One Hundred Ways to Invent the Sexes: Theoretical and Observed Paths to Dioecy in Plants

Dioecy, the presence of male and female flowers on separate individuals, is both widespread and uncommon within flowering plants, with only a few percent of dioecious species spread across most major phylogenetic taxa. It is therefore safe to assume that dioecy evolved independently in these different groups, which allows us to ask questions regarding the molecular and developmental mechanisms underlying these independent transitions to dioecy. We start this review by examining the problem from the standpoint of a genetic engineer trying to develop dioecy, discuss various potential solutions, and compare them to models proposed in the past and based on genetic and evolutionary considerations. Next, we present recent information regarding candidate sex determinants in three species, acquired using newly established genomic approaches. Although such specific information is still scarce, it is slowly becoming apparent that various genes or pathways can be altered to evolve dioecy.