Perhaps no taxa are as promising for enhancing both our understanding 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 postulates 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 transformative 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, particularly 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 ?-proteobacteria that spends its entire life in social groups (Velicer and Vos, 2009). It is a predatory bacterium that hunts other bacteria 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
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 explanation we must turn to natural variation in wild fruiting bodies. In Chapter 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 examined 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 evolution 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 underlying 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
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 investigated 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 species 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 operates on these systems in nature.