Microbial Co-op in Evolution

Courtesy of Michiel Vos, Max Planck Institute, Tübingen, Germany  FRUIT FOR THE HUNGRY: Myxococcus fruiting bodies emerge from soil particles in response to starvation. Approximately 100,000 cells aggregate and communicate via chemical signals to form the bodies, and a portion of the population differentiates into stress-resistant spores. Most microbiologists consider used flasks, laden with splotches of colonizing bacteria, simply more dishes to wash. Paul Rainey sees more. For Rai

By | October 6, 2003

Courtesy of Michiel Vos, Max Planck Institute, Tübingen, Germany
 FRUIT FOR THE HUNGRY: Myxococcus fruiting bodies emerge from soil particles in response to starvation. Approximately 100,000 cells aggregate and communicate via chemical signals to form the bodies, and a portion of the population differentiates into stress-resistant spores.

Most microbiologists consider used flasks, laden with splotches of colonizing bacteria, simply more dishes to wash. Paul Rainey sees more. For Rainey, ecologist and evolutionary biologist at the University of Auckland in New Zealand and the University of Oxford in Britain, those clusters of bacteria may help demonstrate evolutionary principles at work. He is one of many investigators looking to microbes to see how cooperation and sociality evolve.

Microorganisms within a colony may produce by-products not for their own feeding or reproduction, but simply to promote the colony. That individuals in a clonal, or even closely related, microbial population would incur costs to benefit their neighbors' genes and increase the group's fitness is a compelling notion. So-called cheaters may opt out of the "group mentality" for individual gain, but understanding the cooperative mechanisms and even the ways in which cheaters are dealt with could lend new insight into how complex organisms evolve. In particular, such studies may elucidate the transition from individual to multicellular life.

Typically, cooperation studies involve social insects such as bees, ants, and wasps, but microbes offer some major advantages. Bacteria in particular are easy to manipulate; reproduce asexually, allowing for generations of identical genotypes; and evolve quickly. Another advantage: Researchers can freeze and store ancestral types, reviving and mixing them to see how they compete with more evolved strains. Also, microbial studies have potential, practical importance for understanding pathogen life cycles and treating diseases.

STICKING TOGETHER For years, Rainey, working with Pseudomonas fluorescens, has taken advantage of a simple phenomenon: Bacteria in broth, left alone and unshaken in a glass vial--such as those seen in used, dirty lab flasks--will evolve, by mutation and selection, into several niche-adapted types. Some bacteria, for example, colonize the vial's rim, while others prefer the bottom. Some colonize the air-liquid interface, a sort that Rainey nicknamed "wrinkly spreaders."

Recently, Rainey reported how these wrinkly spreaders adapt as a group to their niche in order to maximize their exposure to both oxygen-rich air and nutrient-rich broth1--an impossible feat for an individual bacterium. The key: A mutation allows for polymer glue production, allowing the bacteria to literally stick together and form a mat on the broth's surface. In producing this glue, an individual bacterium incurs a cost, but the group as a whole benefits. Rainey suggests that it's some of the first empirical evidence demonstrating an evolutionary transition in which the selection level shifts from individuals to groups.

In a companion paper to Rainey's, Gregory Velicer, a group leader at the Max Planck Institute for Developmental Biology in Tübingen, Germany, demonstrated a somewhat similar phenomenon.2 Using the well-known social bacterium Myxococcus xanthus, he showed how bacteria can regain lost social behavior through mutation and selection. Velicer knocked out a gene necessary for making the pili that drive M. xanthus' normal social motility, a feature that allows the bacteria to swarm its prey. After about 36 weeks, they re-evolved the ability to swarm socially, but without pili. Much like Rainey's P. fluorescens, they instead stick together, but in this case by the increased production of an extracellular fibril matrix.

CHEATER, CHEATER As with most groups, not every one cooperates. In looking to bacteria to investigate roots of collaboration, researchers must also account for those selfish individuals, the so-called cheaters. As part of his recent paper, Rainey reported that his sticky P. fluorescens group is susceptible to cheating types--those that do not produce the glue but reap the group benefit--and that they can prosper (i.e., be favored by selection) and eventually cause the group's demise. Analogous dynamics can be seen in everything from societies to cancer, where certain cells divide unchecked to the detriment of the tissue, organ, or even organism.

A major case study outside of the bacterial world is that of the social amoebae Dictyostelium discoideum in work pioneered by Rice University evolutionary biologists David Queller and Joan Strassmann. After working with social insects for most of their careers, Strassmann and Queller turned their attention to the social amoebae, whose largely sequenced genome enables the group to take advantage of knockout and transformation technologies. "Both of our studies," says Velicer, "would be examples of how predictions from general evolutionary theory were found to actually be borne out in these systems."

