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Population bottlenecks promote cooperation in bacterial biofilms.

Brockhurst MA - PLoS ONE (2007)

Bottom Line: However, the size of the population bottleneck is likely to play a crucial role in determining the success of cooperation.In line with predictions, the frequency of evolved cheats within the populations increased with increasing bottleneck size.This result highlights the importance of ecologically mediated population bottlenecks in the maintenance of social traits in microbes.

View Article: PubMed Central - PubMed

Affiliation: School of Biological Sciences, University of Liverpool, Liverpool, United Kingdom. michael.brockhurst@liv.ac.uk

ABSTRACT
Population bottlenecks are assumed to play a key role in the maintenance of social traits in microbes. Ecological parameters such as colonisation or disturbances can favour cooperation through causing population bottlenecks that enhance genetic structuring (relatedness). However, the size of the population bottleneck is likely to play a crucial role in determining the success of cooperation. Relatedness is likely to increase with decreasing bottleneck size thus favouring the evolution of cooperation. I used an experimental evolution approach to test this prediction with biofilm formation by the bacterium Pseudomonas fluorescens as the cooperative trait. Replicate populations were exposed to disturbance events every four days under one of six population bottleneck treatments (from 10(3) to 10(8) bacterial cells). In line with predictions, the frequency of evolved cheats within the populations increased with increasing bottleneck size. This result highlights the importance of ecologically mediated population bottlenecks in the maintenance of social traits in microbes.

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The effect of population bottleneck size on total population density.Dots represent mean ± SEM population density on day 16 of the experiment.
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pone-0000634-g002: The effect of population bottleneck size on total population density.Dots represent mean ± SEM population density on day 16 of the experiment.

Mentions: As predicted, the frequency of evolved SM cheats inhabiting the biofilm increased with increasing bottleneck size (Figure 1, Table 1; F1,19 = 5.72, P = 0.027), suggesting that reductions in relatedness caused by larger bottlenecks disfavoured cooperation. SM is also able to inhabit the broth phase of microcosms, however no significant growth in the broth phase was observed during this study (data not shown), this is in line with previous work performed under this disturbance frequency where the majority of evolved SM inhabited the biofilm [15]. There was also a significant effect of founding genotype on the frequency of evolved SM cheats (F3,19 = 4.32, P = 0.018), suggesting, as has been previously observed [18], that independent WS genotypes differed in their susceptibility to invasion by evolved SM cheats. It is possible that these results could have arisen through differences in population density between bottleneck treatments, however this is unlikely as there was no significant correlation between the proportion of cooperators and population density (Pearson's r = 0.029, P = 0.9). Furthermore, no significant effect of bottleneck size on total population density was observed (Figure 2, Table 2; F1,19 = 3.07, P = 0.096), although a significant effect of founding genotype was observed (F3,19 = 4.42, P = 0.016). This confirms previous work suggesting that independent WS genotypes vary in terms of productivity [18].


Population bottlenecks promote cooperation in bacterial biofilms.

Brockhurst MA - PLoS ONE (2007)

The effect of population bottleneck size on total population density.Dots represent mean ± SEM population density on day 16 of the experiment.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC1919422&req=5

pone-0000634-g002: The effect of population bottleneck size on total population density.Dots represent mean ± SEM population density on day 16 of the experiment.
Mentions: As predicted, the frequency of evolved SM cheats inhabiting the biofilm increased with increasing bottleneck size (Figure 1, Table 1; F1,19 = 5.72, P = 0.027), suggesting that reductions in relatedness caused by larger bottlenecks disfavoured cooperation. SM is also able to inhabit the broth phase of microcosms, however no significant growth in the broth phase was observed during this study (data not shown), this is in line with previous work performed under this disturbance frequency where the majority of evolved SM inhabited the biofilm [15]. There was also a significant effect of founding genotype on the frequency of evolved SM cheats (F3,19 = 4.32, P = 0.018), suggesting, as has been previously observed [18], that independent WS genotypes differed in their susceptibility to invasion by evolved SM cheats. It is possible that these results could have arisen through differences in population density between bottleneck treatments, however this is unlikely as there was no significant correlation between the proportion of cooperators and population density (Pearson's r = 0.029, P = 0.9). Furthermore, no significant effect of bottleneck size on total population density was observed (Figure 2, Table 2; F1,19 = 3.07, P = 0.096), although a significant effect of founding genotype was observed (F3,19 = 4.42, P = 0.016). This confirms previous work suggesting that independent WS genotypes vary in terms of productivity [18].

Bottom Line: However, the size of the population bottleneck is likely to play a crucial role in determining the success of cooperation.In line with predictions, the frequency of evolved cheats within the populations increased with increasing bottleneck size.This result highlights the importance of ecologically mediated population bottlenecks in the maintenance of social traits in microbes.

View Article: PubMed Central - PubMed

Affiliation: School of Biological Sciences, University of Liverpool, Liverpool, United Kingdom. michael.brockhurst@liv.ac.uk

ABSTRACT
Population bottlenecks are assumed to play a key role in the maintenance of social traits in microbes. Ecological parameters such as colonisation or disturbances can favour cooperation through causing population bottlenecks that enhance genetic structuring (relatedness). However, the size of the population bottleneck is likely to play a crucial role in determining the success of cooperation. Relatedness is likely to increase with decreasing bottleneck size thus favouring the evolution of cooperation. I used an experimental evolution approach to test this prediction with biofilm formation by the bacterium Pseudomonas fluorescens as the cooperative trait. Replicate populations were exposed to disturbance events every four days under one of six population bottleneck treatments (from 10(3) to 10(8) bacterial cells). In line with predictions, the frequency of evolved cheats within the populations increased with increasing bottleneck size. This result highlights the importance of ecologically mediated population bottlenecks in the maintenance of social traits in microbes.

Show MeSH
Related in: MedlinePlus