Functional markers as genetic approach to study ethylene production and fruit softening in apple ( Malus * domestica Borkh.)

Functional markers, a new class of molecular marker, are based on the polymorphism present within the gene. We report the allelic variability and allele mining of markers obtained on four genes controlling specific traits. In order to investigate particular aspects of fruit quality, four functional markers for the ACO and ACS genes, which are involved in the ethylene pathway, and for the expansin and polygalacturonase genes, which play a key role in fruit softening, were positioned on the linkage maps of 'Fuji' * 'Mondial Gala' and 'Prima' * 'Fiesta' and evaluated for their involvement in phenotype from the 'Fuji' * 'Braeburn' cross. The four markers are: (1) Md-ACO1 mapped on linkage group 10 on the border of the 5% confidence region of a known QTL for firmness (King et al., 2000); (2) Md-ACS1 mapped on LG15, but it did not appear associated with any known QTL for firmness. Offspring homozygous for allele Md-ACS1-2 always showed low levels of ethylene synthesis and extremely good retention of fruit firmness and shelf-life. (3) Md-EXPDCA1 mapped on L01 at approximately 9 cM of the Vf gene was strongly correlated to a known QTL for crispiness and juiciness; and (4) Md-PG1 also mapped on LG10 and SNP polymorphism were associated with fruit softening.