How the gender of a flower or plant is determined is an important question in plant developmental biology. Understanding this process has also practical applications, as the gender of a flower or plant determines how it is bred and cultivated. In particular, one of the remaining challenges in plant breeding is to optimize the production of F1 hybrid seeds in many crops.
F1 hybrid cultivars have made major contributions to the world's food supply. In many cases, hybrid varieties have much superior performance in grain, fruit yield or biomass. The production of F1 hybrid seeds requires controlled pollination to ensure that the seed indeed results from crossing, but not from self-fertilization. To this end, the female parental line that will produce the hybrid seeds should carry strictly female flower to avoid self-pollination. On the other hand, the male line must produce enough pollen. In most crops, producing pure female plants is a challenge that often requires the tedious hand emasculation of bisexual flowers by a skilled workforce. This process is only applicable on a small scale and usually associated with high abortion rates.
The majority of flowering plants are hermaphrodites, producing only bisexual flowers. However, about 10% of species, among which major crops such as maize and cucumber display different sexual morphs. Monoecious species exhibit male and female flowers on the same plant while dioecious species have separate male and female individuals. Gynoecious plants bear only pistillate flowers, androecious ones only staminate flowers. Andromonoecious plants exhibit both male and bisexual flowers.
The Cucurbitaceae family is in the top list for the number of species consumed as human food and used for cultural, medicinal, and botanical purposes. Melon, cucumber, watermelon and zucchini are broadly cultivated in western countries. In total, 34 species are cultivated worldwide. Most of them are monoecious or andromonoecious. Thus, it is not possible to use of two mutually exclusive sexual morphs for F1 hybrid production and the process requires hand emasculation and hand pollination of the female parental line.
The foundation of HybridSeed is the knowledge of how unisexual and hermaphrodite flowers develop, which was built during the previous ERC-funded project, SEXYPARTH. In that research, key genes involved in flower sex determination were identified: CmWIP1, a zinc finger transcription factor that causes gynoecy; CmACS-7, involved in ethylene production and controlling andromonoecy; CmACS11, linked to androecy; and CmHB40, responsible for a second form of andromonoecy.
The HybridSeed project set out to solve major challenges in producing F1 hybrid seeds in cucurbit crops like melon and cucumber. Traditional seed production methods depend on manual flower emasculation and controlled pollination, which are labor-intensive, expensive, and often unreliable due to unstable flower development in parent plants. To address this, HybridSeed developed a new approach using sex determination genes to create female-only (gynoecious), male-only (androecious) and bisexual (hermaphroditic) plants. These modified plants were investigated to ease hybrid seeds production, while maintaining important fruit traits like shape and size.
