Evolution of wind pollination in Leucadendron (Proteaceae) : experimental evidence and floral trait shifts.

Abstract

Evolutionary transitions from insect to wind pollination are thought to have occurred many times during the angiosperm radiation. This transition is commonly associated with a suite of distinctive floral traits such as reduction of flower size and a transition to dry pollen. In the dioecious genus, Leucadendron (Proteaceae), evolutionary shifts from insect to wind pollination have been postulated based on floral morphology features. In this study, I aimed to experimentally test the potential for wind versus insect pollination in several Leucadendron species and document a variety of floral traits (pollen size, inflorescence size, scent, colour, etc.) in order to determine their functional significance whilst utilizing phylogenetic comparative methods to test the statistical significance of evolutionary associations between particular floral traits and pollination systems. Fifteen representative Leucadendron species were investigated to verify insect and wind pollination in as many clades as possible. Insect exclusion experiments confirmed that five Leucadendron species, L. rubrum, L. salicifolium, L. dubium, L. coniferum and L. teretifolium are indeed wind-pollinated. Pria cinerascens (Nititulidae) was found to be the main pollinator of the insect-pollinated Leucadendron species due to their abundance, high stigmatic contact and relatively pure Leucadendron pollen loads. Overall, however, the abundance of insects visiting inflorescences was not significantly different between insectand wind-pollinated species, which highlights the importance of conducting insect exclusion experiments to evaluate whether a species is wind- or insect-pollinated. From the previously determined pollination systems of 17 Leucadendron species, floral traits associated with the shift to wind pollination were investigated to determine whether transitions from insect to wind pollination were accompanied by modifications of pollination-relevant floral traits. In a wind tunnel, pollen grains of wind-pollinated species were found to be more motile than those of insect-pollinated species. Phylogenetic analyses suggest that transitions from insect to wind pollination occurred at least four times during the diversification of Leucadendron and that, compared to insect-pollinated cogeners, wind-pollinated Leucadendron species are characterized by increased production of smaller pollen grains, higher inflorescence density, less attractive visual and olfactory cues, and a greater degree of sexual dimorphism for these visual and olfactory cues. In conclusion, this study experimentally confirms that there were several shifts from insect to wind pollination in Leucadendron and identifies floral traits that were evolutionarily modified during these shifts.