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A model system for group behavior of biofilaments

Birds on the µm-scale

For the casual observer it is fascinating to watch the ordered and seemingly choreographed motion of hundreds or even thousands of fish, birds or insects. However, the formation and the manifold motion patterns of such flocks raise numerous questions fundamental to the understanding of complex systems. A team of physicists from Technische Universitaet Muenchen (TUM) and LMU Muenchen has developed a versatile biophysical model system that opens the door to studying these phenomena and their underlying principles. Using a combination of an experimental platform and theoretical models, more complex systems can now be described and their properties investigated. The Munich researchers report on their findings in the current issue of the journal Nature.

“Everything flows and nothing abides,” is a saying ascribed to the Greek philosopher Heraclites. Large groups of individuals may show collective behavior where the individuals’ actions appear to be coordinated: Flocks of birds move through the air without a conductor, as if they were choreographed, and shoals of fish change their direction instantaneously when a shark appears. Yet science is still puzzled: Do all these systems obey the same universal laws? Does complex group behavior emerge from simple interactions between individuals intrinsically and inevitably? A team of researchers headed by Professor Andreas Bausch, Chair of Biophysics at TUM and Professor Erwin Frey, Chair of Statistical and Biological Physics at LMU, are unraveling the mystery.



© Nature 467 S. 73-77 (2. September 2010)



Volker Schaller (Bausch, TUM) has developed a experimental model system that makes it possible to carry out targeted high-precision experiments under controlled conditions. To this end, Volker Schaller fixed biological motor proteins to a microscope coverslip in such a way that they could drive filaments of the muscle protein actin suspended loosely over them, in any direction. The filaments measure about seven nanometers across, i.e. seven millionths of a meter, and are about ten micrometers long, i.e. a ten thousandth of a millimeter. The movement of the filaments is visualized using high-resolution microscopy.

In the experiments described in Nature, the actin filaments began to move as soon as ATP – the fuel for the motor proteins – was added. With low concentrations of actin filaments, the motion remained completely chaotic. Once the density crossed a threshold of five actin filaments per square micrometer, the filaments began to move collectively in larger clusters – with an astonishing resemblance to flocks of birds or shoals of fish. Structures like waves, swirls or ordered clusters seem to appear spontaneously during the experiments. Some of these structures grow to a size of almost one millimeter and remain stable for up to 45 minutes before they dissolve again.

Based on these observations, IDK-NBT student Christoph Weber developed theoretical models to describe the experimental results. With the combination of extensible theoretical models and a precisely controllable experiment, the physicists have set out to tackle more difficult problems and unravel their underlying principles. The model published is a cellular automaton where each filament moves on a hexagonal lattice. In the limit of low filament densities each filament performs a persistent random walk. Increasing the density interactions between the filaments become more and more important. We included steric interactions as well as a local alignment field. We found a phase transition to collective motion for certain parameter regimes, where the emerging clusters spread into the transversal direction to form wave patterns – as observed in the experiment.

Press Release TU Munich

movies to simulation results

"Polar patterns of driven filaments."
V. Schaller, C. Weber, C. Semmrich, E. Frey und A. R. Bausch:
Nature 467 S. 73-77 (2. September 2010)

Christoph Weber

since 2009
PhD student in the group of Prof. Erwin Frey, LMU Munich

2007 - 2008
Diploma Thesis in the group of Prof. Erwin Frey, LMU Munich
Topic of Diploma Thesis: “Modeling of Free Flow Isoelectric Focusing

2003 - 2008
Diploma in Physics at the LMU Munich

Selected Publication

V. Schaller, C. Weber, C. Semmrich, E. Frey und A. R. Bausch:
Polar patterns of driven filaments.
Nature 467 S. 73-77 (2. September 2010)