Diversity of cucumber fruit tip is attributed to the epistatic QTL pairs Ft4.1 and Ft6.1

Background: The shape of fruit tip is critical to appearance quality and commodity value of cucumber ( Cucumis sativus L.). Although diversity of cucumber fruit tip is rich, the genetic basis and molecular mechanism were poorly understood. In this study, the morphological and histological features of fruit tip were investigated to accurately evaluate the diversity in cucumber germplasm. QTLs of fruit tip were genetically mapped by using F 2 and RILs populations. Candidate genes of major-effect QTLs were predicted. Results: The appearance of cucumber fruit tip was generally divided into sharp, oval-round and blunt-round categories, but morphological measurement showed that the angle of fruit tip was continuously distribute in range of 99~173° and the fruit tip index (the ratio of diameter and length of tips) was in range of 1.4~3.5. Histological analysis suggested that the different structures of fruit tips were owed to the different distribution of cell number in vertical and horizontal direction, rather than difference in cell size. In total, thirty fruit tip related QTLs were identified from two mapping populations including a F 2 population derived from EC1 (blunt round tip) × 8419s-1 (sharp tip) and a RILs population derived from IL52 (oval round tip) × CCMC (sharp tip). Two major-effect QTLs were detected, of which Ft4.1 was detected in both mapping population, while Ft6.1 was only identified in F 2 population. The phenotypes of Ft4.1 were epistatically suppressed by dominant Ft6.1 genotype, suggesting that Ft6.1 has epistatic dominance interaction to Ft4.1 . The candidate genes of Ft4.1 were predicted based on genome re-sequencing of parental lines, including CsEXT (Extension-like family protein), CsPRK (leucine-rich repeat protein kinase protein) and CsRNApase (RNA polymerase beta subunit protein), of which CsEXT and CsPRK play important functions during cell division and expansion. Conclusions: Different shapes of fruit tip were owed to the different frequency and orientation of cell division in tip structures. The fruit tip variation in cucumber maybe controlled by the major QTLs Ft4.1 and Ft6.1 through epistatic interaction.