Pollination biology of the columnar cactus Pachycereus pecten-aboriginum in north-western México

Columnarcacti in tropical deserts depend on nectar-feeding bats for their reproduction while species from extra-tropical deserts show a relatively generalized pollination system with both nocturnal and diurnal pollinators. Pachycereus pecten-aboriginum is a columnarcactus with a broad distribution along the Pacific coast of México, from Oaxaca to Sonora. Along its distribution, the nectar-feeding bat, Leptonycteris curasoae, changes from resident within the tropics to migratory in the Sonoran desert. If bat unpredictability has been an important force in the evolution of pollination systems in columnarcacti, P. pecten-aboriginum is expected to show a relatively generalized system in northern populations. We studied the pollinationbiology of P. pecten-aboriginum in two northern populations in the state of Sonora. Hand pollination experiments showed that this species has a self-incompatible, hermaphroditic breeding system. Although flowers open at night, they remain open and continue secreting nectar during the morning, allowing visitation by both nocturnal and diurnal pollinators. One population showed evidence of strong pollinator limitation while the results from both populations indicated that diurnal pollinators are more important than nocturnal pollinators. These results are discussed in terms of specialization vs. generalization in the pollinationbiology of columnarcacti in tropical and extra-tropical deserts.