- German version - deutsche Fassung -

The staminal lever arm mechanism – a key innovation for adaptive radiation in the genus Salvia (Lamiaceae)?

The genus Salvia, with more than 900 species one of the largest genera in angiosperms, is characterised by the well known `staminal lever arm mechanism´. Our hypothesis is that the evolution of this specific pollen transfer mechanism within the Lamiaceae-Nepetoideae has been essential for the adaptive radiation in Salvia.

How does this mechanism function, what is its significance for reproductive isolation among sympatric species and to what extent does it influence speciation within the genus?

To answer these questions we are investigating the biomechanics of the staminal lever arm mechanism, its function and diversity:

1. For the first time, the forces and energies which are necessary to move the lever arms are measured quantitatively by means of a special device. These data are related to those of the forces and energies of possible pollinators which are being determined at the University of Freiburg at the same time. In this way we are testing whether through physical force certain animals are excluded from pollination.
2. By means of comparative functional-morphological and morphometric investigations we are studying the structure of the stamens and the diversity of the floral structures involved in the process of pollen-transfer. We assume that already small changes in structural proportions may clearly influence the process and precision of the lever movement which may then result in reproductive isolation and speciation by an altered interaction between flowers and pollinators. To check our assumption the process of pollen transfer by a pollinator is imitated experimentally. Thereby, both the pattern of pollen deposition on the pollinator’s body and the receptiveness of the stigma are analysed.
3. Little is known about reproductive systems and diversity of the pollination mechanisms in Salvia. As the genus has no genetic self-incompatibility system, sympatric species are reproductively separated by mechanical, saisonal and ecological isolation. In this context the lever arm mechanism may have a key function. Field investigations into bee pollinated and bird pollinated species are to clarify the functional interrelations between floral construction, pollen transfer mechanism, flower class and outcrossing.

All data are collected in an ACCESS database and are thus available for future phylogenetic analyses.

The interaction between pollinators and the staminal levers in the genus Salvia: biomechanics, functional morphology and exact pollen placement

Thomas Speck, Freiburg and Regine Claßen-Bockhoff, Mainz

For an integrative understanding of the importance of flower structures, lever arm mechanics and exact pollen positioning for reproductive isolation and speciation processes in the genus Salvia, the interaction between pollinators and flowers will be studied. In our experimental approach we will concentrate on insect pollinators, as bees and bumble-bees. As part of our interdisciplinary project, two groups of experiments have been designed:

(1) In the first set of experiments, we want to test if the forces and energies required to get to the nectar during a visit of a Salvia-flower are relevant for the foraging behaviour of the pollinators. For this purpose, the insects will be trained to 'artificial flowers' with sugar water as food source, and the forces and energies, which are exerted by the insects in order to get to the food source will be determined. The exerted forces and required energies will then be compared to the forces and energies necessary for the movement of the staminal levers and for the deformation of the flowers in Salvia necessary to get access to the nectar. The latter forces and energies will be measured with a specific, very sensitive force-distance measurement device of different design at the University of Mainz at the same time.

(2) In a second set of experiments, the hypothesis will be tested that the staminal lever mechanism has a key function in stabilising sympatric Salvia-species via a precise, portioned and species-specific transfer of pollen on pre-defined regions of the head or body of the pollinators. We will study experimentally if the process of pollen deposition on different species of pollinators is precise enough to prevent or to minimise the transfer of pollen on the stigma of other visited sympatric species of Salvia. Therefore, pollen transfer processes between different flowers of the same Salvia-species and flowers of different, sympatric Salvia-species will be analysed quantitatively.

These experiments will for the first time render quantitative data for testing the hypothesis of importance of staminal lever structures for avoiding hybridistaion between sympatric Salvia-species and thereby stabilising these species. On the other hand, if a hybridisation has occurred and if the staminal structure of the new hybrid species differs morphologically or functionally from that of the parental species, the same mechanism would stabilise this hybrid species by reproductive isolation.