Micromorphology of the labellum and structure of the osmophore in a group of closely related species of the sexually deceptive Orchid genus Ophrys (orchidaceae)

Tese de doutoramento, Biologia (Biologia Celular), Universidade de Lisboa, Faculdade de Ciências, 2016

The study of the micromorphology of flowers is crucial to understand the complex process of pollination and the intimate interaction between flowers and pollinators. This plant‐pollinator interaction is a key factor to the diversification and evolution of angiosperms and is particularly tight in species pollinated by sexual deception, almost all of which belonging to the orchid family. The highly specific pollination system of the sexually deceptive species is based on a precise mimicry of the visual, tactile and chemical signals exhibited by the females of the pollinator species (usually a single or a few related insect species), thereby attracting males, which, in most cases, perform copulatory attempts with the flower leading to pollination. In the Euro‐Mediterranean sexually deceptive orchid genus Ophrys, most research has been focused on the chemical mimicry between certain components of the odour bouquet emitted by the flowers and the female sex pheromone of their specific insect pollinators. In most cases, these compounds were shown to be mainly long‐chained hydrocarbons, which are found in the cuticular waxes spread over the surface of the strongly modified median petal typical of orchid flowers – the labellum. However, numerous compounds of higher volatility were shown to act as long‐range attractants for male pollinators, although their site of synthesis and emission (presumably an osmophore) remains unknown. Likewise, little is known about the micromorphological features of the labellum and their importance in providing visual and tactile signals for pollinator attraction. In the present thesis, we investigated the location, structure and secretion of the osmophore and the labellum micromorphology and anatomy of flowers, at three developmental stages, of six closely related Ophrys taxa from natural populations occurring in Portugal, using scanning electron microscopy, light microscopy and histochemistry. We demonstrated the occurrence of an osmophore, which synthesizes a terpene‐rich lipophilic secretion, in the apical region of the labellum in the investigated Ophrys taxa, located mainly at the labellum margin and/or in an apical appendix exhibited by some species. A great diversity of epidermal cell types was found in the adaxial surface of the labellum, which form well‐defined areas with distinct visual and tactile properties. The detailed comparative analysis of these different areas allowed us to establish potential homologies between the labellum areas of the studied taxa. The Ophrys osmophore probably plays an important role in pollination, by synthesizing highly volatile long‐range pollinator attractants, which are possibly complementary to the cuticular waxes spread over the labellum surface for the success of pollination by sexual deception. Findings of the micromorphological and anatomical study were used to build a morphoanatomical data matrix, using objective criteria, as a basis for a morphological phylogenetic analysis of the unresolved clade of Ophrys formed by the six studied taxa, using maximumparsimony and Bayesian inference. Our phylogenetic hypothesis rejects an earlier morphological hypothesis and favours the existing molecular phylogenetic trees based on nuclear ITS data rather than plastid data. Furthermore, the present analysis brought some insights into the floral trait evolutionary trends in Ophrys orchids.

Fundo Social Europeu (FSE), III Quadro Comunitário de Apoio (2000-2006), Programa Operacional Ciência e Inovação 2010 (POCI 2010)