Summarized by: Josie Russo
Stephanie Kay Fong, Joseph Kawash, Yifei Wang, Jennifer Johnson-Cicalese, James Polashock, and Nicholi Vorsa
Tree Genetics & Genomes 17, 4 (2021). https://doi.org/10.1007/s11295-020-01482-8
- Commercial cranberry cultivars have high concentrations of acids, about five times that of fruit eaten fresh, that contribute to high titratable acidity (TA)—or tartness within the fruit.
- To offset the tartness for consumers, higher amounts of sugar need to be added in the cranberry products, such as sweetened-dried-cranberries and juices. The added sugar has deterred consumers from increasing cranberry product consumption despite cranberry health benefits.
- Within the cranberry germplasm collection at Rutgers University, a unique cranberry plant was identified with fruit having a lower percent TA than is normally found in commercial cultivars and may offer cranberry products with less added sugar.
- Crosses using this plant identified a naturally occurring genetic trait that produced fruit with a lower malic acid, which impacts TA.
- Utilizing these populations—segregating for the low malic acid traits—it was possible to generate effective genetic markers for use in breeding of cranberry cultivars with TA.
- Locus: fixed position on a chromosome where a genetic marker is located
- Allele: one of two or more versions of a gene
- Heterozygous: two different versions of a gene present in an individual.
- Homozygous: two copies of the same version of a gene in an individual
- Cranberries are known for their health benefits, but due to the fruit’s relative tartness, most cranberry products typically contain up to 40% added sugars (Ocean Spray Cranberries Inc. 2019).
- Cranberry fruit has three organic acids that contribute to tartness or TA:
- malic (MA)
- citric (CA)
- quinic acid (QA)—though to a lesser extent
- Modeling MA in peach showed that both MA and CA are significant contributors to TA (Lobit et al. 2002).
- A germplasm accession was collected from a native cranberry population in Suffolk County, NY which exhibited lower MA in a subsequent germplasm screen.
- If TA is affected by the concentration of MA, then reducing the latter will reduce overall acidity/tartness in cranberry fruit, thus reducing the amount of added sugar needed in cranberry products.
Objectives of this Study
- As the first characterization of the genetics of a qualitative low MA trait in cranberry fruit, the objectives of this study were to:
- describe the inheritance of the mala allele (a low MA allele), its effect on TA and MA, and its relationship with CA and QA.
- identify, develop, and validate molecular markers that could be used to select for the low MA trait.
- determine the effect of the mala among other cranberry genotypes that have the cita allele (a low CA allele)
- A germplasm accession, NJ93-57, was collected and exhibited lower MA in a subsequent germplasm screen.
- An initial cross, of the germplasm accession with cv. Mullica Queen (MQ), was made in 2004 to generate the CNJ04-52 population (Fig. 1). Between 2004 and 2012, various crosses were made to produce varying populations.
- Crosses were made manually during April and May in the greenhouse during 2004–2012.
- Seedlings were maintained in the greenhouse and in the field for at least three years.
- 2017 and 2018
- During the spring and summer, flowering plants were taken outside for bee open pollination.
- Fruits were collected from each individual in each population once a year during late August and September in 2014–2018 for analysis of TA and organic acids with HPLC .
- Leaf tissue was collected in the spring of 2017 for DNA extractions.
- This study characterized a low MA trait in cranberry fruit derived from a native germplasm accession (NJ93-57) and determined its interaction with genotypes at the CITA locus.
- NJ93-57 was determined to be heterozygous (Mala/mala). In an F2 population, derived from an F1 Mullica Queen × NJ93-57, progeny with a MA concentration as low as ~ 2 mg/g were recovered (compared to the typical 6–8 mg/g range).
- This progeny has the lowest level MA phenotype reported in cranberry and is a result of a mala/mala homozygous genotype.
- The high correlation of MA levels across years indicates a strong qualitative genetic effect of the mala/mala genotype.
- However, significant differences in MA and TA between populations with the low MA trait, indicate general genetic background also influences acidity to some extent.
- To characterize and map the low MA trait, mala, three populations (119 unique individuals) were phenotyped and genotyped. The three populations segregated for the low MA trait (~ 2 mg/g FW) consistent with a single, co-dominant gene in a Mendelian pattern and we named the locus MALA.
- The mala allele had a significant effect on QA and CA as well as TA.
- Although the mala/mala genotype yields TA < 1%, there is a caveat, as the homozygous mala plants have a dwarf-like growth habit which is likely not commercially viable.
- A low MA trait was characterized, and a genetic locus was identified that presents a decreased TA to below 1%—within the range of fruit that is consumed fresh (Kallio et al. 2000).
- Other than variation in acidity, there were no other apparent fruit quality traits affected.
- Markers have been identified and are being developed for genetic screening, including screening the germplasm collection for low MA.
- Identifying the genes controlling CA and MA accumulation would allow greater ability for marker-assisted selection and gene editing applications in the future. This will allow seedlings of no value to be culled—saving space, time, and money.
- Continued work will contribute to breeding efforts to develop commercially viable cranberry cultivars that require less added sugar.