Evolution of Dioecism in Lepechinia Willd. sect. Parviflorae (Lamiaceae)

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Abstract
Species of Lepechinia sect. Parviflorae (Lamiaceae) are gynodioecious and dioecious; the flowers of these species are white, less than 8.0 mm long, and pollinated by short-tongued bees and flies. Species of Lepechinia sects. Salviifoliae and Speciosae that are phylogenetically most closely related are gynodioecious and hermaphroditic; the flowers of these species are blue or violet, 11.540 cm long, and pollinated by long-tongued insects and birds. Hermaphroditic and gynodioecious species have larger corollas than do dioecious species. In the dioecious species of Lepechinia sect. Parviflorae the pistillate flowers have comparatively larger calyces, but smaller corollas than do the staminate flowers. The importance of phylogeny in understanding the relative roles of genetic vs. ecological factors in the evolution of dioecism is discussed. The results of phylogenetic analysis of Lepechinia sects. Parviflorae and Salviifoliae are included. Dioecism appears to have evolved from gynodioecism at least twice. The commonly held idea that gynodioecism is necessarily stable in the Lamiaceae does not apply in Lepechinia. Dioecism also seems correlated with xeric habitats and weediness.