ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

EVOLUTION OF ADHESION PROPERTIES IN YEAST
T. Odenthal 1 , B. Smeets 1 , J. Christiaens 2 , K. Verstrepen 2 , E. Tijskens 1 ,
and H. Ramon 1
1 KU Leuven - MeBioS, Kasteelpark Arenberg 30, 3001 Heverlee
2 KU Leuven - CMPG Lab for Genetics and Genomics, Gaston Geenslaan 1, B-3001 Heverlee
1 ABSTRACT
Using the particle-based simulation software DEMeter++ , we created an off-lattice individualcell
based model to investigate the role of adhesion mechanics in the cell sorting phenomenon of
Saccharomyces Cerevisiae. Simulations in three dimensions show that only a small difference in
adhesion, caused by the expression of FLO genes, is required to create sorted colonies. In these
colonies, f lo − cells remain unattached or in the outer layers of the flock, and FLO + cells are all
found in the center. Cell adhesion is described by a modified Hertz-potential with adhesion, in
which the attracting force is proportional to the contact area of two adhering cells. The cells are
modeled as suspended in liquid and move by Brownian motion. Furthermore, the effect of the
budding mechanism in growing colonies on the cell sorting is being addressed.
2 INTRODUCTION
In budding yeast, Saccharomyces Cerevisiae, the expression of “FLO genes” has been identified
as a cause of flocculation in colonies. For yeast, flocculation is a protection mechanism in stress
conditions. Cells expressing the FLO1 gene preferably stick to other cells that also express this
gene. Because they stick more strongly, FLO1 + cells aggregate to the center of the flock, where
they are physically shielded against toxics in the medium, including ethanol and antimicrobials.
From a socio-evolutionary viewpoint, FLO1 can be considered a “green beard gene”, which promotes
co-operation between individuals that express this gene [2], p.89. It is known that the FLO
genes code for a number of surface proteins influencing inter-cellular interactions. The following
work substantiates the hypothesis [10], that the “kin-recognition” mechanism relies solely on
the fact that cells expressing the FLO1-protein adhere more strongly to other cells expressing the
protein than to cells which do not express it.
3 MATERIALS AND METHODS
3.1 Model structure
To investigate the flocculation behavior of yeast cells in solution in silico, we construct a mathematical
model which is as simple as possible while capturing the flocculation dynamics well. To
that end, an Individual-cell Based, mesh-free model (IBM) is used. We approximate the cell shape
to be spherical, since the shape of yeast cells in solution deviates only slightly from spheres.
We opt for a fully three-dimensional, mesh-free implementation to avoid artifacts in the number
of possible contacts a cell can have, since these direct contacts with “next neighbors” strongly
influence the dynamics of the system.
To model the “kin-recognition” process, it is crucial to capture the movements of the cells with a
precision high enough to resolve the dynamics of single contact-events.