I am interested in understanding the evolutionary processes involved in the evolution of wind pollination in the angiosperms, and the mechanisms that promote reproductive success and outcrossing in wind-pollinated plants. Wind pollination is a derived condition in the angiosperms and has arisen independently in many unrelated families. A recent survey estimated that 10% of angiosperm species rely on wind pollination, with 65 independent transitions from biotic to wind pollination. The repeated evolution of wind pollination from animal pollination suggests that wind pollination is a more effective means of pollen dispersal in some environmental and demographic situations. Although the shift from animal pollination to wind pollination represents one of the major transitions in angiosperm reproduction, we have very little knowledge about the evolutionary mechanisms responsible. Moreover, in sharp contrast to biotic pollination, we know very little about the ecology of wind pollination and mating strategies in anemophilous plants.
My research addresses three key issues pertaining to the ecology and evolution of wind pollination: 1) what conditions promote the evolution of wind pollination? 2) is wind pollination as inefficient as often claimed? and 3) how do wind-pollinated plants promote outcrossing?
Evolution of wind pollination
The ecological and evolutionary mechanisms responsible for transitions from animal to wind pollination are poorly understood in comparison with other major reproductive transitions in angiosperms. Anemophily tends to be associated with particular ecological conditions. For example, regions of higher latitude, dry temperate environments and open vegetation have the highest proportions of wind-pollinated plants. A suite of morphological traits is also associated with wind pollination, including inconspicuous flowers, lack of nectar and petals, and single ovules. To investigate correlations between wind pollination and a range of ecological and morphological characters, I am using a recent large-scale molecular angiosperm phylogeny to analyse the presence of trait correlations and the order of trait acquisition.
To complement the comparative biology approach, I am using theory to investigate the conditions that favour the evolution of wind pollination from animal pollination. The evolution of wind pollination could occur in several ways. Under certain ecological conditions, it may be advantageous for plants to reduce the proportion of pollen dispersed by animal vectors, and to increase the proportion of wind dispersed pollen. Beginning with an outcrossing, insect-pollinated ancestor, a decline in pollinator abundance or effectiveness could impose selective pressure for an alternative pollination mechanism. Using an ESS modelling approach with various male gain curves, I am exploring the proportion of reproductive fitness that is realised through animal pollination versus wind pollination. Combined, these approaches provide an understanding of the context in which wind pollination evolves, which is fundamental to further investigations into the ecology and evolution of wind pollination.
Efficiency of wind pollination
Traditionally, abiotic pollen vectors are viewed as random and relatively poor modes of pollen dispersal. I answer the second question by conducting a survey of pollination efficiency in 18 wind-pollinated species. I show that wind pollination is not less efficient than animal pollination (the percentage of pollen produced by plants that reaches stigmas ranges from 0.01 to 1.02 % , which is slightly lower but well within the magnitude of the range observed for animal-pollinated plants with granular pollen (0.07-2.16 %)). These results contradict several published assertions about the reproduction of wind-pollinated species. The association between anemophily and uni-ovule flowers has been presumed to be because of the very small chance of multiple pollen grains landing on each stigma. My results from the pollination efficiency study demonstrate that a substantial portion of stigmas capture more than one pollen grain, and the majority of ovules develop into mature seed. Why, then, are wind-pollinated plants frequently uni-ovulate?
Several morphological and aerodynamic features of wind-pollinated plants may favour an optimal strategy of fewer ovules per flower. The relatively low cost of producing flowers in wind-pollinated plants may favour a packaging strategy with fewer ovules per flower and more flowers per plant. Also, the stochastic nature of wind pollination may favour single ovules as a mechanism to increase pollen-tube competition and promote sexual selection. Finally, by producing more flowers with fewer ovules, the spatial separation of flowers may increase the volume of air sampled by a plant and the probability of capturing pollen grains. A more mechanistic understanding of the functional relation between wind pollination and ovule number would be informative. I am currently developing a model to test these ideas.
Outcrossing in wind pollinated plants
In this part of my research, I use experimental approaches to determine the mechanisms that promote reproductive success in wind-pollinated species.
Flowers of anemophilous plants often separate sexes spatially and/or temporally. Particularly, it appears that there is an association between protogyny (female phase first) and wind pollination, and also monoecy and wind pollination. Both these strategies are typically viewed as mechanisms to reduce selfing. The genus Carex (Cyperaceae) is entirely wind-pollinated and has consistently unisexual flowers. Many of the monoecious species are dichogamous. Using seven Carex species, I investigate the function of protogyny and monoecy in promoting outcrossing. I use emasculation experiments to determine whether intact monoecious plants have higher selfing rates than emasculated plants. I also sample stigmas during both the female phase and the combined sex phase and show that the protogynous phase during flowering allows some pollen to be captured, but significantly less than during the combined sex phase. These experiments show that monoecy is ineffective at promoting outcrossing (all species are highly selfing) and that protogyny may provide an opportunity for plants to first capture outcross pollen, but this represents a very small fraction of seed set.