Gender dimorphism has evolved at least twice in the genus Lycium (Solanaceae) and occurs in three species that are present in southwestern North America. Plants of L. californicum, L. exsertum, and L. fremontii are either male-sterile (i.e., female) or perfect-flowered (i.e., hermaphroditic) and populations are morphologically gynodioecious. Eleven floral characters were measured on female and hermaphroditic plants for the three North American dimorphic species to characterize sexual dimorphism between flowers on female and hermaphroditic plants. Despite notable shifts in floral size and shape among the dimorphic species, the general pattern of floral dimorphism between females and hermaphrodites was similar for all three species and consistent with the single origin of gender dimorphism in North America. Specifically, flowers on female plants are smaller than on conspecific hermaphrodites and have a long style equal to or slightly exerted from the corolla tube and reduced stamens with abortive anthers. Flowers on hermaphrodites are more flaring than on females, have a style of variable length and long stamens with fertile anthers that are typically exerted from the mouth of the corolla tube. However, the degree of sexual dimorphism between females and hermaphrodites varied among the dimorphic species, suggesting that flowers on females and hermaphrodites of the three species may be specialized for gender function to different degrees. In addition, morphometric measurements for the dimorphic species were compared to five species of cosexual (i.e., hermaphroditic) Lycium. These data, along with the presence of a phylogenetic hypothesis, allowed historical reconstruction of the shifts in floral size and shape across the transition from cosexuality to gender dimorphism.

Key words: floral morphology, gynodioecy, Lycium, sexual dimorphism, Solanaceae