MAPPING IN THE REALM OF POLYPLOIDY - THE WHEAT MODEL

Wheat is an allopolyploid containing three distinct but genetically related (homoeologous) genomes, A, B and D. Because of polyploid inheritance and large genome size (16x10(12) bp), the wheat genome is thought to be intractable to map-based cloning of agronomic and other genes of interest. We propose a targeted genetic mapping strategy that combines linkage and physical mapping and may facilitate map-based cloning. High-density linkage maps are either generated in wheat or in diploid Triticum tauschii, the donor of the D genome to wheat. Molecular marker-based chromosome maps are constructed, using an array of deletion lines in wheat. The conventional genetic linkage maps are aligned with chromosome maps to construct cytogenetic ladder maps (CLMs). The CLMs allow region-specific mapping and convert genetic distances into physical distances. The information from CLMs suggests that many genes in wheat are present in clusters that are highly recombinogenic, small, and may be amenable to cloning by chromosome walking. Therefore, the effective genome size of wheat is relatively small in comparison to the whole genome. The utility of using the smaller genome of rice for mapping and homologous gene cloning is discussed.