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

Abstract

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 9 PI (Plant Introduction) 613988] and GMAL 4590 (Royal Gala 9 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 lowacid 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.

Apple fruit acidity is genetically diversified by natural variations in three hierarchical epistatic genes: MdSAUR37, MdPP2CH and MdALMTII

“…Of the four genes in qtl08.3, a 36‐bp insertion 639 bp upstream of the start codon (ATG) of the MdSAUR ‐like gene (MDP0000153382) displayed highly significant distortion of segregation ( P = 1.81 × 10 −20 ) between extremes (high or low) of malate content, so this gene was determined to be a candidate gene in qtl08.3. In addition, MdALMTII (MDP0000252114) in the qtl16.1 region was consistent with a previously reported Ma locus (Xu et al ., ; Ma et al ., 2015b), and the maximum marker segregation distortion ( P = 1.73 × 10 −17 ) between malate content extremities (high or low) was an A/G SNP 2579 bp downstream of the start codon (ATG) in MdALMTII . Therefore, it was also selected as a candidate gene.…”

Identification and validation of a QTL influencing bitter pit symptoms in apple (Malus × domestica)

“…The data reported by Xu et al (2012) on desirable and undesirable alleles for the selection of genotypes heterozygous at the Ma locus could be used in conjunction with the data presented here to enable the ideal haplotypes to be selected from a progeny segregating for both levels of fruit titratable acidity and bitter pit incidence. Selection of seedlings associated with elevated levels of certain polyphenolic compounds in progenies also segregating for bitter pit incidence could prove more problematic, since the two loci are tightly linked.…”

“…As well as presenting data on the genetics of bitter pit incidence, Kumar et al (2013) also reported significant associations between markers in this region and fruit splitting, and to a lesser extent, internal browning and weighted cortical intensity. Maliepaard et al (1998) first reported the mapping of the Ma locus (controlling fruit pH) to the top of LG16 and the characterization of this locus was performed in greater detail by Xu et al (2012), who performed fine mapping of the region and located the locus in a narrow genetic interval between SSR markers Hi22f06 and Hi02h08, equating to the 4.7 cM of genome immediately flanking the QTL interval identified as containing the Bp-2 locus in this investigation. Recently, the genetic analysis of 17 fruit polyphenolic compounds identified QTLs in seven stable clusters, and identified a major locus controlling the content of flavanols catechin, epicatechin and procyanin B2, and five unknown procyanadim oligomers in the fruit that was located in the region of the bitter pit locus (Chagne et al 2012).…”

Molecular-level and trait-level differentiation between the cultivated apple (Malus× domesticaBorkh.) and its main progenitorMalussieversii

“…The profiles of P values, in terms of 2 log 10 (p), for all the tested SNPs for each trait are shown in Fig. explaining 8% variation) on TA resides within the leucoanthocyanidin reductase (LAR1) gene (MDP0000376284) and the next-best SNPs (ss475881686 and GDsnp01588) are located close to the Ma gene (Xu et al, 2012;Khan et al, 2013). Clustering of significant SNPs for OCOL (on LG9) and TA (on LG16) suggested the presence of large-effect quantitative trait locus (QTLs) at these positions.…”