CONSTRUCTION OF A BAC CONTIG AROUND THE SCAB RESISTANCE GENE Vf, IN APPLE

¹ Plant Pathology, Institute of Plant Sciences, Swiss Federal Institute of Technology, Universitaetstr. 2, CH-8092 Zürich, Switzerland

² Dipartimento di Colture Arboree, via Filippo Re 8, University of Bologna, I-40126 Bologna, Italy

³ Currently at the Soil and Crop Dept., Texas A&M University, College Station TX 77843-2123 USA

Apple scab is the most important disease occurring in apple orchards. To avoid massive fungicide use, one approach to control the disease is the deployment of resistant cultivars. Scab resistance has been described in several wild Malus species and has been introduced into Malus x domestica by conventional breeding. The most widely used source of resistance to scab is Vf, derived from the selection M. floribunda 821 collected by Crandal in 1926. A single dominant gene controls the resistance. Pathogen attack on Vf-resistant plants does not result in a hypersensitive response, meaning that most probably the underlying resistance mechanism is different from the one hypothesized for other cloned resistance genes. We initiated a positional cloning project that builds on i) an existing linkage map including markers tightly linked to Vf ii) an apple BAC library of a Vf-resistant cultivar and iii) our previous estimation that the physical distance separating the two markers is less than 870kb. Here we report the construction of a 550Kb BAC contig spanning the genomic region containing Vf. Techniques such as the screening of the BAC library, the estimation of the overlap and orientation of the clones, and the clone end-rescue have been optimized. Moreover, the analysis of large segregating populations enabled further narrowing of the region where Vf should be located. The present work is a collaboration between the Plant Pathology group of the Swiss Federal Institute of Technology (ETH) and the Dipartimento di Colture Arboree, University of Bologna. The aim of the collaboration is the identification and characterization of the Vf gene.