The ecology, evolution, and biogeography of dioecy in the genus Solanum: with paradigms from the strong dioecy in Solanum polygamum, to the unsuspected and cryptic dioecy in Solanum conocarpum

• Premise of the study: Island plants are over-exploited and "under-explored." Understanding the reproductive biology of plants, especially rare species, is fundamental to clarifying their evolution, estimating potential for change, and for creating effective conservation plans. Clarification of sexual systems like dioecy, and unusual manifestations of it in specific studies within Solarium, helps elucidate evolutionary patterns and genetic and ecological control of sex expression. • Methods: Studies of reproductive systems of two Caribbean endemics, S. polygamum and S. conocarpum, combined multifaceted analyses of field populations and of multiple generations of greenhouse plants. • Key results: The dioecy in both species is, like that in other solanums, largely cryptic, although the gender of S. polygamum flowers is obvious. The rare S. conocarpum is recognized as dioecious; floral gender is not obvious. Variation in sex expression facilitated experiments and promoted hypotheses on control and significance of morphological features and sex expression. • Conclusions: Confirmed dioecy in at least 15 solanums is distributed across the genus, with perhaps 6 independent origins, and with crypticity in the form of morphologically hermaphroditic, but functionally unisexual, flowers characterizing all species. Dioecy is not more strongly associated with islands. Inaperturate pollen in pistillate flowers characterizes almost all, but not the two dioecious species studied herein. Dioecy in both species indicates leakiness (rare hermaphroditic flowers on male plants) that helps explain island colonization and radiation. Leakiness allowed confirmation—usually impossible for dioecious species—of self-compatibility for S. polygamum, and thus support for the hypothesis that dioecy evolved to promote outcrossing.