Evolutionary transitions between hermaphroditism and dioecy have occurred numerous times in the land plants. We briefly review the factors thought to be responsible for these transitions, and we provide a synthesis of what has been learned from recent studies of the annual herb Mercurialis annua, in which dioecy (males and females), monoecy (functional hermaphrodites), and androdioecy (males and hermaphrodites) occur in different parts of its geographic range. Previous research on M. annua has revealed the importance of genome duplication and hybridization in the origin of much of the observed variation. Here we show, however, that spatial transitions in the sexual system also occur within the same ploidy level. In particular, we present an analysis, using flow cytometry data, of ploidy variation across a previously unstudied transition between hermaphroditism and androdioecy, in which we find that the sexual-system transition is uncoupled from the shift in ploidy levels. We review recent research that shows that such transitions between sexual systems in M. annua are consistent with differential selection at the regional level for reproductive assurance during colonization. We also present new experimental data that highlight both the importance of the resource status of plants and that of their local mating context in regulating gender strategies and sex ratios. The studies reviewed and the new results presented emphasize the role that shifts in the ecological and genetic context of plant populations may play in causing transitions between sexual systems. © 2008 by The University of Chicago. All rights reserved