The CmACS-7 gene provides sequence variation for development of DNA markers associated with monoecious sex expression in melon (Cucumis melo L.)

Most melon (Cucumis melo L.) breeding lines in South Korea display andromonoecious sex expression, which necessitates laborious hand emasculation during F₁ hybrid seed production. Thus, there is a need to develop monoecious sex types in elite germplasm to obviate self-pollination. Sex expression is associated with floral ethylene production, which, in monecious melon plants, is associated with the A locus. Our study was conducted to develop molecular markers for selection of monoecious plants based on sequence variation inherent in the CmACS-7 gene [encoding 1-aminocyclopropane-1-carboxylic acid synthase (ACS)] that is associated with ethylene production. Full-length CmACS-7 sequences were cloned from a monoecious (MO23) and an andromonoecious (AM24) line. The alignment of those CmACS-7 sequences revealed a single nucleotide polymorphism (SNP; C170T) in exon 1 and an 18 bp indel in the 3′-untranslated region (UTR) of between MO23 and AM24, which was then used to develop a cleaved amplified polymorphic sequence (CAPS) (EX1-C170T) and a sequence characterized amplified region (SCAR) marker (T1ex), respectively. The sex expression and the T1ex SCAR-based genotype of 442 F₂ plants derived from a MO23 × AM24 cross was determined. Monoecy and andromonoecy segregated in a 3:1 ratio in F₂ progeny, where the sex type of 429 plants (13 plants not classified) co-segregated with the SCAR marker, demonstrating that sex expression regulated by CmACS-7 is controlled by a single dominant gene and that it confers monoecy in line MO23. Allelic variation in 112 geographically diverse melon lines for CmACS-7 as accessed by CAPS EX1-C170T and SCAR T1ex markers indicated that the: 1) exon 1 of CmACS-7 is highly conserved and the SNP/sex expression association detected is highly predictable making it potentially useful for marker-based selection of monoecious plants, and; 2) 18 bp indel mutation in the 3′-UTR was present in various lengths depending on different monoecious melon germplasm.