Paper: Observing bacteria through the lens of social evolution
Observing bacteria through the lens of social evolution
1Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA
2Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA
3Howard Hughes Medical Institute, 4000 Jones Bridge Road, Chevy Chase, MD 20815, USA
Journal of Biology 2008, 7:27doi:10.1186/jbiol87
The electronic version of this article is the complete one and can be found online at: http://jbiol.com/content/7/7/27
Published: 30 September 2008
© 2008 BioMed Central Ltd
Explaining the evolution of cooperative behavior is a long-standing problem for which much theory has been developed. A recent paper in BMC Biology tests central elements of this theory by manipulating a simple bacterial experimental system. This approach is useful for assessing the principles of social evolution, but we argue that more effort must be invested in the inverse problem: using social evolution theory to understand the lives of bacteria.
The core principle of Darwin's theory of evolution is simple: entities with traits that maximize reproductive success will increase in frequency relative to their competitors. A naive application of this idea might suggest that natural selection should only favor organisms that boost their own output of offspring. However, nature is rife with cooperative behaviors that decrease the actor's reproduction and increase that of recipient organisms, and Darwin himself recognized that cooperation poses a serious challenge for his theory. In many cases, this dilemma can be resolved by considering the replication of genes separately from the reproduction of whole organisms. This perspective was first presented clearly by JBS Haldane and then formalized by WD Hamilton, who showed that a gene responsible for cooperative behavior will increase in frequency in a population if the cost c (decrease in lifetime reproduction) of producing the behavior is less than the benefit b (increase in lifetime reproduction) of receiving the behavior weighted by relatedness r, the likelihood that receivers of cooperation share the gene or genes controlling the cooperative behavior . In short, cooperation can evolve if rb > c. Hamilton's rule may seem quite simple, but r, b, and c are not static parameters; rather, they are dynamic variables that change with the cooperative behavior in question and the environmental circumstances in which that behavior is expressed. Rigorously testing Hamilton's rule therefore requires a highly tractable experimental system, for which culturable unicellular organisms are ideal candidates.
Using microbes to test evolutionary theory
Manipulating microbes to address basic ecological and evolutionary questions has a rich history. In a series of classic experiments, GF Gause used mixed cultures of Paramecium species and Saccharomyces species to test elementary theories of competition and predator-prey interactions. Various authors have also combined microbiology with ecology and evolution to explore topics ranging from host-parasite interactions to long-term experimental evolution (for example, B Levin, E Cox, R Lenski, L Chao, B Bohannan and others). Historically, many studies have assumed that bacteria lead solitary, asocial lives. On the contrary, prokaryotic microbes frequently live in dense populations, termed biofilms, and they interact extensively with each other by secreting a variety of extracellular compounds [2,3]. Some of these compounds, such as the colicins of Escherichia coli and the pyocins of Pseudomonas aeruginosa, are weapons against competing microbial strains or species. Other secreted products may mediate cooperation, including polymers that lend biofilms structural support, chelating agents that sequester nutrients and enzymes that digest complex substrates into smaller units for subsequent import. Bacteria are thus gregarious and social, but their sociality presents a conundrum: extracellular products that are costly to produce and benefit other individuals ('public goods') can be exploited by individuals that reap the benefits of public goods without contributing to the pool . In a recent paper in BMC Biology, Brockhurst et al.  use bacteria to test the efficacy of Hamilton's rule for describing whether cooperation can succeed against exploitation. Taking structural polymer secretion by Pseudomonas fluorescens and siderophore secretion by P. aeruginosa as two examples of public good production, they studied how changes in nutrient availability affect the ability of cooperative cells to resist invasion by exploitative mutants.
When grown in beakers partly filled with liquid medium, strains of P. fluorescens that constitutively produce extracellular polysaccharide (EPS) form large groups on the liquid surface, which affords them better access to oxygen (Figure 1a). In this context, EPS is a public good that binds members of the surface-dwelling bacterial population together and anchors them to the beaker walls. These biofilms are invaded by spontaneous EPS-null mutants that take advantage of the structural polymer produced by others, thereby gaining access to high oxygen concentrations without investing in the public good. By diverting more resources into growth instead of paying the cost of cooperation, exploitative P. fluorescens mutants achieve higher division rates and compete successfully against EPS-producing strains (Figure 1b).
Source: Journal of Biology [Open Access Paper]
Sent to you by
Robert Karl Stonjek