GEOGRAPHIC-VARIATION IN THE BREEDING SYSTEM AND THE EVOLUTIONARY STABILITY OF TRIOECY IN PACHYCEREUS-PRINGLEI (CACTACEAE)

The Sonoran Desert columnar cactus Pachycereus pringlei has a geograph ically variable, non-hermaphroditic breeding system. It is trioecious (separate males, females and hermaphrodites) in the northern two-third s of its range in Sonora, Mexico, and in the southern three-quarters o f its range in Baja California, Mexico, and is gynodioecious (separate females and hermaphrodites) elsewhere. Trioecy occurs near known mate rnity roosts of its major pollinator, the nectar-feeding bat Leptonyct eris curasoae; gynodioecy occurs >50 km from known bat roosts. The obs erved geographic patterns cannot be explained by limited gene flow or by the geographic distributions of diurnal avian pollinators. Our fiel d observations plus a theoretical analysis suggest that the abundance of chiropteran pollinators plays an important role in the maintenance of trioecy in this plant. Under pollinator limitation, trioecy can be a stable breeding system in this species.