A low citric acid trait in cranberry: genetics and molecular mapping of a locus impacting fruit acidity

The fruit of American cranberry (Vaccinium macrocarpon Ait.) is known for its tart, acidic taste. Although some acidity is required for expressing fruit flavor, the high acidity of cranberry requires considerable amounts of “added-sugars” to make cranberry products palatable. Reducing acidity of cranberry would allow for less sugar to be added to cranberry products. In fruit, total acidity is quantified as titratable acidity (TA) in citric acid equivalents and typically ranges from 2.3 to 2.5% in cranberry. This study characterized the genetics of a low acid trait (TA≈1.5%), due to lower citric acid (~ 2 mg/g fresh weight), identified in an undomesticated cranberry germplasm accession. To determine the inheritance of this trait, this accession was used in a series of crosses. The progeny of these crosses exhibited phenotypic segregation consistent with single-gene Mendelian inheritance, with the low citric acid trait being largely recessive. Bulk segregant analysis using simple sequence repeat (SSR) markers identified two SSR markers within < 1 cM that co-segregated with the low citric acid trait. Genotypic variation indicated that the locus, referred to as CITA, was multi-allelic with a semidominant hierarchy of alleles. Further fine mapping, through genotyping-by-sequencing, identified single nucleotide polymorphisms (SNPs) that placed the CITA locus on a distal end of chromosome 1. Competitive allele-specific PCR (KASP) assays generated based on selected SNPs, in combination with the SSR markers identified, will facilitate marker-assisted selection (MAS) for low citric acid phenotypes in cranberry. Utilization of MAS will accelerate the development of reduced acidity cranberry cultivars.