Towards the map-based cloning of Vf: Fine and physical mapping of the Vf region

A map-based cloning scheme is being used to isolate the Vf resistance gene of apple against the fungus Venturia inaequalis. Vf is a major dominant gene that is inherited in a Mendelian manner and influenced by minor genes that modify its activity. The two recently published markers M18 and AL07, bracketing Vf, were tested on 1179 progeny plants of three crosses to fine-map Vf. M18 and AL07 were positioned at 0.2 cM and 1.1 cM from Vf respectively, for a total distance between the two markers of 1.3 cM. Physical mapping by pulsed-field gel electrophoresis, using M18 and AL07 as probes, demonstrated that both markers hybridize to a common 870 kb NotI restriction fragment. We therefore found a relationship between physical and genetic distance of 670 kb/cM in the Vf region. This led us to the conclusion that a chromosome walk using the recently published apple BAC library starting from M18 and AL07 is feasible.