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Geographic differentiation in the pollination system of the columnar cactus Pachycereus pecten -aboriginum¹

²Departamento de Ecología de la Biodiversidad, Instituto de Ecología, Universidad Nacional Autónoma de México, Apartado Postal 70–275 México D.F. C.P. 04510 México; ³Departamento de Ecología de la Biodiversidad, Instituto de Ecología, Universidad Nacional Autónoma de México, Apartado Postal 1354, Hermosillo, Sonora C.P. 83000 México; ⁴Unidad de Biología, Tecnología y Prototipos, FES-Iztacala, Universidad Nacional Autónoma de México, Av. de los Barrios 1 Tlalnepantla, Estado de México C.P. 54090, Apartado Postal 133, México, México; ⁵Centro de Investigaciones sobre Ecosistemas, Universidad Nacional Autónoma de México, Apartado Postal 27–3 (Xangari), Morelia, Michoacán C.P. 58089, México

The pollination biology of the cactus Pachycereus pecten-aboriginum was studied in a tropical location in western Mexico (ca. 18° N latitude) to compare with data from a northern population (ca. 28° N latitude). Throughout this range, the nectar-feeding bat Leptonycteris curasoae is resident within the tropics but migratory in its northern range. The hypothesis was tested that if a predictable bat presence has been an important force in the evolution of pollination systems in columnar cacti, P. pecten-aboriginum will have a specialized pollination system within the tropics and a generalized pollination system in northern populations. In both areas, pollination experiments showed that P. pecten-aboriginum has a self-incompatible, hermaphroditic breeding system. In the tropical area, flowers open at night and close early in the morning. Nectar is secreted only during the night, and flowers are exclusively pollinated by three species of nectar-feeding bats, indicating a specialized pollination system. In contrast, anthesis and nectar secretion in northern populations occur during the night and day, allowing visitation and effective pollination by both nocturnal and diurnal pollinators. This study provides evidence of divergence mediated by pollinator predictability (resident vs. migrant), through shifts from short to long anthesis and nectar production periods from southern to northern populations.

Key Words: Chamela region • columnar cacti • generalization • Leptonycteris curasoae • Pachycereus pecten-aboriginum • pollination biology • Sonoran desert • specialization

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