Viral transduction and the dynamics of bacterial adaptation

Transduction - horizontal gene transfer (HGT) by viruses - is an important macroevolutionary force in prokaryotes, contributing to functional innovation and lineage diversification. In contrast, the role that transduction plays in bacterial microevolutionary adaptation is poorly known. By facilitating the transfer of beneficial alleles between host cells, transduction may accelerate adaptation. But transduction also carries the risk of transferring deleterious alleles, which, in addition to the demographic cost of viral infection, may hinder adaptation. Here we resolve the conflicting effects of transduction on bacterial adaptation in a simple eco-evolutionary model for large populations characterized by a quantitative (resource-use) trait with a single evolutionary optimum. Our model focuses on generalized transduction by virulent phages. Away from the optimum, the effect of transferring beneficial alleles dominates and transduction tends to accelerate adaptation. Close to the optimum, transduction generates a large amount of stochasticity in the population adaptive trajectory, thus hindering adaptation. Under disruptive selection, transduction may either limit (as sexual recombination would) or promote phenotypic diversification, or drive ‘transient optimization’ whereby phenotypic subpopulations recurrently visit the optimum. Our modeling framework paves the way to study complex adaptive feedbacks between bacterial hosts and phages generated by the combination of deterministic and stochastic effects of transduction.