Testing functional hypotheses on poricidal anther dehiscence and heteranthery in buzz-pollinated flowers

Agnes Dellinger, Léa Pöllabauer, Milan Loreti, Janis Czurda, Jürg Schönenberger

Poricidal anther dehiscence is characteristic of pollen-rewarding flowers specialized on buzz-pollinating bees. Pollen dosing as a strategy of avoiding excessive pollen loss as a reward to the bees is believed to have played a major role in the evolution of buzz-pollination. Such dosing strategies should, however, be flexible and allow dispensing schedules to be adjusted to pollinator visitation frequency and flower age in order to avoid risks of dispersing too small amounts of pollen. In addition, many pollen-rewarding flowers have evolved different stamen types within the same flower (heteranthery), usually involving at least one showy and one cryptic type. Heteranthery is commonly explained by 'division-of-labour' between stamen types with one type depositing pollination pollen on the bee's back (which cannot be reached by grooming) and the other depositing feeding pollen on the bee's belly. Recently, however, differential pollen dosing between stamen types as a strategy to maximize male fitness has been proposed as an alternative driver of the evolution of heteranthery. We used three pollen rewarding species (Senna reticulata (Fabaceae), Adelobotrys adscen-dens, Conostegia subcrustulata (both Melastomataceae)) with poricidal anthers to test 1) whether pollen dosing strategies change with flower age and 2) whether the different stamen types of heterantherous species differ in dosing strategies. In A. adscendens, 3) we also tested whether pollen from two different stamen types is deposited in different areas of the pollinator's body (mimicked by a microscope slide) as proposed by the 'division-of-labour' hypothesis. Pollen was dosed in all three species, but pollen release increased with flower age only in S. reticulata. Differential dosing by the different stamen types was strong in both heterantherous species. In A. adscendens, we could not detect pollen deposition in different areas of the pollinator's body, but significantly more pollen was released by the cryptic (feeding) stamen type. Our results suggest that heteranthery may indeed be understood as a complex combination of spatial and temporal 'division-of-labour'. As proposed by other authors, heteranthery may function both in differential pollen placement (spatial aspect) but also in differential dosing between stamen types via 'shift-working' (temporal aspect).

Department of Botany and Biodiversity Research, Department of Behavioral and Cognitive Biology
Acta ZooBot Austria
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Peer reviewed
Austrian Fields of Science 2012
106042 Systematic botany, 106008 Botany, 106012 Evolutionary research
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