Identification of QTLs Underlying Pedicel Fruit Retention Force in Sweet Cherry (Prunus avium L.)

Sweet cherry is one of the most labor-intensive fruit crops due to hand harvesting. To enable adoption of mechanical or mechanical-assisted harvest, it is necessary to develop new sweet cherry cultivars with low retention force between pedicel and fruit.  A range of variability for pedicel fruit retention force (PFRF) exists among current cultivars and breeding progenies thus suggesting genetic determination. Therefore an understanding of the genetic systems controlling PFRF would facilitate development of new cultivars with low PFRF that will require minimal labor for picking thereby increasing profitability. A total of 480 pedigree-linked individuals representing the U.S. sweet cherry Crop Reference Set and the Pacific Northwest Sweet Cherry Breeding Program’s (PNWSCBP) Pedigree Set were used in this study. Fruit samples were hand harvested with pedicels on at physiological maturity determined by color, taste (soluble solids content) and firmness. Five largest fruits from each individual were selected for PFRF measurements (in Newton) in the laboratory using a mechanical force gauge (Imada DPS-11, Northbrook, IL) with a custom fitted polyvinyl chloride attachment for fruit detachment. An Infinium array including approximately 150 cherry single nucleotide polymorphism (SNP) markers with sufficient polymorphism was chosen from a total of 6000 SNPs developed by the RosBREED project (www.rosbreed.org) to provide genome-wide markers for determining marker-locus-trait associations. The FlexQTL™ software was used to integrate both the phenotypic and genotypic data to identify QTLs underlying PFRF variation in sweet cherry. We identified two QTLs on linkage group (LG) 4. The first QTL on top of LG 4 mapped between 0.9cM and 2.9cM and was flanked by SNPs RS4.00234975 and RT4.00725298, respectively. The other QTL mapped between 8.7cM and 21.3cM and was flanked by RS4.0218853 and RS4.05334508, respectively. The haplotypes for these QTLs are discussed in relation to breeding for PFRF in sweet cherry.