Comparative Performance of Diploid and Synthetic Tetraploid Populations of Watermelon in the Light of Ploidy Level Changes and Genome Perturbations

Seedless triploid watermelon is produced by crossing the diploid with a tetraploid line, which is a unique ploidy reconstruction that has to be laboriously carried every growing season. In the current study, a segregating F₂ tetraploid population was generated by selfing a F₁ plant of the cross (Citrullus lanatus var. citroides, PI 482252 × Citrullus lanatus var. lanatus, PI 248178), which was treated with 0.2% colchicine and subsequently screened for ploidy levels. Current research is the first report of molecular characterization of segregating synthetic tetraploid mapping populations and QTL analysis in watermelon. The ploidy level of putative tetraploid watermelon plants was determined by flow cytometry using Ploidy Analyzer PA I (Partec, Germany). The lower epidermis was removed from fully-expanded leaf of 3 to 4 week-old seedlings using a fine forceps, transferred to a microscope slide in a drop of water and covered by a cover slip. The number of chloroplasts per guard cell pair was then counted under 400x magnification. In the current research, AFLP fingerprinting was employed to examine a large number of genomic loci across the synthetic tetraploid population and their corresponding parents. The Findgeno subroutine of the TetraploidMap software (Hackett and Luo, 2002) were used to identify the different types of AFLP markers: a dominant marker that were polymorphic across the offsprings present in one parent in single dose (simplex) and double dose (duplex), or present in both parents (double simplex). A genetic linkage map was constructed using JoinMap 3.0 (van Ooijen and Voorrips, 2001). Composite  interval mapping (CIM) was performed to identify the QTLs related to various important traits using Windows QTL cartographer V2.5 (Wang et al., 2007) with model 6 (standard model) and a LOD threshold score of 3.0. Morphological traits such as shape and size of leaf and flower were different between the progenies of tetraploids and diploids. The performance of diploid and tetraploid progenies along with their parents showed tetraploids to be superior for the commercially useful traits such as rind thickness, rind hardness, mesocarp pressure, endocarp pressure, soluble solids, number of fruits per plant and fruit yield per plant. In the current study, out of 10 significant QTLs identified, 5 had strong effects linked to fruit length-diameter ratio, rind harness, mesocarp pressure, endocarp pressure and soluble solids .