by PhilipJ on 27 September 2006
PLoS Biology is running a series of short discussion papers on the core issues that have arisen in modern biological research. In the September editorial (open access!), Simon A. Levin, a professor in Ecology and Evolutionary Biology at Princeton and chair of this “Challenges” series outlines some of the questions he sees as fundamental, due to the “recognition that all biological systems are what have come to be known as complex adaptive systems, in which macroscopic patterns reflect the collective dynamics of individual units at lower levels of organization and feed back to affect those more microscopic dynamics.” With this in mind, his questions are:
What features convey robustness to systems? How different should we expect the robustness of different systems to be, depending on whether selection is operating primarily on the whole system or on its parts? How does robustness trade off against adaptability? How does natural selection deal with environmental noise and the consequent uncertainty at diverse scales? When does synchrony emerge, and what are its implications for robustness? When and how does cooperative behavior emerge, and can we derive lessons from evolutionary history to foster cooperation in a global commons?
The first paper in the series is Cooperation among Microorganisms by Ned S. Wingreen* (Princeton Molecular Biology) and Levin. They outline that, while it is obvious that cells cooperate (see any higher organism as a perfect example), single-celled organisms (bacteria, fungie, etc) also cooperate. An example of single-cell cooperation is detailed in quorum sensing, where cells excrete and sense small autoinducer molecules. At high cell concentrations, genes for cooperative behaviour are turned on, leading to “the formation of protective biofilms, the expression of virulence factors to attack a host, the production of light, the establishment of competence to exchange DNA (a bacterial form of sexual recombination)”, all of which are more effective when a group of cells act in concert. Another example is that of the cyanobacteria Anabaena, where roughly 1 in 10 cells (which grow together in chains) differentiates into a heterocyst, supplying its neighbours with fixed levels of Nitrogen (Figure 2 from the paper):
Why is this behaviour a fundamental question in biology? From Wingreen and Levin:
Understanding how cooperation arose and is maintained, particularly among large numbers of species, presents a challenge for practitioners of both molecular biology and evolutionary biology, as well as for theorists. Is cooperation best understood as the convergence of the immediate self-interest of multiple parties? Or can evolution lead to stable cases of short-term altruistic behavior, providing long-term benefit for all? These questions have been central in evolutionary biology since the time of Darwin, who regarded apparently altruistic behavior as a challenge for his theory. Especially puzzling was the extreme levels of cooperation and altruism, termed eusociality, in the haplodiploid insects and termites.
The entire field of kin selection has since emerged as an explanation for much of the observed altruistic behaviour in higher organisms, but is this relevant on the cell level?
The authors end by describing some of the techniques that mathematical biologists can use to help attack the problem, namely new methods using dynamical game theories and spatial stochastic processes, and that while some progress has been made, there is still much work to be done.
Finally, as per the original editorial, contributions to the “Challenges” series are encouraged; ideas should be sent to email@example.com. If you’re going to email PLoS, please leave your ideas here in the comments as well!
* If you subscribe to Physics Today, Wingreen is also the author of the September issue’s quick study, “A glossary of cellular components”. Unfortunately not open access.