Science 350, 688–691 (2015)

Most angiosperms produce bisexual flowers, but some species have male and female flowers, either on the same or separate individuals. A newly identified gene responsible for determining the sex of cucumber and melon flowers may show how unisexual flowers and dioecious species develop.

Abdelhafid Bendahmane of the Institut National de la Recherche Agronomique, France, and colleagues identified a gene, ACS11, whose loss in cucumbers and melons resulted in the production of only male flowers. ACS11 encodes a rate-limiting enzyme for ethylene biosynthesis, and the mutant phenotype could be rescued by exogenously supplying ethylene. ACS11 is specifically expressed in the buds and vascular bundles of female flowers, suggesting that ACS11-mediated ethylene production is needed for female flower development.

ACS11 also represses the carpel inhibitor, WIP1, which in turn represses the stamen inhibitor, ACS7. The lack of ACS11 results in exclusively male flowers by allowing WIP1 to directly prevent carpel formation and indirectly stimulate stamens. Expression of ACS11 releases the block on carpel development while restoring the suppression of stamens, thereby leading to female flowers. Disrupting the function of ACS7 allows both carpels and stamens to develop, producing hermaphroditic flowers.

The researchers further created a dioecious mating system by crossing two individuals with different allelic combinations of ACS11 and WIP1, demonstrating a possible path for dioecy to evolve from monoecy.