Biology Reference
In-Depth Information
the sites are always occupied, that is bacteria may replace each other, but may not
leave empty sites. The inhabitants of the sites may differ at three genetic loci: locus C
for cooperation (production of a public good), the other two for QS: locus S for pro-
ducing the signal molecule and locus R for signal response, which includes the signal
receptor and the signal transduction machinery that triggers the cooperative behavior
when the critical signal concentration has been reached. Each of these loci can harbor
either a functional allele denoted by a capital letter (C, S, and R), or an inactive allele
denoted by a small letter (c, s, and r). Thus the bacteria can have 2
3
= 8 different geno-
types, each paying its own metabolic cost of allele expression on the 3 loci (Table 1)
besides the basic metabolic burden M
0
that is carried by all individuals.
Table 1.
The eight possible genotypes of the cooperation-quorum sensing system and the corresponding
total metabolic costs me of gene expression.
Total cost
m
e
(with
m
c
= 30.0)
Total cost
m
e
(with
m
c
= 10.0)
GENOTYPE
PHENOTYPE
csr
"Ignorant"
0.0
0.0
csR
"Voyeur"
1.0
1.0
cSr
"Liar"
3.0
3.0
cSR
"Lame"
4.0
4.0
Csr
"Blunt"
30.0
10.0
CsR
"Shy"
31.0
11.0
CSr
"Vain"
33.0
13.0
CSR
"Honest"
34.0
14.0
Cooperation can be costly (
m
c
= 30.0; left column) or relatively cheap (
m
c
= 1 0.0; right column). Cost of QS signalling:
m
s
= 3.0; Cost of QS re s onse:
m
c
= 1.0.
Fitness Effects of Cooperation
The product of the cooperating C allele is supposed to be an excreted “public good”
molecule such as an exoenzyme for extracellular food digestion. It may increase the
fitness of a bacterium, provided there are at least n
q
bacteria (possibly, but not neces-
sarily, including itself) expressing the C allele as well within its 3 × 3-cell neighbor-
hood; n
q
is the quorum threshold of cooperation. An individual can only obtain a fit-
ness benefit from cooperative behavior in its neighborhood if at least nq cooperators
are present in that neighborhood. On the other hand, cooperation carries a fitness cost
which is always paid by the cooperator whether or not it enjoys the benefits of coop-
eration. The cost of cooperation is the metabolic burden associated with the production
of the public good. That is, cooperation (expressing C) carries an inevitable fitness
cost and a conditional fitness benefit. We study the effects of a high as well as a low
cost of cooperation. Of course for cooperation to be feasible at all the benefit has to
outweigh the cost.
