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bioRxiv · 10.64898/2026.10.14.759074

Spatial structure drives the evolution of parasites into host defense systems

Nazir, S. A.·Ashby, B.·Leeks, A.·van Dijk, B.
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Abstract

Host defense systems in prokaryotes and eukaryotes often appear to be derived from the very parasites they now defend against, yet the evolutionary route for such a transition remains poorly understood. Based on the biology of integrating viruses such as temperate bacteriophages and retroviruses, we develop a multilevel eco-evolutionary framework in which within-host viral replication is treated as a public goods game over a costly, shared replication product. In well-mixed host populations, we predict convergence on a single viral strategy of intermediate replicative investment. However, when we impose spatial structure by restricting host reproduction and virus transmission to local events, the virus population diversifies into two lineages: a horizontally transmitted lineage that parasitises hosts, and a vertically transmitted hyperparasitic lineage that parasitises the horizontally transmitting viruses. The latter "hyperparasite" is a viral cheat that suppresses the replication and propagation of the parasitic virus, thereby acting as a defense system to the hosts. Local interactions concentrate the benefits of this suppression among neighbouring hosts that carry related proviruses, allowing selection on the proviral cheat as a host-level defense. Our results show that evolutionary cheating within parasite populations can drive the evolution of parasites into defense systems in spatially structured host populations.

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Nazir, S. A., Ashby, B., Leeks, A., van Dijk, B.. 2026-10-16. Spatial structure drives the evolution of parasites into host defense systems. https://doi.org/10.64898/2026.10.14.759074

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