Group selection and the evolution of cooperation in plasmids
The origin of cooperative behaviors in biological systems results to be challenging. The classical theory of natural selection implies competition such that better adapted individuals will survive and reproduce more than the others. It has been even suggested that the Darwinian evolution could not explain cooperative behaviors. Cooperation is an extended phenomenon in nature, from subcellular environment to human societies. In the evolutionary context, a cooperator is an individual which pays a cost for another individual to receive a benefit in terms of fitness (survivability and reproduction). It is expected that defectors that pays no cost, have a higher fitness in mixed populations. However, a population of only cooperators has the highest average fitness while populations of only defectors has the lowest (Nowak, 2006).
Plasmids are self-replicating extra-chromosomal DNA usually found in prokaryotes. The replication control genes of bacterial plasmids face selection at two conflicting levels (Paulsson, 2002). On the one hand, more replicative plasmids should have a lower probability of being lost by chance during cell duplication. But, this cells should have also a lower chance of becoming fixed in the bacterial population.
There had been proposed different approaches to explain cooperation (Nowak, 2006). Paulsson had derived a theoretical model for group selection (Paulsson, 2002) applied particularly to the case of bacterial plasmids and its copy number control (CNC). Plasmids are a plausible experimental model for testing the hypothesis of group selection for critical reasons. Firstly, you can define clear groups of plasmids as the single bacteria that contains them, and second, you can differentiate relative cooperativity between groups and populations. Here we are going to call cooperators to plasmids that tend to produce a lower number of copies due to a tighter CNC.
The model defines the conditions in which cooperative behaviors could appear, considering the benefit for the bacterial population, cost for cooperative plasmids, and plasmid groups size and bacterial population size. The benefit is related to the relative growth rate (µ) of the bacterial population as a function of the mean number of plasmids per cell. Cost is determined by the dynamic factors involved in the CNC mechanism. Through the CNC, plasmids can sense constantly their own amount and adjust its replication rate to an optimum given the extracellular environmental conditions (Nördström & Austin, 1989). The CNC consist of a couple of antisense-RNAs codified by the plasmid and expressed in a constitutive way. RNAI is an indispensable precursor of the replication initiation complex. As RNAII is complementary to RNAI, it functions as a repressor of the initiation of the replication of the plasmid (Paulsson, J., Nördström, K. & Ehrenberg, M. 1998). When there is a high copy number, the RNAII concentration raises and replication rate diminishes. And when there is low copy number the amount of activator augments leading to an increase in the replication rate. This constitutes a feedback that stabilize the copy number inside the bacterial cell. The cost of the cooperative behavior as a function of the copy number depends mainly on the ratio between the activator and inhibitor synthesis rates.
We developed an instrument to keep bacteria carrying plasmids evolving at different population sizes for a big number of generations (about 5000). Our research question is if group selection is actually a force leading to the appearance of cooperation among plasmids. Our project consists on an experimental evaluation of this hypothesis. What we expect is that the mean plasmid number per cell (n) varies in respect to bacterial population sizes (m). And that if cooperation emerge in some trait (m), then the resulting copy number after system evolution do not exceed the maximum predicted by the model. It would be relevant because of the lack of empirical evidence for group selection. This would show that group selection, actually exist and that it is useful for determined sceneries, as microbial unstructured populations.
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