Fine-tuned floral filters in melittophilous flowers sort out particular bees

• Applicant: Professor Dr. Klaus Lunau
• Subject Area: Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions ; Sensory and Behavioural Biology
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 315642637

Bees are considered reliable pollinators; this is corroborated by the bees frequency of flower visits and their flower constancy, but challenged by the bees collection of pollen for larval food provisioning. Pollen grains which have been groomed into the pollen transport structures are no longer available for pollination. This allocation of pollen for a purpose other than pollination amounts to a dilemma for bee-pollinated flowering plants. In this project we will investigate three so far unstudied and coherent aspects of this pollen dilemma by testing whether and how flowers manipulate bees for optimization of pollen transfer. 1. Spines of pollen grains are involved in the mechanical defence of pollen grains from being collected by corbiculate bees. We will test the role of spine number, spine length, pollen grain diameter, pollen hydration and added regurgitated nectar in order to understand the mechanism by which spiny pollen is defended against collection by corbiculate bees. In addition we will test if flowering plants use spiny pollen grains to restrict bees to nectar collection, because nectar collecting bees groom themselves less frequently. 2. Pollen grains on safe sites of bees bodies experience a smaller probability of being groomed into the pollen transport structures, from where they cannot pollinate flowers. We will test whether keel blossoms represent an exception and can use the scopa as a kind of safe site of pollinating bees. We will test whether the stiffness of keel flowers, which can only be opened by megachilid bees, represents a floral adaptation to promote transfer of pollen grains from the ventral scopa of the bee to stigma of the flower. This would be the first evidence for pollination by fertile scopal pollen and constitute strong selective pressure for the respective flowers to exclude bees that do not possess a ventral scopa. 3. Many bee-pollinated flowers possess pollen- and stamen-mimicking structures which are considered to function as visual floral guides. We will study the as yet unexplored function of pollen- and stamen-mimicking structures to trigger grooming behaviour in bees by means of tactile stimuli. We will test the hypothesis that stamen-mimicking structures set a stronger trigger for grooming than real stamens. In addition we will test if feeding stamens induce grooming more strongly than pollination stamens in heterantherous flowers. These experiments will show if flowers are able to manipulate the bees grooming by tactile cues in order to promote pollen transfer to conspecific flowers. The coherent aspect of these studies is the fine-tuned floral filter by which bee-pollinated flowers might be able to exclude distinct bees and thereby improve pollination success. These studies will focus on currently understudied antagonistic aspects of the mutualism between flower-visiting bees and melittophilous flowering plants.

• DFG Programme: Research Grants