The acidity content of an apple contributes to its delicious taste and overall enjoyment. Two major loci are known to contribute to the bulk of the heritability of acidity levels in apple. The Ma locus on chromosome 16, with its underlying malic acid transporter gene, accounts for ~30% of the phenotypic variance in Washington apple breeding program (WABP) germplasm. The recently discovered “A” locus on chromosome 8, hypothesized to be responsible for malic acid production, explains another ~20% of observed variation. A DNA test is available and in use for the Ma locus (“Ma-indel”) but not for the A locus. At Washington State University, a new DNA test (“LG8A-SSR”) was developed to help differentiate between apple fruit acidity levels. Primers were developed for microsatellites near the “A” QTL peak. From ten sets of primers, several were chosen that matched functional haplotype patterns obtained from RosBREED’s high-resolution SNP data. By pairing outcomes of LG8A-SSR and Ma-indel, differentiation of five levels of acidity could be made, allowing for a more accurate prediction. In combination, these two tests explain > 50% of phenotypic variation within WABP germplasm and were used in the 2014 marker-assisted seedling selection. The predictive power of these two tests should be verified on breeding germplasm of other regions. The double DNA testing strategy creates a powerful predictive tool helping breeders select for the desired acidity levels.
