According to the Food and Agriculture Organization, China was the largest melon producer and consumer, accounting for 50.04% of the total production worldwide in 2014 [as cited in Sun et al. (2017)]. Compact or dwarf crops have great agricultural advantages, including greater planting density and resistance to storm and lodging, and reduced losses during harvesting operations (Amasino et al., 2003).
Many dwarf crop traits, including short internodes, short MSL, and bush-type growth habits, have been reported in watermelon [Citrullus lanatus (Thumb.)]. In watermelon, two allelic genes (dw-1 and dw-1s) and three independent loci (dw-2, dw-3, and dw-4) have been shown to control dwarfism (Huang et al., 1998; Loy and Liu, 1972; Yang et al., 2010). The recessive gene dw-1 was discovered in the first dwarf watermelon that was mutated from the standard vining watermelon inbred line ‘WB-2’ (Mohr, 1956), which has short internodes formed by abnormal internode cells. The recessive gene dw-2 was identified in the short vining variety “Bush Desert King,” which has substandard internodes. Subsequently, the simply inherited recessive gene dw-1s was detected in the short vining watermelon variety “Somali Local” (Dyutin and Afanas’eva, 1987), whereas dw-4 in the short vining habit was identified in the self-pollinated population of the ‘5-6y’ watermelon (Yang et al., 2010). Furthermore, a potential dwarf candidate gene has been mapped to chromosome 7 and named Cla010726 (Dong et al., 2018). In cucumber, cp and cp2, and dw gene expression have been reported to be responsible for compact and dwarf-type cucumber, respectively (Robinson and Mishanec, 1965; Kauffman and Lower, 1976). A recent study revealed a gene association with short vining habit on chromosome 4 of the cucumber; this gene is highly homologous with cytokinin oxidase (CKX) (Li et al., 2011). A dwarf gene was mapped a dwarf gene to a 2.29 cM with interval using random amplified polymorphic DNA markers in near-isogenic lines (NILs) derived from the pumpkin Ai10 variety (Li et al., 2007). In melon, three recessive loci—si-1, si-2, and si-3—were reported to control compactness or dwarfism in ‘UC Top Mark bush’, ‘Persia202’, and ‘Maindwarf’, respectively (Knavel, 1988, 1990; Paris et al., 1984). Furthermore, Hwang et al. (2014) identified the dwarf gene mdw1 of the melon ‘PNU-WT1’ dwarf mutant on chromosome 7, which is highly homologous with CKX.
Bulked-segregant analysis (BSA) is an advanced method to identify markers rapidly that are linked tightly to causal genes underlying a given phenotype. In 1991, BSA was used for the first time to map disease-resistant genes (Michelmore et al., 1991). Since then, BSA has been widely used for rapid gene mapping and identification of molecular markers linked to a quantitative or qualitative trait of interest in horticultural crops, including watermelon (Dong et al., 2018), melon (Li et al., 2017), cucumber (Zhang et al., 2015), and tomato (Zhao et al., 2016). In our study, we used a combination of BSA and whole-genome resequencing to characterize the molecular inheritance of dwarfing genes associated with the diminutive plant habit of the melon line ‘X090’, and developed molecular markers for marker-assisted breeding of dwarf melon cultivars in China.
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