A simple mathematical model identifies when a temperate phage that encodes a defense system for its bacterial host can invade, persist, or coexist with other phages despite costs to host growth.
A prophage that imposes host costs can still be favored if defense gives it an advantage in competition with other temperate phages.
When defense can be lost by mutation, defensive, non-defensive, and defense-loss variants can coexist through a rock–paper–scissors-like cycle that may produce persistent oscillations.
The model quantifies ranges of parameter space (including growth costs up to roughly 20%) where defensive prophages invade, where bistability occurs, and where stable coexistence emerges.
# Overview
# What the model does The authors build a simplified, population-dynamic model of competition between temperate phages that infect the same bacterial host. The model tracks variants that carry a defense system, variants that lack it, and variants that have lost defense by mutation. It focuses on long-term outcomes rather than short-term infection events.
# Main findings The model identifies several distinct dynamical regimes depending on parameters such as the cost of defense, rates of infection, and mutation that leads to defense loss.
# Why this matters Prophage-borne defense systems are widespread in microbial genomes, but their evolutionary role for the phage itself has been unclear because many defenses slow host growth or carry other fitness costs. This model provides a clear set of conditions under which those costs are offset by competitive benefits between phages. The results give quantitative expectations that can be tested experimentally or in environmental datasets.
# Practical implications and next steps Researchers can use the model's parameter regimes to design experiments that test whether prophage-carried defenses confer a net advantage in multi-phage settings. The work suggests looking for non-transitive interactions among phage variants and measuring rates of defense loss. The authors made the minimal data set and code available on Zenodo (DOI provided in the paper) to reproduce and extend the analysis.
# Bottom line Even costly prophage-borne defenses can be evolutionarily stable when temperate phages frequently compete for the same host species. The simple framework clarifies when defense pays off for the prophage, when historical contingencies matter, and when complex coexistence or oscillatory dynamics should be expected.
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