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in the population, the adoption of QS genes obviously becomes profi table, because
the “Honest” (CSR) genotype takes over, ultimately excluding the “Blunt” one. The
“Honest” takeover renders the stationary population essentially dimorphic: the great
majority of the individuals are either “Ignorant” or “Honest”. The remaining six geno-
types are present at very low frequencies, close to their metabolic mutation-selection
equilibrium. What we end up with is thus the coexistence of cooperating-communicat-
ing cells (“Honest”) and parasitic ones (“Ignorant”).
The Effects of Changing the Quorum Threshold and Diffusion
(1) Cooperation is Costly (m C = 30.0).
Keeping the costs m C , m S , m R, and the cooperation reward r constant, we have system-
atically screened the effects of the quorum threshold n q and the diffusion parameter D
on the evolution of cooperation and QS (Figure 3, left column). Comparison with the
corresponding cases without the possibility of QS (Figure 1, left column) immediately
shows that the QS functions of signaling and responding are selected in a large part of
the parameter space and that they have a positive effect on the relative frequency of
cooperation in the population.
First consider the case of a low quorum threshold (n e = 2). If the population is
not mixed at all (D = 0.0), cooperators do not need an intact QS machinery to have
a reliable cue on the presence of cooperating neighbors: with a high chance at least
one clone mate (mother or daughter) is always around, and that is suffi cient for them
to enjoy the cooperation reward during their next interaction. This is why the great
majority of cooperators have disposed of one or both QS alleles (S and R) at D = 0.0.
Most cooperators are of the “Shy” (CsR) genotype, which responds to quorum signals
and cooperates accordingly, but does not itself produce the signal. Parasites capitalize
on this feature by issuing the signal only, thereby persuading the “Shy” type to cooper-
ate. This results in the parasite population to become, to an overwhelming majority, of
the “Liar” (cSr) type which is the exact complement of the “Shy” one: it possesses the
only functional allele that “Shy” is missing. Since the quorum signal is necessary for
the onset of cooperation in “Shy” individuals, the interaction between these two domi-
nant genotypes can be interpreted both as parasitism and as a peculiar type of “division
of labor”. The latter, less antagonistic component of the interaction immediately disap-
pears with the introduction of the slightest diffusion into the system. At and above D
= 0.1, the diffusion in the population creates already too many neighborhoods that do
not contain the required two C and two S alleles distributed over separate genotypes
(i.e., two “Shy” and two “Liar” types), and the presence of CSR (“Honest”) is selected,
guaranteeing successful cooperation as soon as two such genotypes are present in a
neighborhood. This leaves ample space for csr (“Ignorant”) parasites of course, which
reach high frequencies. This will be true even at fairly high diffusion rates.
The n q = 3 case has already been described in some detail above (Figure 2). The
special feature of this series of simulations is that the QS system is always adopted by
the cooperators, even without diffusion. This might be accounted for by the fact that at
about 50-60% (or less) of the population cooperating, the presence of three coopera-
tors within a 9-individual neighborhood is far from guaranteed, making QS well worth
 
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