Identification of a Novel Major QTL on Chromosome 5 Controlling Female Flowering Frequency in Watermelon

Female flowering frequency is a critical determinant of fruit yield and concentrated harvest outcomes in watermelon (Citrullus lanatus).Although the genetic mechanisms governing sex forms (e.g., monoecy vs. andromonoecy) are well established, quantitative regulation of the female flower node frequency remains largely underexplored.In this study, we investigated the genetic architecture of the female flowering index (FFI) using an F 2 segregating population derived from a cross between a high-female-flowering-frequency line, 'DAPFF01', and a low-female-floweringfrequency line, 'A0228'.A phenotypic analysis of the F 2 population revealed a continuous distribution with transgressive segregation, indicating that FFI is a quantitative trait controlled by polygenic factors.To identify the genomic regions governing this trait, we employed a combined approach involving bulked segregant analysis sequencing (BSA-seq) and high-density genotypingby-sequencing linkage mapping.We identified a novel major quantitative trait locus (QTL), qFFI5.1,located on chromosome 5 within a physical interval of approximately 294 kb (28.66-28.96Mb), showing a peak logarithm of the odds score of 10.47.This locus is distinct from the previously characterized sex-determination loci on chromosomes 3 and 6.High-resolution melting markers developed for single-nucleotide polymorphisms within the candidate region showed co-segregation rates in the range of 71.9-77.5% with the phenotype, validating the robustness of the QTL.Annotation of the genomic interval revealed putative candidate genes encoding MYB transcription factors and auxin-responsive proteins, suggesting that a regulatory pathway is potentially involved in auxin signaling.These findings provide new insights into the genetic control of flowering habits in watermelon and offer knowledge of valuable molecular markers for marker-assisted selection in breeding high-yield cultivars.