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Physical Aspects of the Evolution of Biological Complexity

Final Report Summary - BIOCOMPLEX (Physical Aspects of the Evolution of Biological Complexity)

One of the most fundamental issues in evolutionary biology is the nature of transitions from single cell organisms to multicellular ones, with accompanying cellular differentiation and specialization. Not surprisingly for microscopic life in fluid environments, many of the relevant physical considerations involve diffusion, mixing, and sensing, for the efficient exchange of nutrients and metabolites with the environment is one of the most basic features of life.
This project involved a combination of experimental and theoretical research aimed at some of the key mysteries surrounding transport and sensing by and in complex, multicellular organisms, and the implications of those findings for the explanation of driving forces behind transitions to multicellularity. There were two main components of the research.
The first involved studies of single and multicellular algae which serve as model systems for allometric scaling laws in evolution. Of particular importance are the synchronization dynamics of the eukaryotic flagella that provide motility, enhance nutrient transport, and allow phototaxis in these organisms. Here we took the field from a stage at which there were only schematic qualitative studies of the phenomena to the point where the study of synchronization is now a science. By developing advances in micromanipulation, imaging, and data analysis we have been able to show the extent to which hydrodynamic interactions between flagella drive synchronization, and a role for intracellular elastic couplings as well. Our study of phototaxis reveals how multitudes of individual cells in a multicellular organism can act to create coherent, organism-level motion without the need for intercellular coupling, a crucial first stop in the development of multicellularity.
The second thrust involves investigation of the ubiquitous phenomenon of cytoplasmic streaming in aquatic and terrestrial plants. Despite decades of research, there had been no clear consensus on the metabolic role of this persistent circulation of the fluid contents of cells, nor any quantitative understanding of how it arises from motor protein activity. Using a combination of state of the art experimental methods such as magnetic resonance velocimetry, microfluidics, and theoretical fluid mechanics, we have developed a quantitative understanding of streaming in large plant cells and insect oocytes, as well as the possible self-organization that underlies the various streaming patterns seen in nature.
Finally, using the same green algae as in the studies of multicellularity, we initiated a new research direction in the area of morphogenesis by developing the first quantitative understanding the process of 'embryonic inversion', perhaps the simplest topological transition in biology.