Flowering Time in Watermelon Is Associated with a Major Quantitative Trait Locus on Chromosome 3

Abstract

U.S. watermelon (Citrullus lanatus) production is worth ≈$0.5 billion annually to growers and nearly all of them are dependent on reliable synchronized flowering time of triploid cultivars and diploid pollenizers in their production fields. One aspect of this synchronization is time to flowering, the change from the vegetative to reproductive phase of a plant. Flowering time has emerged as one of the key traits in horticultural and agronomic crops to breed for escape from biotic and abiotic stresses. However, very little is known about the control of flowering time in watermelon. The number of genes involved, mode of inheritance, heritability, and the possible candidate genes are all unknown. In this study, quantitative trait loci (QTL) associated with days to first male flower (DMF), days to first female flower (DFF), and the female-male flower interval (FMI) were identified in a ‘Klondike Black Seeded’ × ‘New Hampshire Midget’ recombinant inbred line population over 2 years. Heritability for DMF, DFF, and FMI were 0.43, 0.23, and 0.10, respectively. Control of flowering time was oligogenic with a major, stable, colocalized QTL on chromosome 3 responsible for ≈50% of the phenotypic variation observed for DMF and DFF. This region of the draft genome sequence contains 172 genes, including homologs of the flowering locus T (Cla009504) and tempranillo 1 (Cla000855) genes associated with flowering time in other species. Cla009504 and Cla000855 represent excellent candidate genes toward the development of a functional marker for marker-assisted selection of flowering time in watermelon. In addition to the major QTL on chromosome 3, two other QTL were identified for DMF (chromosomes 2 and 3) and DFF (chromosomes 3 and 11) and one for FMI on chromosome 2. Understanding the genes involved in this trait and the ability to select efficiently for flowering time phenotypes is expected to accelerate the development of new watermelon cultivars in changing environmental conditions.