Comparative labellum micromorphology of the sexually deceptive temperate orchid genusOphrys: diverse epidermal cell types and multiple origins of structural colour

Abstract

Labellum micromorphology was imaged via scanning electron and light microscopy in 32 microspecies and one artificial hybrid of the European terrestrial orchid genus Ophrys, together representing all ten macrospecies circumscribed in the genus via molecular phylogenetics. Imaging of homologous regions of the adaxial surface, paying particular attention to the diagnostic feature of the comparatively reflective speculum, revealed the presence of between three and seven epidermal cell types on each labellum, the less complex labella being plesiomorphic. Epidermal protuberances range from short, domed papillae to long, twisted unicellular filaments. Multiple origins are inferred for pale labellar margins, large yellow appendices (both putative osmophores exuding pseudopheromones), broad flat labella and long lateral horns. Homoplasy in the speculum is manifested in unusually complex or simple outlines and the presence or absence of a pale margin or iridescence. The reflectivity of the speculum is caused by a combination of chemical and physical colour, whereas iridescence can be caused only by physical properties. The specula of most microspecies studied bear striated trichomes, albeit maturing comparatively late in ontogeny and being sufficiently narrow to allow light to reach the flat polygonal trichome bases. Reflectivity appears to be negatively correlated with the convexity and degree of cuticular corrugation shown by these epidermal cells. Two clades (the Speculum + Tenthredinifera + Bombyliflora group and Bertolonii subgroup of the Sphegodes group) have lost specular trichomes and include the most iridescent species; their flat, polygonal, nonstriated cells resemble those observed on the paired pseudoeyes that bracket the stigmas of all Ophrys except the Fusca group. The smooth thick-layered cuticle and dense layers of organelles and starch bodies revealed by preliminary transmission electron microscopy study provide alternative candidates for the primary reflective surface of the speculum; in contrast, the trichomes and conical cells that dominate Ophrys labella, and occur on the specula of all but the most reflective species, absorb and/or diffuse light. Multiple MYB family genes are hypothesized to control epidermal micromorphology. The relative contributions of olfactory, visual and tactile cues to the sophisticated pseudocopulatory pollination mechanism that characterizes Ophrys remain unclear, but the degree of reproductive isolation achieved, and thus the speciation rate, have certainly been greatly exaggerated by most observers.