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Part II
COOPERATION WRIT SMALL: MICROBES
P
erhaps no taxa are as promising for enhancing both our under-
standing of cooperation and our understanding of the organisms
themselves as are microbes. Early work on microbes concentrated
on purifying and isolating them for growth in pure culture. The postu -
lates by Koch (1893) required this and were important for determining
exactly which microbes caused which disease. But in nature microbes live
in complex multispecies structured environments. Social interactions are
profound, because microbes perform many functions (such as digestion)
extracellularly that animals perform inside. One of the recent transfor-
mative elements of the study of microbes has been an appreciation of
the importance of their social interactions. Many of the types of social
interactions found in animals have their counterparts in microbes. Some
cooperative interactions are much more easily studied in microbes, par-
ticularly if the goal is to illuminate the genetic basis of behavior or to use
the power of experimental evolution.
Perhaps the best-studied social bacterium is Myxococcus xanthus, a
species of d-proteobacteria that spends its entire life in social groups
(Velicer and Vos, 2009). It is a predatory bacterium that hunts other bacte-
ria in social packs, dissolving its prey in pools of cooperatively produced
enzymes before ingesting them. Movement usually is based on Type IV
pili and is fundamentally social. When food is scarce, individual bacteria
aggregate into a fruiting body. In this stalkless fruiting body, most or
nearly all cells lyse, perhaps to the benefit of the remaining few, which
form hardy spores. Experimental evolution has shown us much about the
87
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88 / Part II
nature of sociality in M. xanthus. For example, when food was patchily
distributed, the species evolved more efficient group hunting techniques
(Hillesland et al., 2009). Under other circumstances, social cheaters can
drive population crashes (Velicer and Vos, 2009; Fiegna and Velicer, 2005).
In one fascinating case, a new cooperator evolved from the social cheater.
But this work does not tell us how natural these events are; for that expla-
nation we must turn to natural variation in wild fruiting bodies. In Chap -
ter 5, Suzanne Kraemer and Gregory Velicer explore natural phenotypic
variation in social traits of distinct clones within a fruiting body. They
took 10 fruiting bodies from nature, and from them isolated 48 individual
clones and examined their social phenotypes. These clones varied within
fruiting bodies in swarming and in spore production, genetic traits likely
to have arisen recently because the clones from the same fruiting body
were nearly genetically identical. This fascinating work will shed light
on the nature of sociality in the absence of a single cell bottleneck, where
variations that benefit single clones within the group can spread, even at
the cost of other group members.
One advantage to studying microbial social systems is that attributes
that are strong but sometimes hard to measure in animals are easily exam-
ined in experimental systems. One such attribute can be called “restraint.”
It may not be easy to determine whether or not a cow in a herd is eating all
it could or is holding back so that others may eat. If it were holding back,
this would be a social trait that would benefit others, and thus would be
expected to evolve under kin selection only if the genes for that trait are
also present in others, and benefit accordingly. In an ingenious experiment
described in Chapter 6, Joshua Nahum and colleagues examine the evolu-
tion of restraint in a nontransitive hierarchy often described by the rock-
paper-scissors game in which no one type consistently dominates. They
used Escherichia coli clones and the colicin system (Riley and Wertz, 2002).
Colicins are costly to produce and resist, but sensitive strains are killed
when producers release these substances. The researchers engineered
double colicin producers and resisters so production and resistance would
not be lost or gained in their system, and then, they asked how the three
types of clone would fare under different migration schemes compared
with how the resistor performed on its own. The authors found that the
resistor strain exhibited the most restraint with restricted migration in the
presence of all three strains, just the conditions where their models expect
cooperation to evolve.
Cooperation among clonemates arises easily because the genes under-
lying cooperation are present in both partners. In microbes, cooperation
often takes the form of extracellular secretions, including those used for
quorum sensing, iron scavenging, and fruiting body formation. Therefore,
a key question involves what favors the formation of clonal patches such
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Cooperation Writ Small: Microbes / 89
that cooperation can be promoted. One answer involves the physical
structure of the environment. For example, microorganisms growing on
substrates are more likely to be in contact with clonemates than those
living in a more fluid environment. Another possibility, and one investi -
gated by Sara Mitri and colleagues in Chapter 7, is that other species can
generate structure that favors within-species clonality. The authors use
a modeling approach to understand how additional species can change
interactions within species for the case of a growth-promoting secretion.
This agent-based modeling approach uses one other species to stand in
for all competing species. The authors’ models indicate that other species
can insulate secretors from selfish nonsecretors, even when the other spe-
cies can use the secretions themselves. Other factors such as the role of
dispersal and nutrient levels are also addressed in these models, which
begin the important task of considering microbial sociality and ecology
simultaneously, because these factors must influence how selection oper-
ates on these systems in nature.
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