Construction of a Cyto-molecular Map of Chestnut (Castanea spp.) Using Physically and Genetically Mapped BACs

The American chestnut (Castanea dentate, 2n = 2x = 24), once known as the “King of the Appalachian Forest” was nearly extirpated by chestnut blight (caused by Cryphonectria parasitica) accidentally introduced to the United States on nursery stock imported in the late 1800s from Asia.  The Chinese chestnut, in contrast to American chestnut, is resistant to chestnut blight and it is being used as a source of resistance in backcross breeding by the American Chestnut Foundation.  In addition collaborators are sequencing the Chinese chestnut genome (J.E. Carlson, Penn State University) and resistance candidate genes are being transformed into American chestnut (W.A. Powell, State University of New York and S.A. Merkle, University of Georgia) for functional testing.  Plant genomes in general contain large amounts of repetitive DNA that interfere with physical map construction and genome sequence assembly.  To overcome these issues we are using genetically and physically mapped BAC clones from all 12 linkage groups as fluorescence in situ hybridization (FISH) probes to directly locate their cytological position on their respective chromosomes. The resulting cyto-molecular map will be essential for completing a reference genome sequence for chestnut and properly locating and characterizing candidate genes.  In addition we are testing the hypothesis that reciprocal translocation polymorphisms exist between or within the two chestnut species, a phenomenon often encountered during interspecific breeding.  One early result shows that BAC contig 3296 on linkage group E is located distal to BAC contig 1231, not proximal as it was positioned on the integrated genetic/physical map. Details of our research results towards these objectives will be presented and discussed.