QTL mapping of parthenocarpic fruit set in North American processing cucumber.

Through a novel phenotyping method, four QTLs were consistently associated with increased parthenocarpic fruit set in North American processing cucumber that accounted for over 75 % of observed phenotypic variation. Parthenocarpy is a desirable trait with potential for increasing yield and quality in processing cucumber production. Although many successful parthenocarpic fresh market cucumber varieties have been developed, the genetic and molecular mechanisms behind parthenocarpic expression in cucumber remain largely unknown. Since parthenocarpy is an important yield component, it is difficult to separate the true parthenocarpic character from other yield related traits. In the present study, we developed a novel phenotypic approach for parthenocarpic fruit set focusing on early fruit development. Two hundred and five F3 families derived from a cross between the highly parthenocarpic line 2A and low parthenocarpic line Gy8 were phenotypically evaluated in three greenhouse experiments. Seven QTLs associated with parthenocarpic fruit set were detected. Among them, one each on chromosomes 5 and 7 (parth5.1 and parth7.1) and two on chromosome 6 (parth6.1 and parth6.2) were consistently identified in all experiments, but their relative contribution to the total phenotypic variation was dependent on plant growth stages. While each of the four QTLs had almost equal contribution to the expression of the trait at commercial harvest stage, parth7.1 played an important role in early parthenocarpic fruit set. The results suggested that parthenocarpic fruit set can be accurately evaluated with as few as 20 nodes of growth. The QTLs identified in this study for parthenocarpic fruit set are a valuable resource for cucumber breeders interested in developing parthenocarpic cultivars and to researchers interested in the genetic and molecular mechanisms of parthenocarpic fruit set.