The interplay between relatedness and horizontal gene transfer drives the evolution of plasmid-carried public goods.

Plasmids carry a wide range of genes that are often involved in bacterial social behaviour. The question of why such genes are frequently mobile has received increasing attention. Here, we use an explicit population genetic approach to model the evolution of plasmid-borne bacterial public goods production. Our findings highlight the importance of both transmission and relatedness as factors driving the evolution of plasmid-borne public goods production. We partition the effects of plasmid transfer of social traits into those of infectivity and the effect of increased relatedness. Our results demonstrate that, owing to its effect on relatedness, plasmid mobility increases the invasion and stability of public goods, in a way not seen in individually beneficial traits. In addition, we show that plasmid transfer increases relatedness when public goods production is rare but this effect declines when production is common, with both scenarios leading to an increase in the frequency of plasmid-borne public goods. Plasmids remain important vectors for the spread of social genes involved in bacterial virulence thus an understanding of their dynamics is highly relevant from a public health perspective.

The evolution of conflict resolution between plasmids and their bacterial hosts.

It has recently been proposed that mobile elements may be a significant driver of cooperation in microorganisms. This may drive a potential conflict, where cooperative genes are transmitted independently of the rest of the genome, resulting in scenarios where horizontally spread cooperative genes are favored, whereas a chromosomal equivalent would not be. This can lead to the whole genome being exploited by surrounding noncooperative individuals. Given that there are costs associated with mobile elements themselves, infection with a plasmid carrying a cooperative trait may lead to a significant conflict within the host genome. Here, we model the mechanisms that allow the host to resolve this conflict, either by exhibiting complete resistance to the mobile element or by controlling its gene expression via a chromosomally based suppressor. We find that the gene suppression mechanism will be more stable than full resistance, implying that suppressing the expression of costly genes within a cell is preferable to preventing the acquisition of the mobile element, for the resolution of conflict within a genome.

Horizontal gene transfer and the evolution of bacterial cooperation.

Bacteria frequently exhibit cooperative behaviors but cooperative strains are vulnerable to invasion by cheater strains that reap the benefits of cooperation but do not perform the cooperative behavior themselves. Bacterial genomes often contain mobile genetic elements such as plasmids. When a gene for cooperative behavior exists on a plasmid, cheaters can be forced to cooperate by infection with this plasmid, rescuing cooperation in a population in which mutation or migration has allowed cheaters to arise. Here we introduce a second plasmid that does not code for cooperation and show that the social dilemma repeats itself at the plasmid level in both within-patch and metapopulation scenarios, and under various scenarios of plasmid incompatibility. Our results suggest that although plasmid carriage of cooperative genes can provide a transient defense against defection in structured environments, plasmid and chromosomal defection remain the only stable strategies in an unstructured environment. We discuss our results in the light of recent bioinformatic evidence that cooperative genes are overrepresented on mobile elements.