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In the Light of Evolution: Volume V: Cooperation and Conflict (2011)

Chapter: Part II: COOPERATION WRIT SMALL: MICROBES

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Suggested Citation:"Part II: COOPERATION WRIT SMALL: MICROBES." National Academy of Sciences. 2011. In the Light of Evolution: Volume V: Cooperation and Conflict. Washington, DC: The National Academies Press. doi: 10.17226/13223.
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Part II

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COOPERATION WRIT SMALL: MICROBES

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

Suggested Citation:"Part II: COOPERATION WRIT SMALL: MICROBES." National Academy of Sciences. 2011. In the Light of Evolution: Volume V: Cooperation and Conflict. Washington, DC: The National Academies Press. doi: 10.17226/13223.
×

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

Suggested Citation:"Part II: COOPERATION WRIT SMALL: MICROBES." National Academy of Sciences. 2011. In the Light of Evolution: Volume V: Cooperation and Conflict. Washington, DC: The National Academies Press. doi: 10.17226/13223.
×

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.

Suggested Citation:"Part II: COOPERATION WRIT SMALL: MICROBES." National Academy of Sciences. 2011. In the Light of Evolution: Volume V: Cooperation and Conflict. Washington, DC: The National Academies Press. doi: 10.17226/13223.
×

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Suggested Citation:"Part II: COOPERATION WRIT SMALL: MICROBES." National Academy of Sciences. 2011. In the Light of Evolution: Volume V: Cooperation and Conflict. Washington, DC: The National Academies Press. doi: 10.17226/13223.
×
Page 87
Suggested Citation:"Part II: COOPERATION WRIT SMALL: MICROBES." National Academy of Sciences. 2011. In the Light of Evolution: Volume V: Cooperation and Conflict. Washington, DC: The National Academies Press. doi: 10.17226/13223.
×
Page 88
Suggested Citation:"Part II: COOPERATION WRIT SMALL: MICROBES." National Academy of Sciences. 2011. In the Light of Evolution: Volume V: Cooperation and Conflict. Washington, DC: The National Academies Press. doi: 10.17226/13223.
×
Page 89
Suggested Citation:"Part II: COOPERATION WRIT SMALL: MICROBES." National Academy of Sciences. 2011. In the Light of Evolution: Volume V: Cooperation and Conflict. Washington, DC: The National Academies Press. doi: 10.17226/13223.
×
Page 90
Next: 5 Endemic Social Diversity Within Natural Kin Groups of a Cooperative Bacterium--SUSANNE A. KRAEMER and GREGORY J. VELICER »
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Biodiversity--the genetic variety of life--is an exuberant product of the evolutionary past, a vast human-supportive resource (aesthetic, intellectual, and material) of the present, and a rich legacy to cherish and preserve for the future. Two urgent challenges, and opportunities, for 21st-century science are to gain deeper insights into the evolutionary processes that foster biotic diversity, and to translate that understanding into workable solutions for the regional and global crises that biodiversity currently faces. A grasp of evolutionary principles and processes is important in other societal arenas as well, such as education, medicine, sociology, and other applied fields including agriculture, pharmacology, and biotechnology. The ramifications of evolutionary thought also extend into learned realms traditionally reserved for philosophy and religion.

The central goal of the In the Light of Evolution (ILE) series is to promote the evolutionary sciences through state-of-the-art colloquia--in the series of Arthur M. Sackler colloquia sponsored by the National Academy of Sciences--and their published proceedings. Each installment explores evolutionary perspectives on a particular biological topic that is scientifically intriguing but also has special relevance to contemporary societal issues or challenges. This book is the outgrowth of the Arthur M. Sackler Colloquium "Cooperation and Conflict," which was sponsored by the National Academy of Sciences on January 7-8, 2011, at the Academy's Arnold and Mabel Beckman Center in Irvine, California. It is the fifth in a series of colloquia under the general title "In the Light of Evolution." The current volume explores recent developments in the study of cooperation and conflict, ranging from the level of the gene to societies and symbioses.

Humans can be vicious, but paradoxically we are also among nature's great cooperators. Even our great conflicts-wars-are extremely cooperative endeavors on each side. Some of this cooperation is best understood culturally, but we are also products of evolution, with bodies, brains, and behaviors molded by natural selection. How cooperation evolves has been one of the big questions in evolutionary biology, and how it pays or does not pay is a great intellectual puzzle. The puzzle of cooperation was the dominant theme of research in the early years of Darwin's research, whereas recent work has emphasized its importance and ubiquity. Far from being a rare trait shown by social insects and a few others, cooperation is both widespread taxonomically and essential to life. The depth of research on cooperation and conflict has increased greatly, most notably in the direction of small organisms.

Although most of In the Light of Evolution V: Cooperation and Conflict is about the new topics that are being treated as part of social evolution, such as genes, microbes, and medicine, the old fundamental subjects still matter and remain the object of vigorous research. The first four chapters revisit some of these standard arenas, including social insects, cooperatively breeding birds, mutualisms, and how to model social evolution.

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