The sex-control system involves several mechanisms in melon. The present study identified a novel bisexual flower control gene from the hermaphroditic melon germplasm, different from the previously recognized one. Genetic analysis showed that a single recessive gene in the newly identified locus b controlled the bisexual flower phenotype in melons. We generated 1431 F2 segregating individuals for genetic mapping of locus b, which was delimited to a 47.94 kb region. Six candidate genes were identified in the delimited interval, and candidate No. 4 encoding melon CPR5 protein was selected as the suitable one for locus b and was denoted CmCPR5. CPR5 reportedly interacted with ethylene receptor ETR1 to regulate ethylene signal transduction. Moreover, the ethephon assays showed that the parental lines (unisexual line and bisexual line) had contrasting expression patterns of CmCPR5. The BiFC and LCI assays also confirmed that CmCPR5 interacted with CmETR1 in 0426 but not in Y101. However, crossover tests showed that CmETR1 functioned normally in both parental lines, suggesting CPR5 malfunction in Y101. This study proposed a corollary mechanism of bisexual flower regulation during stamen primordium development in which the inhibition of stamen primordia development was prevented by the malfunctioning CmCPR5, resulting in bisexual flowers.
Keywords: bisexual flower; ethylene; ethylene receptor; melon; unisexual flower.