Competition favours reduced cost of plasmids to host bacteria

Abstract

Conjugative plasmids of Gram-negative bacteria have both vertical and horizontal modes of transmission: they are segregated to daughter cells during division, and transferred between hosts by plasmid-encoded conjugative machinery. Despite maintaining horizontal mobility, many plasmids carry fertility inhibition (fin) systems that repress their own conjugative transfer. To assess the ecological basis of self-transfer repression, we compared the invasion of bacterial populations by fin þ and fin À variants of the plasmid R1 using a computational model and co-culture competitions. We observed that the fin þ variant had a modest cost to the host (measured by reduction in growth rate), while the fin À variant incurred a larger cost. In simulations and empirical competitions the fin À plasmid invaded cultures quickly, but was subsequently displaced by the fin þ plasmid. This indicated a competitive advantage to reducing horizontal transmission and allowing increased host replication. Computational simulations predicted that the advantage associated with reduced cost to the host would be maintained over a wide range of environmental conditions and plasmid costs. We infer that vertical transmission in concert with competitive exclusion favour decreased horizontal mobility of plasmids. Similar dynamics may exert evolutionary pressure on parasites, such as temperate bacteriophages and vertically transmitted animal viruses, to limit their rates of horizontal transfer.

A cooperative virulence plasmid imposes a high fitness cost under conditions that induce pathogenesis

“…Expression of the plasmid conjugal machinery used to transfer the plasmid between bacterial cells can also impose a dramatic fitness burden on host cells [29,32,38]. This cost establishes a trade-off between horizontal and vertical transmission of a plasmid because conjugation of the plasmid comes at the expense of lower cell division rates, and thus a lower efficiency of plasmid vertical transmission [32].…”

How Do Species Interactions Affect Evolutionary Dynamics Across Whole Communities?

“…Because vectors are under selection to spread independently of host genomes, there could be complex interactions between the ecological impacts and the evolutionary benefits of HGT vectors (Harrison & Brockhurst 2012). For example, competition between coexisting plasmids or hosts could generate selection for reduced HGT rates (Haft et al 2009). …”

What traits are carried on mobile genetic elements, and why?

“…When should a mobile element provide a benefit and when should it harm its host, and how will these evolutionary decisions in turn shape mobility? The answer to these questions are likely to depend on the details of the tradeoffs between horizontal transmission, vertical transmission and virulence (Ferdy and Godelle, 2005;Haft et al, 2009). The more an element transmits vertically, the more it will depend on the reproductive value of the host, and selection will therefore favour genes that are beneficial to the host, whereas higher HGT rates will exert an effect to the detriment of the host (subject to tradeoffs).…”

“…Infectious elements such as plasmids are likely to face a tradeoff between horizontal transfer and vertical transmission, mediated by the costs that they impose on their hosts (Turner et al, 1998;Turner, 2004;Haft et al, 2009). Any tradeoff between HGT and costs to the host can be viewed as a form of the much-discussed virulence-transmission tradeoff (Alizon et al, 2009).…”