[Sex determination in cucurbits].

Sex determination in plants leads to the development of unisexual flowers from an originally bisexual floral meristem. Cucurbits are not only species of agronomic interest but they also represent model species for the study of plant sex determination, because of their ability to harbor different sexual types. Such sexual forms are controlled by the identity of the alleles at the following loci: andromonoecious (a) and gynoecious (g) in melon, or androecious (a), Female (F), and Monoecious (M) in cucumber. We firstly showed that the andromonoecious a gene in melon encodes for an ACC synthase (CmACS7) and demonstrated that andromonoecy results from a mutation in the active site of the enzyme. Expression of the active enzyme inhibits the development of the male organs and is not required for carpel development. Because the a gene in melon and M gene in cucumber control the same sexual transition, monoecy to andromonoecy, we isolated the andromonoecy M gene in cucumber using a candidate gene approach in combination with genetic and biochemical analysis. We demonstrated the co-segregation of CsACS2, a close ortholog of CmACS7, with the M locus, and showed that the cucumber andromonoecious phenotype is also due to a loss of ACS enzymatic activity. CsACS2 is expressed specifically in carpel primordia of female flowers and should play a similar role to that of CmACS7 in melon in the inhibition of stamina development. Finally, we also showed that the transition from male to female flowers in the gynoecious lines results from epigenetic changes in the promoter of a C(2)H (2) zinc-finger transcription factor, CmWIP1. This epigenetic change is elicited by the insertion of a DNA transposon, which causes the spreading of DNA methylation to the CmWIP1 promoter. Expression of CmWIP1 leads to carpel abortion, resulting in the development of unisexual male flowers. From all these results, we built a model in which CmACS7 and CmWIP1 interact to control the development of male, female and hermaphrodite flowers in melon.