Mining the cucumber core collection: phenotypic and genetic characterization of morphological diversity for fruit quality characteristics

Commercial value of cucumber is primarily driven by fruit quality. However, breeding goals frequently focus on production constraints caused by biotic and abiotic stresses. As sources of resistances are often present in unadapted germplasm, we sought to provide morphological and genetic information characterizing the diversity of fruit quality traits present in the CucCAP cucumber core collection. These 388 accessions representing >96% of the genetic diversity for cucumber present in the US National Plant Germplasm System harbor important sources of resistances and extensive morphological diversity. Data were collected for skin color, length/diameter ratio (L/D), tapering, curvature, and spine density for young fruits [5-7 days postpollination (dpp)], and length, diameter, L/D, skin color, netting, seed cavity size, flesh thickness, hollowness, and flesh color for mature fruits (30-40 dpp). Significant associations of single nucleotide polymorphisms (SNPs) with each trait were identified from genome-wide association studies. In several cases, quantitative trait loci (QTL) for highly correlated traits were closely clustered. Principal component analysis, driven primarily by the highly correlated traits of fruit length, young and mature L/D ratios, and curvature showed a clear divergence of East Asian accessions. Significant SNPs contributing to the longest fruits, including development-stage specific QTL, were distributed across multiple chromosomes, indicating broad genomic effects of selection. Many of the SNPs identified for the various morphological traits were in close vicinity to previously identified fruit trait QTL and candidate genes, while several novel genes potentially important for these traits were also identified.