Plant Genome III Conference

APPLE MAPPING: PROGRESS AND GOALS.

Initial maps of two apple cultivars were published last year, and we have been using these maps for further genetic analysis as well as expanding our studies to other crosses. Our most complete map for apple is that for the crabapple 'White Angel', which, because of its interspecific parentage, displays a very high level of heterozygosity. Approximately 350 markers have been placed on this map, arranged in 21 linkage groups (n = 17 for Malus). The map has proven extremely valuable for documenting the genetic basis of numerous characters and locating the genes influencing these characters. In most cases, White Angel appears to be heterozygous for a gene that is dominant to the gene or genes in Rome Beauty. Most apple varieties do not exhibit as much heterozygosity as White Angel, but still possess sufficient heterozygosity to permit a linkage map to be developed using less than 100 random 10- mer primers. Bulked segregant analysis provides an efficient method for identifying markers tightly linked to specific genes of considerable interest to the breeding program. Seedlings can now be screened using DNA markers for resistance to scab, resistance to scald, fruit color, and the columnar growth habit (Wijcik). At least one cluster of scab resistance genes has been identified, indicating that these genes may have a similar structure to clusters of fungal resistance genes found in lettuce, barley and other plant species. These successes have encouraged us to start analyzing fruit quality traits such as acidity, sugar content, fruit size, and total yield. As these are quantitative traits that appear to be controlled by a number of loci, they do not lend themselves to quick improvement through transformation, and the elucidation of the position and function of QTLs affecting these traits should be of particular importance for future breeding programs.