Wheat Radiation Hybrids For Functional Genomics and Developing High Resolution Physical Maps For Anchoring Sequence Scaffolds.

Radiation hybrid (RH) mapping, which uses radiation-induced chromosomal breaks rather than genetic recombination to map markers onto chromosomes, is a powerful tool for mapping markers and genes at a much higher and more uniform resolution across the genome, including low recombination regions. In the present study, we developed two RH panels of more than 4,000 lines for wheat D-genome progenitor Aegilops tauschii and reference hexaploid wheat ’Chinese Spring’. Mapping resolution of these RH panels were estimated, using markers of known physical distance, to be less than 140kb. Utilizing a total of 178 lines from Aegilops tauschii RH panel, 610 markers (DArT and SSR) were mapped to the seven D-genome chromosomes, which when compared to the past studies, suggested that RH approach was able to map nearly 10 times more markers than bi-parental genetic populations. These RH maps covered a total of 14,435.1 cR in length with an average distance of 23.7 cR (or 1.7 cM or 8.1Mb) between any two marker loci. The average estimates for Mb/cR was 0.34, while cM/cR it was 0.07, suggesting almost 17 times higher resolution compared to available genetic maps. When RH is used to anchor BAC contigs and sequence scaffolds, we were able to separate markers which co-segregated in genetic maps. Moreover, significantly more sequence scaffolds which could not be anchored to the chromosomes using genetic maps, possibly due to lack of polymorphism, were anchored using RH maps. We have now genotyped ~850 RH lines with a 45K NimbleGen array and the data is being used to construct high-resolution RH-based physical maps that will be used to anchor the BAC based contigs and sequence scaffolds for wheat D-genome assembly. These radiation hybrids are also valuable resources for fine mapping and map based cloning studies of genes present on the D- genome.