Genetic diversity of the MQ8 locus, a major QTL conferring apple fruit acidity
Acidity is a major contributor to apple fruit quality. Apples with titratable acidity (TA) of 0.30-1.00% are acceptable for flesh consumption, whereas those with a TA beyond this range are either too tart or flat. Breeding apple varieties of optimum acidity levels is challenging as seedlings bearing low acid apples often account for >25% in the breeding populations. Malic acid is the predominant organic acid in mature fruit and largely determines acidity levels. The major QTLs (quantitative trait loci) that control fruit acidity are Ma (malic acid) and MQ8 (Malic QTL8), which are located on chromosome 16 and 8, respectively. The Ma QTL has been recently isolated and characterized, but MQ8 remains to be understood. The difficulty has been that the effect of MQ8 is detectable only in ‘Fiesta’, an uncommon apple variety not available in the USDA’s Malus collection. To identify the gene(s) underlying MQ8, a population genomics approach is taken to investigate the genomic region harboring MQ8. DNA was isolated from 470 Malus accessions in two sets and was fingerprinted with two markers linked to MQ8. The data reveals a tremendous diversity in these Malus accessions. Interestingly, there are 16 apple accessions of identical genetic patterns as ‘Fiesta’.