Construction of High-resolution Physical Maps for Large Plant Genomes: Wheat Genomes and Chromosomes

Uneven distribution of recombination and significant variation in genetic to physical distance ratios dramatically affect the capacity to order physical contigs in large portions of chromosomes. Hence, methods for physical mapping, which do not rely on meiotic recombination, are necessary for species with large genomes like wheat (Triticum aestivum L.). Physical mapping using a radiation hybrid (RH) mapping approach has proved valuable in a number of non-plant and plant systems. Our results based on RH maps of several wheat chromosomes and the entire D-genome indicate that an average map resolution of ~ 500 kb can be achieved with populations of relatively small size.  RH maps of various chromosomes indicate cR (physical)/cM (genetic) ratios of 4 - 136:1 or an average ratio of 11:1, effectively allowing separation of genetically linked but physically separate marker loci.  Markers anchored to BAC contigs confirmed the calculated resolution and the potential of RH panels for physical mapping.  We have developed several procedures enabling even quicker means of generating RH panels.  Also, we have tested several high-throughput genotyping platforms that allow rapid assignment of thousands of loci to a physical location in the wheat genome.  This work has now been expanded to include the development of RH panels for various individual chromosomes, in collaboration with the members of IWGSC, as well as for the entire A, B and D genomes, with an objective of developing high-resolution RH based physical maps as a prerequisite to complete genome sequence assembly and for gene cloning in wheat (http://147.49.40.110/RHmapping/).