Extended physical maps and a consensus physical map of the homoeologous group-6 chromosomes of wheat (Triticum aestivum L-em Thell.)

Extended physical maps of chromosomes 6A, 6B and 6D of common wheat (Tritic um aestivum L. em Thell., 2n=6x=42, AABBDD) were constructed with 107 DNA c lones and 45 homoeologous group-6 deletion lines. Two-hundred and ten RFLP loci were mapped, including three orthologous loci with each of 34 clones, two orthologous loci with each of 31 clones, one locus with 40 clones, two paralogous loci with one clone, and four loci, including three orthologs an d one paralog, with one clone. Fifty five, 74 and 81 loci were mapped in 6A , 6B and 6D, respectively. The linear orders of the mapped orthologous loci in 6A, 6B and 6D appear to be identical and 65 loci were placed on a group -6 consensus physical map. Comparison of the consensus physical map with ei ght linkage maps of homoeologous group-6 chromosomes from six Triticeae spe cies disclosed that the linear orders of the loci on the maps are largely, if not entirely, conserved. The relative distributions of loci on the physi cal and linkage maps differ markedly, however. On most of the linkage maps, the loci are either distributed relatively evenly or clustered around the centromere. In contrast, approximately 90% of the loci on the three physica l maps are located either in the distal one-half or the distal two-thirds o f the six chromosome arms and most of the loci are clustered in two or thre e segments in each chromosome.