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Herbert Braunschmid (Salzburg University)

Herbert Braunschmid's Master thesis was awarded with the Prize for the Best Plant Science Master Thesis, which was carried out at Salzburg University in the year 2016.

Title of the awarded thesis

"Divergent floral traits and pollinators of Cypripedium calceolus L."

The work analyses whether divergent floral traits of the lady's-slipper orchid (Cypripedium calceolus) along an altitudinal gradient are driven by pollinator mediated selection.

Background and Aims
Studies of floral adaptations in response to divergent pollinators are important for understanding floral evolution and the speciation of plants. Cypripedium calceolus is a lady's-slipper orchid found from near sea level to altitudes of 2500 meter. Its successful pollination depends on small bees and flies temporarily trapped in the labellum and finally escaping the slippery cavern via an exit of similar height as the insect itself. Thereby, insects pass the stigma and the anthers to deposit and collect the pollen, respectively. This raises the question whether the flowers’ size vary across altitude and might be adapted to the available pollinators, which might vary in size at different altitudes too. Comparison of vegetative and floral traits gives hint whether diversification is pollinator driven or a result of environmental factors.

Methods
Differences in plant traits were characterized by measuring floral and vegetative traits at five sites between 450 and 1450 m above sea level. At three of these sites flower visitors were observed, collected and identified at family or genus level. Their exit mode was recorded, i.e. whether they escaped through the posterior exit hole and had pollen on their back (pollinators), escaped through the labellum mouth, or died in the labellum. Trait sizes of the flower visitors were measured together with the sizes of the plants and flowers in which they were caught to gain plant-visitor correlations at different altitudes.

Key Results
Sizes of floral and vegetative traits of Cypripedium calceolus differed in the study populations and their change in size correlated with altitude. This applied for both, floral and vegetative traits. The trait sizes of the flower visitors and pollinators didn't differ between the sites. Sizes of plant traits were not correlated with the sizes of their respective pollinators. In the lowland most of the flower visitors were bees, whereas in high altitudes two thirds of them were Syrphidae. However, almost all pollinators were bees, mostly Lasioglossum spp. (32), Andrena (5) and Nomada (2). Only three Syrphidae were characterized as pollinators, whereas most flies left the flower trough the labellum mouth or died in the labellum.

Conclusions
Sizes of Cypripedium calceolus differ among sites and correlate with altitude, but we cannot deduce that this is caused by a variable pollinator climate and a result of pollinator-mediated selection. Floral and vegetative traits alter in a similar way, while pollinator sizes don’t differ.

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Herbert Braunschmid conducted this work at the Institut für Ökologie und Evolution at Salzburg University in the working group of Prof. Stefan Dötterl.


Floral measurements: Length and height of the labellum, width of the labellum mouth (a), and posterior exit height (b; with a bee leaving through the posterior exit). Photo: Herbert Braunschmid