David Timerman, Ph.D. student
Department of Ecology & Evolutionary Biology
University of Toronto
25 Willcocks Street
Toronto, Ontario, Canada
Office: Earth Science Building 2042, 416-978-8508
David at, The cirque along King’s Throne trail,
Kluane National Park & Reserve, Yukon.
||Biology, Concordia University, Montreal QC.
Thesis title: Pollen clumping and release mechanisms in wind pollinated plants. (2013)
||Environmental Sciences, Concordia University, Montreal QC.
Thesis title: Pollen concentration and seed set in Picea mariana (black spruce). (2010)
|| Department of Ecology & Evolutionary Biology, University of Toronto
Thesis title: Mechanistic studies on the evolution of wind pollination
Download my Botany 2015 Research Poster (pdf)
My research interests broadly include plant evolutionary biology, plant biomechanics and pollination biology. I am fascinated by the evolution and mechanics of wind pollination, both of which are under-explored areas of plant reproductive biology. More specifically, wind pollination has evolved from animal pollination at least 65 times in the angiosperms, but little is known about the ecological conditions or the biomechanical pathways involved in the transition (Friedman and Barrett, 2009). For my Ph.D. thesis, I am using a combination of empirical and theoretical studies to develop a mechanistic understanding of this evolutionary transition. Specifically, my thesis will address the following questions:
- What selective forces favour the transition from animal to wind pollination?
- What is the morphological and biomechanical pathway from animal to wind pollination?
- Do populations vary in the relative importance of insect versus wind pollination and are there predictable morphological differences between populations that can account for this variation?
- How is paternal fitness gained from investment in wind pollination?
My previous research examined various aspects of the wind pollination process including pollen release and capture. For instance, many wind-pollinated plants have long, flexible stamens that vibrate readily even under mild wind conditions. Theoretical scaling analysis has shown that wind-pollinated stamens interact with high frequency eddies whose passage frequencies approach the fundamental frequency of stamens causing them to resonate and release pollen (Urzay et al., 2009). As part of my M.Sc. thesis, I confirmed using a combination of laboratory experiments and field observations, that this is the prevailing pollen release mechanism in the herbaceous weed Plantago lanceolata (see figure below). My results demonstrated that variation in wind speed and not the mean is important in pollen release. Further, biomechanical optimization of stamens is necessary since eddies in this portion of the turbulence spectrum contain low mechanical energy. This is perhaps one reason why animal-pollinated plants do not lose large amounts of pollen to wind gusts.
Vibration induced pollen shedding in Plantago lanceolata. The arrow tracks the motion of a single stamen over time (0.000 s – 0.017 s) and shows the perturbation from equilibrium caused by a gust of wind. (0.025 s) shows the stamen returning to its equilibrium position and releasing pollen. Spectral analysis indicated that the stamen was resonating as there was a significant harmonic at ~40 Hz and this corresponded to the range of fundamental frequencies measured for stamens in the laboratory.
The strength of adhesion forces between pollen and the anther may also be related to aerodynamic pollen release, with weak forces expected for wind-pollinated plants and strong forces expected for animal-pollinated plants. The mechanisms of pollen adhesion (see Pacini, 2000) also cause grains to cohere and this may be beneficial for animal-pollinated plants (Harder and Johnson, 2008) and detrimental to wind-pollinated plants (Hall and Walter, 2011). An intermediate amount of pollen aggregation is expected for species relying on both vectors (i.e., ambophilous plants), as these plants must balance aerodynamic pollen release with conserving pollen for visitors (Stelleman, 1984). Surprisingly, the hypothesis that pollen aggregation follows a quantitative gradient from wind to animal pollination has not been rigorously examined. I tested this hypothesis in my M.Sc. thesis using 9 wind-pollinated, 22 animal-pollinated and 1 putative ambophilous species (Plantago lanceolata). Since pollen is easily disaggregated due to drying or mechanical agitation, I developed a procedure for generating standardized distributions of pollen clumps by applying the same disaggregating force to pollen samples. I hypothesized that the resulting distribution would be lognormal, the expected distribution for a random disaggregation process with many steps. As expected, my results demonstrated significantly more clumping among animal-pollinated plants. Interestingly, P. lanceolata had an intermediate amount of clumping which may be related to the fact that this species has been reported to be ambophilous (Stelleman, 1984). Furthermore, the vast majority of samples fit a lognormal distribution. The resulting manuscript is currently in review at the International Journal of Plant Sciences.
Dave doing field work
Timerman, D., Greene, D.F., Ackerman, J.D., Kevan, P.G. & Nardone, E. (2014) Pollen aggregation in relation to pollination vector. International Journal of Plant Sciences 175(6): 681-687 (pdf)
Timerman, D., Greene, D.F., Urzay, J. & Ackerman, J.D. (2014) Turbulence-induced resonance vibrations cause pollen release in Plantago lanceolata L. (Plantaginaceae). Journal of the Royal Society Interface 11(101) (pdf)
Friedman, J. & Barrett, S.C.H. (2009) Wind of change: new insights on the ecology and evolution of pollination and mating in wind-pollinated plants. Annals of Botany, 103, 1515–1527.
Hall, J.A. & Walter, G.H. (2011) Does pollen aerodynamics correlate with pollination vector? Pollen settling velocity as a test for wind versus insect pollination among cycads (Gymnospermae: Cycadaceae: Zamiaceae). Biological Journal of the Linnean Society, 104, 75–92.
Harder, L.D. & Johnson, S.D. (2008) Function and evolution of aggregated pollen in angiosperms. International Journal of Plant Sciences, 169, 59–78.
Pacini, E. (2000) From anther and pollen ripening to pollen presentation. Plant Systematics and Evolution, 222, 19–43.
Stelleman, P. (1984) Reflections on the transition from wind pollination to ambophily. Acta Botanica Neerlandica, 33, 497–508.
Urzay, J., Llewellyn Smith, S.G., Thompson, E. & Glover, B.J. (2009) Wind gusts and plant aeroelasticity effects on the aerodynamics of pollen shedding: A hypothetical turbulence-initiated wind-pollination mechanism. Journal of Theoretical Biology, 259, 785–792.