APPROACHES TO PHYSICAL MAPPING OF QUALITATIVE AND QUANTITATIVE GENES IN APPLE

Plant Genetics & Biotechnology Dept Horticulture Research International Wellesbourne Warwick CV35 9EF, UK

The European Apple Genome Mapping Project led to development of a detailed genetic linkage map for Malus with good initial coverage of the genome. This provided the basis for assignment of functional loci for a range of important agronomic traits. We are now using the map and populations as a basis for detailed characterisation of genes contributing to previously identified quantitative variation in fruit firmness and crispness. In order to ensure that we develop the most efficient approach to genomic analysis within this species, we are currently carrying out a focused analysis of a single dominant locus conferring aphid resistance. This is providing necessary information on the number of recombinants required, relationship between genetic and physical map distances, and speed of contig development. We have accumulated a comprehensive set of data to enable QTL analysis of fruit texture in apple. Data from different sources (sensory, penetrometer, acoustic resonance, wedge and compression materials testing) have been analysed to provide the necessary confirmation that the QTL detected account for a large and highly significant proportion of the variation. We have decided to focus efforts in one region of the genome. A locus on linkage group 16 contributes to variation in crispness, and appears to be in the same region as the major locus contributing to low pH (acidity). We have carried out initial AFLP analysis based on bulked segregant analysis to provide more markers in this region, prior to contig development. The analysis of a dominant aphid resistance locus has involved identification and screening of additional recombinants from the existing large number of Fiesta and Cox segregants available in mapping and breeding populations. Bulked segregant analysis was successful in identifying new AFLP markers within 1cM of the functional locus, together with 3 existing RFLPs within 1-2cM. Phenotypic analysis with aphids allowed identification of additional recombinants and ordering of markers flanking the gene. We have now initiated contig construction using an apple BAC library obtained from Texas A&M, using these markers as an anchor. These markers and initial BACs identified are also being used to investigate the feasibility of using collinear information from the Arabidopsis physcial map.