The amoebae live on the forest floor and feed on bacteria, but if they're starving, they send out a cAMP pulse to neighboring amoebae. They aggregate, move to a favorable location, form a stalk, and produce long-lasting spores that can survive until favorable growth conditions return. In the process of stalk formation, 20% to 30% of the cells die. By monitoring this life cycle and different genotypes, Strassmann and Queller have shown that amoebae do cheat; one clone, for example, might contribute less than its share to the stalk and more than its share to the spore.3 Also, they've shown that relatedness is a factor, meaning groups of relatives cooperate better than nonrelatives. Strassmann and Queller, having already identified some of the genetics responsible for the amoebae's social behavior, are now using microarrays to further that search; they hope to find the genes that turn on when the amoebae are with relatives.

Velicer, whose lab studies the various circumstances under which cheaters arise in the social M. xanthus, notes that bacteria are much more social than previously thought, but that sociality, in principle, is always threatened by cheaters. Evolutionary theory predicts that any cooperative system relying on behaviors costly to the individual will be susceptible to cheats, unless there are mechanisms in place to exclude them.

Rainey says studying cheaters is key, and recent work is only the first step toward empirically demonstrating the evolution of multicellularity. The sticky P. fluorescens, with its simple undifferentiated groups, has taken a small step toward group living. "But that's a far cry from multicellularity with all of its differentiated cell types," says Rainey. He's now investigating whether his bacteria might evolve some "self-policing" system that would somehow recognize and destroy or punish cheaters. His rationale comes from observing multicellular organisms whose cell types, despite being in constant conflict, cooperate due to policing agents like hormones and other cellular signals.

Courtesy of Kevin Foster, Rice University
 COME TOGETHER: Cell aggregation streams in the slime mold Dictyostelium discoideum as they go through the transition from single celled to multicellular life. Cells are labeled with fluorescent beads.

A GROUP MENTALITY Some suggest that such demonstrations of sociality in bacteria are just a few steps away from the study of the long-controversial notion of group selection, in which natural selection operates on groups rather than (and at the expense of) individuals or genes. Essentially, group selection favors altruism among individuals, even if they're unrelated. The fittest group, and not necessarily the fittest individuals, will survive.

William Hamilton's formulation of kin selection in the 1960s--the notion that altruism is rooted in individuals trying to pass on their genes by helping relatives who share some of those genes--explained previous group selection observations with more scientific rigor. This brought backlash against group selection theory, but some have revisited and even embraced the notion in recent years. "When you see a social interaction, it doesn't mean that group selection must be occurring," says Charles Goodnight, a professor of biology at the University of Vermont. But it does suggest, he says, that such phenomena in social bacteria are a good place to look for group selection.

Using organisms such as flour beetles and chickens, researchers have demonstrated group selection in the lab, but they still tread carefully when evoking the term. David S. Wilson, an evolutionary biologist at Binghamton University in New York and a well-known proponent of the existence of group selection, says there's a strong bias. "Writing openly about group selection is asking for trouble as far as the publication process is concerned," he says. "It is common for people to talk with me openly about levels of selection and then avoid using the 'G-word' in their papers."

Goodnight notes that because of the sometimes negative connotation of group selection, and because the term implies a discrete group rather than a continuous population (a more apt description of bacterial lawns), the term "multilevel selection" is often used to describe the shift from individual- to group-selection levels. "It's not the fitness of an individual, but the fitness of an individual in the context of its neighbors that probably determines whether the cells divide or not," he says.

Rainey says that exploring group and multilevel selection avenues of research is an important next step. "Once you see that there are a group of cells with a single identity, that they function as a group, then everything is set up to ... look at selection between groups." He notes, though, that sociality studies in bacteria, which are clone-mates, are entirely consistent with kin selection. Strassmann says there's no reason to invoke group selection at all: "To me, that is more easily understood as a function of their relatedness than as a function of the fact that they're in a group near to each other." But, perhaps tradition and not science loads these terms. In the end, says Rainey, "a lot of the controversy is just semantic."

Eugene Russo (erusso@the-scientist.com) is a freelance writer in Takoma Park, Md.

References
1. P.B. Rainey, K. Rainey, "Evolution of cooperation and conflict in experimental bacterial populations," Nature, 425:72-4, Sept. 4, 2003.

2. G.J. Velicer, Y.N. Yu, "Evolution of novel cooperative swarming in the bacterium Myxococcus xanthus," Nature, 425:75-8, Sept. 4, 2003.

3. J.E. Strassmann et al., "Altruism and social cheating in the social amoeba Dictyostelium discoideum," Nature, 408:965-7, 2000.

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