Fine Mapping of the Co Locus of Malus x domestica by Molecular Markers

The very compact growth habit of columnar apple tree cultivars and the associated possibility of high density planting is a valuable resource in apple breeding. A single dominant allele of the Co gene, located on chromosome 10, is responsible for the columnar habit. The identification of closely linked molecular markers provides the possibility of developing PCR assays, which can be used in breeding programs as well as for positional cloning approaches. We used the DNA of 100 trees (50 columnar, 50 non-columnar) from the F-1 progeny of a cross between a columnar and a non-columnar cultivar to identify new molecular markers displaying a 100% linkage to the Co allele. The markers were created using sequences obtained from BAC-clones and sequences of the published 'Golden Delicious' genome. In our segregating population (100 individuals tested) the developed markers exhibit up to 100% linkage to the columnar phenotype. These markers allow a fast and efficient PCR-based selection of trees carrying the Co allele.