Does gene mobility promote the persistence of a bacterial social trait?

The mer operon confers bacteria with resistance to mercury toxicity whilst simultaneously allowing them to bioremediate their local environment. This ‘collective’ effect of mercury resistance provides the potential for social conflict wherein mercury sensitive cells may act as ‘cheats’ benefiting from the detoxification performed by resistant cells whilst bearing none of the cost; it is challenging therefore to explain how mercury resistance is maintained. One potential solution for the maintenance of social traits is for their location on mobile genetic elements allowing infectious transfer of the trait and enforcement of cooperation on cheat cells. This study explores the dynamics of mobile versus non-mobile mercury- resistance in populations of Pseudomonas fluorescens SBW25. When the mer operon is located on a costly plasmid the short-term dynamics show that mercury resistance is a Snowdrift Game. Intermediate concentrations of mercury select for the stable coexistence of plasmid-carrying and plasmid-free cells due to negative frequency dependent selection. In the longer term, environmental mercury selects for resistance to be encoded on the bacterial chromosome allowing cells to eliminate the costly plasmid. However, in the absence of mercury, resistance is maintained at a significantly greater proportion of the population when located on a mobile plasmid, as conjugative transfer promotes the spread of mercury resistance. These findings suggest that gene mobility plays an important role in the dynamics of microbial social traits. In the short term, location on a costly plasmid enables the persistence of ‘cheats’, however, in the longer term conjugative transfer can overcome these costs by enforcing cooperation on the entire population.