Biology Reference
In-Depth Information
Chapter 2
Quorum Sensing Drives the Evolution of
Cooperation in Bacteria
Tam á s Cz á r á n and Rolf F. Hoekstra
INTRODUCTION
An increasing body of empirical evidence suggests that cooperation among clone-mates
is common in bacteria. Bacterial cooperation may take the form of the excretion of
“public goods”: exoproducts such as virulence factors, exoenzymes, or components
of the matrix in biofilms, to yield significant benefit for individuals joining in the
common effort of producing them. Supposedly in order to spare unnecessary costs
when the population is too sparse to supply the sufficient exoproduct level, many bac-
teria have evolved a simple chemical communication system called quorum sensing
(QS), to “measure” the population density of clone-mates in their close neighborhood.
Cooperation genes are expressed only above a threshold rate of QS signal molecule
re-capture, that is, above the local quorum of cooperators. The cooperative popula-
tion is exposed to exploitation by cheaters, that is mutants who contribute less or nil
to the effort but fully enjoy the benefits of cooperation. The communication system
is also vulnerable to a different type of cheaters (“Liars”) who may produce the QS
signal but not the exoproduct, thus ruining the reliability of the signal. Since there is
no reason to assume that such cheaters cannot evolve and invade the populations of
honestly signaling cooperators, the empirical fact of the existence of both bacterial
cooperation and the associated QS communication system seems puzzling. Using a
stochastic cellular automaton (CA) approach and allowing mutations in an initially
non-cooperating, non-communicating strain we show that both cooperation and the
associated communication system can evolve, spread and remain persistent. The QS
genes help cooperative behavior to invade the population, and vice versa ; coopera-
tion and communication might have evolved synergistically in bacteria. Moreover, in
good agreement with the empirical data recently available, this synergism opens up
a remarkably rich repertoire of social interactions in which cheating and exploitation
are commonplace.
Cooperation behavior that benefi ts other individualsis not easy to explain from an
evolutionary perspective, because of its potential vulnerability to selfi sh cheating. A
classic example is formed by the so-called tragedy of the commons [1]. A commons
pasture is used by many herders, and the best strategy for an individual herder is to add
as many cattle as possible, even if this eventually causes degradation of the pasture.
The unfortunate outcome follows from the fact that the division of the costs and ben-
efi ts of adding additional animals is unequalthe individual herder gains all of the ad-
vantage, but the disadvantage is shared among all herders using the pasture. Therefore,
 
 
 
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