A bacterial artificial chromosome (BAC) library of Malus floribunda 821 and contig construction for positional cloning of the apple scab resistance gene Vf

The apple scab resistance gene Vf, originating from the wild species Malus floribunda 821, has been incorporated into a wide variety of apple cultivars through a classical breeding program. With the aim of isolating the Vf gene, a bacterial artificial chromosome (BAC) library consisting of 31 584 clones has been constructed from M. floribunda 821. From the analysis of 88 randomly selected BAC clones, the average insert size is estimated at 125 kb. If it is assumed that the genome size of M. floribunda 821 is 769 Mb/haploid, the library represents about 5x haploid genome equivalents. This provides a 99% probability of finding any specific sequence from this library. PCR-based screening of the library has been carried out using eight random genomic sequence-characterized amplified regions (SCARs), chloroplast- and mitochondria-specific SCARs, and 13 high-density Vf-linked SCAR markers. An average of five positive BAC clones per random SCAR has been obtained, whereas less than 1% of BAC clones are derived from the chloroplast or mitochondrial genomes. Most BAC clones identified with Vf-linked SCAR markers are physically linked. Three BAC contigs along the Vf region have been obtained by assembling physically linked BAC clones based on their fingerprints. The overlapping relatedness of BAC clones has been further confirmed by cytogenetic mapping using fiber fluorescence in situ hybridization (fiber-FISH). The M. floribunda 821 BAC library provides a valuable genetic resource not only for map-based cloning of the Vf gene, but also for finding many other important genes for improving the cultivated apple.