Project Title:
Aerodynamic features of saccate pollen: Evolutionary implications for wind-pollinated plants

Project Description (short):
The pollen grains of many wind-pollinated conifers have one to three air-filled sacci, which have been thought to add surface area, yet add minimal weight, thereby increasing dispersal distance. However, no published studies have tested this hypothesis. Using the saccate pollen grains of three extant conifers (Pinus, Falcatifolium, Dacrydium), electron microscopy, and mathematical modeling, a computational model has been developed to study pollen flight. The model uses structural characters of pollen grains to calculate terminal settling velocity. Examples of characters utilized in the model include: lengths, widths, and depths of the main body and sacci; angle of saccus rotation; thicknesses of the saccus wall, endoreticulations, intine, and exine; and surface ornamentation. Settling speeds predicted by the model have been compared and validated with terminal settling velocity data obtained by other methods, such as stroboscopic photography. Modeling pollen both with and without sacci indicates that sacci can increase dispersal range. The model affords the opportunity to study pollen flight in three dimensions while controlling factors such as temperature and wind speed. The advantage of a mathematical model that is based on structural characters is that flight properties can be measured without physically testing pollen, providing the opportunity to model flight dynamics of fossil pollen. Several fossils have been studied, including non-saccate (Monoletes), mono-saccate (Gothania), and bi-saccate (Pteruchus, Caytonanthus, Pinus) pollen types. A technique to mathematically expand compressed fossil pollen to their original shape has been developed. Although some studies of extant conifers indicate that sacci have a buoyancy function once grains reach a pollination drop, the present study provides the opportunity to further evaluate the adaptive significance of saccate pollen by correlating structural and aerodynamic features. (A longer project description is [here].)

Skills needed:
Experience with or interest in learning computational tools such as Mathematica or Matlab.

Start Date:
Ongoing, but we're looking for one biology student

End Date:
Ongoing

Mentors:
Prof. Jeffrey Osborn (Biology), josborn@truman.edu
Prof. Scott Thatcher (Mathematics), thatcher@truman.edu

Past Students:

• Andrew Schwendemann, 2005-2006 (Biology)
• George Wang, 2005-2006 (Mathematics & Computer Science)
  Meredith Mertz, 2006 (Biology)
  

  Accomplishments:
  

  • Andrew Schwendenmann and George Wang. "Aerodynamics of Saccate Pollen and Its Implications for Wind Pollination." Truman Student Research Conference. April 2005.
  • Andrew Schwendenmann, George Wang, Jeffry Osborn, Scott Thatcher. Poster. European Conference on Theoretical and Mathematical Biology. Dresden, Germany. July 18-22, 2005.
  • Andrew Schwendenmann and George Wang. "Computational Modeling of Flight Characteristics for Extant and Fossil Saccate Pollen Grains" Truman State University Student Research Conference. April 2006.
  • Meredith L. Mertz, Andrew B. Schwendemann, George Wang, Scott L. Thatcher, and Jeffrey M. Osborn Poster: "Computational Modeling of Flight Characteristics for Extant and Fossil Saccate Pollen Grains." Botany 2006, California State University, Chico. July 28-August 2 2006.
  • Meredith Mertz, George Wang, Andrew Schwendemann, Jeffrey M. Osborn, and Scott L. Thatcher. Poster: "Aerodynamics of Saccate Pollen and Its Implications for Wind Pollination." Truman State University Student Research Conference. April 2007.
  • Mertz, Meredith, Wang, George. "Computational Modeling of Flight Characteristics for Extant and Fossil Saccate Pollen Grains" Truman State University Student Research Conference. April 2007.
  • Andrew B. Schwendemann, George Wang, Meredith L. Mertz, Ryan T. McWilliams, Scott L. Thatcher, and Jeffrey M. Osborn. "Aerodynamics of Saccate Pollen and Its Implications for Wind Pollination." Accepted for publication in the American Journal of Botany.

  About Prof. Osborn:
  Jeffrey M. Osborn joined the faculty of Truman State University in 1991, where he currently serves as Professor and Convener (Chair) of Biology, Director of The Next STEP Program, Co-Prinical Investigator of the Mathematical Biology Initiative, and Chair of Truman’s Undergraduate Research Committee. He is a recipient of the Walker and Doris Allen Fellowship for Faculty Excellence, Truman’s highest award for recognizing outstanding faculty members who have greatly contributed to the success of Truman and its students in achieving the liberal arts and sciences mission of the University.

