Differential pollination modes between distant populations of Unonopsis guatterioides (Annonaceae) in Minas Gerais and Amazonas, Brazil

- U. guatterioides is an Annonaceae understory tree pollinated by bees.
- Euglossine bees pollinate Amazonian plants and non-euglossine bees Cerrado plants.
- Amazonian and Cerrado populations differ also in phenology, floral events and size.
- Cerrado plants lack scent compounds present in other euglossine-pollinated Unonopsis.
- Differences in pollination and flower features indicate eventual speciation.

Differentiation in angiosperms is often linked to small changes in floral features including morphology, but also phenology and floral resources, which select different arrays of pollinators and result in disassortative mating. Unonopsis is a bee-pollinated genus in the mostly beetle-pollinated tropical family Annonaceae. We studied Unonopsis guatterioides, a species widespread in South American forests, from the Amazon to São Paulo. A population in Uberlândia, Minas Gerais state, Central Brazil was investigated for its phenology and floral biology, including anthesis, floral scent, pollinator behavior, floral resources, and breeding system. The plants in Uberlândia were bee-pollinated and self-compatible. Its flowers were protogynous, and their anthesis lasted 30 h. Pistillate and staminate stage flowers mainly released monoterpenoids, among them 1,8-cineole and limonene. A comparison with the previously studied population of U. guatterioides in Amazonas state indicated remarkable differences between the two populations, with regard to flower characters, anthesis, and pollination process. There was a more diverse, unspecialized pollination by several small Halictidae and Apidae bee species in Uberlândia, while in Manaus there was a highly specialized pollination by perfume-collecting males of a single orchid bee species, Eulaema bombiformis (Apidae-Euglossini). The profound differences of floral biology in U. guatterioides in Uberlândia and Manaus indicate that these two populations eventually represent different species.