation points were determined by pollen analysis as described in the previous report (Sako et al.1997). Comparing the genetic map with the physical map, it was clearly shown that the genetic distances well correlated with the physical distances (Fig. 1). This indicated that recombination unifonuly occurred within the wx locus of rice. The average frequency of recombination was 27.1 kb/cM, and this value was about 10 times higher than the average recombination frequency in the whole rice genome (250-300 kb/cM; Nakamura et al. 1997).