A CHROMOSOME REGION-SPECIFIC MAPPING STRATEGY REVEALS GENE-RICH TELOMERIC ENDS IN WHEAT

A strategy is described for rapid chromosome region-specific mapping i n hexaploid wheat (Triticum aestivum L. em. Thell., 2n=6x=42, AABBDD). The method involves allocation of markers to specific chromosome regi ons by deletion mapping and ordering of probes by high resolution gene tic mapping in Triticum tauschii, the D-genome progenitor species. The strategy is demonstrated using 26 chromosome deletion lines for wheat homoeologous group-6. Twenty-five DNA probes from the T. tauschii gen etic linkage map and six wheat homoeologous group-6 specific probes we re mapped on the deletion lines. Twenty-four of the 25 probes from 6D of T. tauschii also mapped on wheat homoeologous group-6 chromosomes, and their linear order in wheat is the same as in T. tauschii. A conse nsus physical map of wheat group-6 was constructed because the linear order and the relative position of the probe loci was the same among t he three group-6 chromosomes. Comparison of the consensus physical map with the genetic map demonstrated that most of the recombination occu rs in the distal ends of the wheat chromosomes. Most of the loci mappe d in the distal regions of the chromosomes. The probes were mostly eit her PstI genomic clones or cDNA clones indicating that the undermethyl ated single-copy sequences are concentrated in the distal ends of the wheat chromosomes. Fifteen loci are uniformly distributed in the dista l 11% of the group-6 chromosomes. Physically, the region spans only 0. 58 mum, which in wheat translates to about 40 Mb of DNA. The average d istance between the markers is, therefore, less than 2.7 Mb and is in the range of PFGE (pulsed-field gel electrophoresis) resolution. Any g ene present in the region can be genetically ordered with respect to t he markers since the average recombination frequency in the region is very high (> 90 cM genetic distance).