Plant Genome III Conference

APPLE GENOME MAPPING

For most plant geneticists apple would not rank in the top 10 choices for a genome mapping project. However, the obvious complications are at least partly balanced by a high level of heterozygosity present in most varieties and the opportunity to enjoy the fruits of ones labor. The high heterozygosity characteristic of apple genomes pen-nits genetic analyses to be performed on the generation produced by crossing two varieties, approximately nine months after making the cross. The progeny segregates as a double pseudotestcross, so that a linkage map for each parent is generated. These maps can be combined using loci heterozygous in both parents. We have developed such maps for several varieties, each map possessing over 200 segregating markers. Our first maps were developed for the crabapple White Angel' and the well known variety 'Rome Beauty.' In this analysis linkage groups were anchored by segregating isozyme or RFLP markers and filled in using RAPDS. As loci are expected to segregate 1: 1, the phenotype of the marker usually reflected the genotype and the dominant nature of the RAPD markers was not a problem. Although well over 500 RAPDs have been mapped in this cross, relatively few (<10%) were heterozygous in both parents. A much greater fraction (20-30%) are heterozygous in both parents in crosses within the cultivated apple germplasm. The breeding program at Cornell is pursuing a number of specific goals in which molecular markers have a significant role. We have used the linkage map for apple, as well as other techniques such as bulk segregant analysis and the conversion of RAPD markers to SCARs to develop convenient DNA tags for fruit color, scab resistance, columnar habit, and powdery mildew resistance. We have been able to arrive at a relatively good understanding of the genes affecting branching habit, and have identified some of the genes influencing initial bud break, flowering time, scald resistance, and time of maturity. We are very optimistic about the possibility of dissecting the genetics of fruit quality traits and other characters of interest to breeders, as well as eventually cloning specific genes coding for disease resistance.