  Dr. Osborn received a B.S., with honors, from Texas State University–San Marcos, where he majored in Biology. He remained at Texas State University to complete an M.S. in Botany and then earned a Ph.D. in Plant Biology from Ohio State University.

  Dr. Osborn’s teaching has covered a broad range of areas, including introductory-level Biology and Botany courses. He has also taught numerous upper-level classes and seminars; some of these include Comparative Plant Morphology, Paleobotany, Plant Anatomy, several Microscopy-based courses, and Understanding Biology through Art.

  Dr. Osborn and his students conduct research in the broad area of plant evolutionary biology, studying both fossil and living plants. The majority of their work is phylogenetically oriented and considers evolutionary relationships among seed plants based principally on the study of pollen development and morphology. He is also interested in pollination biology, including the evolution of pollination mechanisms and the functional role that pollen plays in particular syndromes. He has received the Outstanding Researcher Award from the Kirksville Chapter of Sigma Xi. Dr. Osborn has mentored numerous undergraduate research students, and he has authored many research articles, book chapters, book reviews and abstracts.

  Professor Osborn has spent research leaves from Truman at the Swedish Museum of Natural History, the University of Alberta, the University of Kansas, and conducting fieldwork in Antarctica.

  Dr. Osborn recently served two, three-year terms as the Program Director and member of the Executive Board of the Botanical Society of America (BSA). He currently serves on the External Advisory Board of the State of Oklahoma’s IDeA Network of Biomedical Research Excellence Program, funded by the National Institutes of Health, and has recently served on the Peer Review Committee for the Council for International Exchange of Scholars–Fulbright Senior Scholar Program.

  For more information about Dr. Osborn's research, his students, a full list of publication, etc, please visit h is web site at: http://www2.truman.edu/~josborn/.

  About Prof. Thatcher:
  Dr. Scott Thatcher has taught at Truman since the fall of 2000. He received a Ph.D. in Mathematics from Northwestern University in 2000 and a B.A. in Physics and Mathematics from Carleton College in 1993. Academic interests include geometry, topology, and (more recently) low Reynolds number fluid dynamics as it applies to plant pollen. Other interests include open source software, typography and (more recently) his new son Thomas.

  Project Description (long):
  The immobility of plants has resulted in the evolution of some extraordinary methods of pollination among seed plants. Many angiosperms are zoophilous, and these biotic pollination syndromes can be highly specialized and are thought to be very efficient. In contrast, conifers primarily utilize wind pollination. This form of abiotic pollination has historically been considered inefficient, as copious amounts of pollen are produced, liberated, and presumably reach the ovulate cones in a random fashion. Wind pollination, however, has been demonstrated to be much more efficient than previously thought. For instance, some pines have ovulate cones that are aerodynamically shaped to direct the wind to fertile areas on the cone. The rhythmic motion of cone-laden branches, as well as the structure and arrangement of needles surrounding cones can contribute to this. Furthermore, it is believed that the structural aspects of conifer pollen are also aerodynamically adapted for wind pollination, allowing the "right" pollen grain to land on just the "right" spot on the ovulate cone.

  Pollen grains of many wind-pollinated plants appear to be well-adapted for anemophily. For protection, the gametophytic cells are contained within a spheroidal main body that is surrounded by a two-layered pollen wall. The inner intine is solid, whereas the outer exine contains supportive, plate-like ingrowths. These create air pockets that presumably reduce weight. Most conifer pollen grains have another feature with ramifications for wind pollination. Flanking the main body are one to three air-filled bladders, or sacci, that differ in size and internal structure among species. Sacci are expanded portions of the exine and are lightweight, yet increase the surface area that the wind can push against. The pollen grain's lightweight and minute proportions, perhaps aided by surface characteristics, cause them to follow the wind, rather than fall immediately). This is reflected in the pollen grain's low Reynolds-number, a measure used to describe an object's interaction with surrounding fluid.

  Several studies have also shown that sacci play another reproductive role as well. When the pollen grain settles on the pollination droplet within the ovulate cone of some extant conifers, the sacci may act as buoys to orient the pollen grain when it is drawn into the micropyle. Tomlinson and Runions have suggested that sacci primarily function in floatation and have discounted their adaptive significance in anemophily. However, no published studies have addressed if, and how well, the sacci of different species actually assist in flight. Furthermore, many other aspects of pollen structure may affect flight dynamics, but little is known about how characters such as surface ornamentation, exine thickness, intine thickness, or the internal structure of sacci correlate with flight efficiency. As such, a preliminary project to empirically investigate and computationally model the aerodynamic properties of a select number of saccate pollen grains has been initiated.

