Genetic characterization of the Ma locus with pH and titratable acidity in apple

Apple fruit flavor is greatly affected by the level of malic acid, which is the major organic acid in mature apple fruit. To understand the genetic and molecular basis of apple fruit acidity, fruit juice pH and/or titratable acidity (TA) were measured in two half-sib populations GMAL 4595 [Royal Gala x PI (Plant Introduction) 613988] and GMAL 4590 (Royal Gala x PI 613971) of 438 trees in total. The maternal parent Royal Gala is a commercial variety and the paternal parents are two M. sieversii (the progenitor species of domestic apple) elite accessions. The low-acid trait segregates recessively and the overall acidity variations in the two populations were primarily controlled by the Ma (malic acid) locus, a major gene discovered in the 1950s (Nybom in Hereditas 45:332-350, 1959) and later mapped to linkage group 16 (Maliepaard et al. in Theor Appl Genet 97:60-73, 1998). The allele Ma has a strong additive effect in increasing fruit acidity and is incompletely dominant over ma. QTL (quantitative trait locus) analyses in GMAL 4595 mapped the major QTL Ma in both Royal Gala and PI 613988, the effects of which explained 17.0-42.3% of the variation in fruit pH and TA. In addition, two minor QTL, tentatively designated M2 and M3, were also detected for fruit acidity, with M2 on linkage group 6 of Royal Gala and M3 on linkage group 1 of PI 613988. By exploring the genome sequences of apple, eight new simple sequence repeat markers tightly linked to Ma were developed, leading to construction of a fine genetic map of the Ma locus that defines it to a physical region no larger than 150 kb in the Golden Delicious genome.