Strategies for map-based cloning in apple

The apple, Malus × domestica Borkh, is one of the most important temperate fruit crops grown around the world. Although the apple is a diploid (2n = 34), it originated as an allopolyploid and has a relatively small genome size of 750 Mb/haploid. The apple is self-incompatible, highly heterozygous, and displays a juvenile period of 6 to 10 years or more. These characteristics contribute to long-term and difficult efforts to breed new cultivars of apple. However in the past 15 years, significant advances have been made to develop molecular markers linked to traits of economic interest for apple. In turn, these molecular markers have been used to map, clone, and isolate genes. Efforts to pursue map-based cloning in apple have been quite successful in identifying and characterizing target genes, particularly those controlling disease and pest resistance. Strategies of chromosome walking and chromosome landing have provided viable approaches to locate, isolate, and clone some of these genes. In this review, we provide detailed information for these strategies and their use to clone some of these target genes controlling qualitative traits.