Biology Reference
In-Depth Information
Fitness Effects of Quorum Sensing
Cells carrying genotype S. (for the genotype notation, see Table 1) produce the quo-
rum signal molecule, whereas R genotypes will respond to a sufficient amount of sig-
nal in their immediate environment. Both the expression of S and of R imply a fitness
cost as well, because producing the signal and running the response machinery takes
metabolic resources, although less than cooperation itself [13, 18]. The fitness benefit
of a QS system is an indirect one: communication using a signaling system may spare
unnecessary costs of futile attempts to cooperate whenever the local density of po-
tential cooperators is lower than the quorum n
q
. For this communication benefit to be
feasible, the QS machinery altogether has to be much cheaper (in terms of metabolic
costs) than cooperation itself, otherwise constitutive (unconditional and permanent)
cooperation would be a better option for the bacterium, and resources invested into QS
would be wasted. Thus the ordering of the metabolic fitness costs of cooperation and
QS are assumed to be m
C
>> m
S
≥ m
R
. The inactive alleles c, s, and r carry no metabolic
cost: m
c
= m
s
= m
r
=0.
The Effect of the Quorum Sensing Genes on the Cooperation Gene
The quorum signal is supposed to be the regulator of cooperation: bacteria with a C.R
genome (i.e., those carrying a functional cooperation allele C and a working response
module R) will actually express the C gene (i.e., cooperate) only if there is a sufficient
quorum n
q
of signalers (S. individuals) within their neighborhood. That is, C.R cells
wait for a number of “promises” of cooperation in their 3 × 3-cell neighborhood before
they switch to cooperating mode (produce the public good) themselves. The C.r geno-
types do not have a functioning response module, therefore they produce the public
good constitutively.
Selection
Individuals compete for sites. Competition is played out between randomly chosen
pairs of neighboring cells, on the basis of the actual net metabolic burdens M(1) and
M(2) they carry. The net metabolic burden M(i) of an individual i is calculated as the
sum of the basic metabolic load M
0
carried by all individuals and the total metabolic
cost m
e
(i) of the actual gene expressions at the three loci concerned (see Table 1),
multiplied by the unit complement of the cooperation reward parameter (1-r) if it is
surrounded by a sufficient quorum of cooperators:
M
(
i
)
m
e
(
i
)]
if
#
of cooperators in neighborhood
is below quorum threshold n
q
m
(
i
)
=
[
M
0
+
=
[
M
0
+
m
e
(
i
)](1
−
r
)
otherwise
(0
<
r
<
1).
Thus, successful cooperation reduces the total metabolic burden in a multiplica-
tive fashion. The relative fi tness of individual i is defi ned as its net metabolic burden
relative to the basic metabolic load as M
0
/M(i). In practice, the outcome of competi-
tion is determined by a random draw, with chances of winning weighted in proportion
to the relative fi tness. The winner takes the site of the loser, replacing it by a copy of
itself.