  PRELIMINARY RESULTS: Using combined light, scanning electron, and transmission electron microscopy, structurally different saccate pollen grains of three extant conifers ( Pinus, Falcatifolium, and Dacrydium) have been critically examined. Several key characters have been documented, including overall size, main body size, saccus size, surface ornamentation, wall thicknesses, wall infrastructure, saccus infrastructure, overall mass, and wall mass. These characters have been incorporated into a preliminary mathematical model that calculates flight properties for the pollen grains. The model has been verified by stroboscopic photography of actual Pinus pollen. Using this model, structural data have been incorporated from two fossils to predict how these ancient grains may have flown. Taxa included the monosaccate pollen of Gothania and the bisaccate grains of Caytonanthus.

  Although a few studies indicate that sacci play a buoyancy role once pollen grains reach a pollination drop in some extant conifers, the preliminary results provide the first empirical evidence that sacci do in fact play an important role in increasing the distance that pollen grains can travel in the wind. The interdisciplinary scope of this project provides the opportunity to further evaluate the adaptive significance of saccate pollen by correlating structural and aerodynamic features. The preliminary mathematical model takes into consideration both the biologically based structural features, as well as several salient physical parameters such as height at time of dispersal, wind speed, and wind direction; however, it does not currently evaluate finer-scale aspects of pollen grain architecture, turbulent wind conditions, or model flight in three dimensions.

  RESEARCH OBJECTIVES: The proposed project will comparatively investigate the aerodynamic characteristics of structurally different saccate pollen grains from a broad range of taxa. The primary objectives of the project are to: 1) Document the external micromorphology and sizes for the various pollen; 2) Document the internal ultrastructure and wall thicknesses for all pollen types; 3) Develop and enhance the mathematical model such that it includes characters at a smaller scale of resolution (e.g., surface ornamentation) and evaluates flight dynamics in three-dimensions and in turbulent wind conditions; 4) Evaluate the accuracy of the model by empirically determining terminal velocity for actual saccate pollen grains.

  MATERIALS AND METHODS: To document the external pollen micromorphology, samples will be dehydrated, critical-point dried, sputter-coated, and imaged using scanning electron microscopy. The images will be used to determine overall size, central body size, sac size, and surface ornamentation. To document the internal pollen ultrastructure, samples will be dehydrated, embedded, and sectioned using an ultramicrotome. Ultrathin sections will be collected and stained. Using transmission electron microscopy, images will be studied that provide data on wall and saccus thickness and infrastructure.

  The preliminary computational model incorporates data on pollen grain structure, including the presence or absence of sacci and the calculation of grain density and volume, but the final dynamic model assumes a spheroidal grain geometry. The computational model will first be enhanced to include an approximation to actual large-scale grain geometry, with attention paid to dynamic properties of the grain in turbulent airflow. Further enhancements will then include data on small-scale

  exine features, such as surface ornamentation. Equations to model the dynamics of low-Reynoldsnumber objects are known and will be applied in this project. Another strategy for modeling the dynamics of pollen dispersion will be to employ a distribution function that describes the density of pollen grains released from a single source. To test the model’s accuracy, stroboscopic photography will be used to determine terminal velocities.

  SIGNIFICANCE: Using a comprehensive mathematical model it will be possible to accurately discern flight properties of pollen without having to physically test actual grains. This has geat potential to better understand pollen biology. For example, the aerodynamic effects of saccus size can be evaluated — how large or small do they need to be to significantly change flight dynamics? Similarly, what about overall size of the pollen grain? Furthermore, how do the endoreticulations within the sacci affect flight dynamics of the grain—is there a significant difference between sacci with endoreticulations crossing the entire saccus cavity vs. sacci with endoreticulations extending only a short distance into the saccus cavity?

  The mathematical model relies strictly on structural data for determining aerodynamic properties because this is paramount to understanding the flight dynamics of ancient pollen grains. The pollen of many extinct plants are different from modern pollen, varying in number of sacci, overall shape, thickness of walls, and type of endoreticulations. It is possible that some fossil pollen grains were not aerodynamically efficient, and this played a role in selection leading toward extinction. One way to shed light on this question is to model the flight of fossil pollen. However, fossil pollen contains only an exine layer that resists decay, and many are often highly compressed. Because of these limitations, the only data that can be obtained from fossil pollen are structural data such as wall thicknesses, wall ultrastructure, and surface sculpture. Because those values are the only data required for the computational model, the flight dynamics of fossil pollen grains can in fact be rigorously examined.

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