Fine mapping of ClACS7 gene regulating bisexual flowers in watermelon (Citrullus lanatus L.)

Monoecious and andromonoecious cultivars of watermelon are characterised by the production of male and female flowers, or male and hermaphrodite flowers, respectively. Although the gene responsible for andromonoecy, designated as the a locus, has been identified, its fine-scale genetic mapping remains incomplete. In this study, we built upon our initial mapping of the a locus by performing QTL analysis on F2 populations from spring 2021 (187 plants), and autumn 2021 (137 plants), revealing a stable preliminary region for the a locus on Chr3: 29,252,650–30,021,268 bp. By expanding the ZXG1555 × COS–F2 population to 710 plants, we fine-mapped the a locus to an 82.10 kb interval (Chr3: 29,488,876–29,570,980 bp), which contains five candidate genes. Sequence comparison and transcriptional expression analysis indicated that ClACS7, which encodes 1-aminocyclopropane-1-carboxylate synthase 7, is the most probable candidate gene for this locus. Furthermore, we genotyped 32 natural watermelon populations (2 andromonoecious and 30 monoecious) using the molecular marker Chr03_29539131, which is closely linked to the andromonoecious trait. The results demonstrate that Chr03_29539131 can be effectively used for screening andromonoecy in watermelon and for molecular marker-assisted breeding.