Reproductive division into two sexes (i.e. dioecy) is a rare condition in flowering plants. Dioecy in angiosperms is often associated with sexual differences in floral traits other than the sexual organs. In their new study published in AoBP, Brandt et al. focus on the neotropical orchid genus Catasetum, which produce unisexual flowers characterized by remarkable morphological sexual dimorphism. They highlight that this morphological dimorphism is the prerequisite for one of the most unusual and fascinating pollination mechanisms reported in angiosperms: when a male euglossine bee (also known as orchid bees) collects perfume from a male flower of Catasetum, a catapult-like mechanism is triggered. This action results in the attachment of pollen to the bee’s body and its prompt expulsion from the flower. When the same bee subsequently enters into a female flower, again to collect perfume, the pollinen is deposited on the stigma, resulting in pollination. Although the role of floral perfumes of Catasetum in attracting euglossine pollinators is well investigated, questions remain, specifically (1) do male and female flowers of Catasetum orchids differ in floral scent chemistry and emission? And, if so, (2) does sexual dimorphism in floral scents influence the behaviour of male euglossine pollinators?
Brandt et al. addressed these questions by collecting floral scent samples from Catasetum arietinum and analysed these using gas chromatography mass spectroscopy. Their results provide the first documented case of sexual dimorphism reported in orchid floral perfumes. The chemical make-up of floral perfumes differed between male and female flowers, with one compound found exclusively in male flowers and seven compounds found only in female flowers. Female flowers were found to emit more scent than male flowers and the amount of scent emitted from each sex was also found to vary across the different times of sampling. Behavioural observations showed that pollinating Euglossa bees were able to discriminate male from female flowers. The authors discuss the influence of sex-specific floral scents on the behaviour of euglossine pollinators and conclude that their results offer new insights into the ecological and evolutionary significance of divergence in floral scents.
Katharina Brandt completed her Master’s degree at the University of Ulm, Germany. Since this time she has been interested in pollination ecology, paying close attention to olfactory communication within animal-plant interactions. In the framework of her PhD studies, Katharina spent over two years in the Northeast of Brazil investigating the role of floral scents in the Neotropical orchid genus Catasetum and the responses these scents elicit in their male euglossine pollinators. Currently she is finishing her PhD thesis and works as scientific assistant at the Institute of Evolutionary Ecology and Conservation Genomics at the University of Ulm.