Chemical and molecular ecology of orchid bees (Euglossini)

Chemical signaling in neotropical orchid bees (Hymenoptera: Apidae) is outstanding since males of these bees do not use self-synthesized odors but collect volatile chemicals from flowers and other odoriferous substrates to store them in specialized hind leg pockets. This behavior is the basis of the “euglossine pollination syndrome”, where ~700 orchid species are exclusively pollinated by scent-seeking males. Male orchid bees combine the collected odors to form predictable fragrance blends, which are exposed during pre-mating display and believed to function as pheromone analogues. The present dissertation (based on three publications and three manuscripts) focuses on the species-specificity of male fragrance blends, their potential function in courtship behavior and species isolation, as well as on orchid bee conservation genetics. An analysis (gas chromatography/mass spectrometry) of male fragrances of 15 species of sympatric Euglossa revealed significant chemical distinctness with the greatest disparity between the most closely related species, which is consistent with a role of the fragrances in mate recognition, ensuring premating isolation. Analyses of two morphotypes of Euglossa viridissima Friese in southern Mexico revealed component-specific differences in male fragrances, and also laid open a shift in antennal sensitivity of males of the two forms to the relevant components, suggesting that peripheral olfaction has driven fragrance differentiation and divergence of the two lineages. Further morphological, chemical and genetic characterization led to the description of one of the “morphotypes” as a new species, E. dilemma sp. nov., cryptic sibling to E. viridissima. Paternity analyses of brood of three Euglossa species using microsatellite markers revealed that females of all three species were mated with only a single male, suggesting that monandry is the rule in orchid bees and might have been the ancestral state in corbiculate bees. Monandry is consistent with the idea that orchid bee females choose a single best mate based on its fragrance blend, and that male volatile collection evolves through sexual selection. Concerning conservation genetics I conducted population genetic analyses with microsatellite markers that revealed no or only weak genetic structure among populations of euglossine bees in southern Mexico, suggesting substantial gene flow between them. Habitat fragmentation appears not to have caused loss of allelic diversity within or differentiation between orchid bee populations of eight different Euglossa species. Noticeable genetic differentiation was only found across substantial geographic distances, e.g. between populations on the Yucatan peninsula and those in Veracruz. Finally, a comprehensive analysis of orchid bee males of 27 different species from all over the neotropical region detected extremely low frequencies of (infertile) diploid males, contradicting earlier studies based on allozyme analysis, and rejecting the hypothesis that euglossine bees decline due to the genetic mechanism of Hymenopteran sex determination (haplodiploidy, complementary sex determination).