Final Report Summary - SEXYPARTH (Unraveling sex determination and parthenocarpy mechanisms to improve crops)
How the gender of a flower or plant is determined is an important question in plant-developmental biology. Understanding this process also has practical applications, as the gender of a flower or plant often limits how the plant is bred and cultivated. In melon, floral primordia are initially bisexual with sex determination occurring by the selective developmental arrest of either the stamen or the carpel primordia, resulting in unisexual flowers. This sexual organ arrest is genetically governed by the interplay of alleles of the andromonoecious (M), androecious gene (A) and gynoecious (G) genes. Within the ERC project SEXYPARTH we cloned and characterized M, G and A sex determination genes in different cucurbit species. We have also proposed a mechanistic model in which spatial and temporal expression of the three sex determination genes lead to male female or hermaphrodite flowers. To take forward this work we exploited different combinations of “omics” technologies and genetic screens. Systematic phenotyping of mutant collections and germplasms led to identification of sex determination transition mutants. Cloning of the causal mutations and epistasis analysis led to the identification of new components of the sex determination pathway. In the omics task we set up an approach based on laser capture micro-dissection (LCM) coupled to high depth RNA-seq an epigenetic analysis. Thanks to this LCM approach and analysis of sex transition mutants, we demonstrated that the female promoting gene, A, represses the expression of the male promoting gene G via deposition of H3K27me3. Our results demonstrated that the sex determination genes recruit histone modifiers to orchestrate unisexual flower development. We have also demonstrated that M, G and A functions are conserved at least in Cucumis and Citrulus genera that diverged more than 20 million years ago. In all together our mechanistic model has been considered as a cornerstone by many evolutionists working in the field of sex determination. In 2016 our work on sex determination obtained the prize the best work published in the field of biology from the Magazine La Recherche. To translate the output of the project into breeding traits we generated large mutant collections from melon, cucumber, watermelon, zucchini and bottle gourd. We also developed SENTINEL, software that help to identify induced mutations in large mutant collections. Using the developed tools, we succeeded to validate the function of a list of candidate genes for sex determination. We were also able to engineer new plant sexual morphs for specific breeding applications. The developed tools and resources were shared with the scientific communities.