Quantitative approach of the genetic determinism of sex expression in monoecious hemp (Cannabis sativa L.), and its relationship with flowering phenology and stem and seed yields

The sex expression is a major concern for the genetic improvement of hemp. Although naturally dioecious and bearing heteromorphic sex chromosomes, the species displays a highly variable sexual phenotype. The monoecious cultivars allow the mechanical harvest of both stem and seed; however, their seed production requires a strict control of the sex expression. This thesis investigated the genetic determinism of sex expression in monoecious hemp and established its relationship with flowering phenology and stem and seed yields. The existence of genotypic variability for the sex expression in monoecious hemp was evidenced despite the high environmental sensitivity of the trait. Sex expression and earliness varied mostly consistently among cultivars. Higher seed yields were associated with (mid-) early feminised cultivars. Monoecious hemp was demonstrated to bear XX chromosomes, which showed homologies with both the X and Y ones of dioecious hemp. Three F1 segregating populations were created, and genetic maps were constructed. The distribution of male and female flowers in monoecious plants was modelled. A relatively high number of genomic regions (QTLs) showed variation associated with the model parameters. Some of them were likely located on the sex chromosomes. Closely linked QTLs for sex expression, earliness and yields were found. Their putative implications for breeding were discussed. In conclusion, the present quantitative approach successfully provided fundamental insights to improve our understanding of the genetic determinism of sex expression in monoecious hemp and to support its cultivation for a dual production.