Nonclassical phase diagram for virus bacterial co-evolution mediated by CRISPR

TL;DR

This paper explores the complex phase diagram of virus-bacteria co-evolution mediated by CRISPR, revealing novel coexistence regimes, non-monotonic extinction probabilities, and recombination strategies for viral escape, with implications for understanding microbial dynamics.

Contribution

It introduces a detailed phase diagram for virus-bacteria co-evolution under CRISPR, highlighting new coexistence regions and escape mechanisms not previously characterized.

Findings

Virus extinction probability exhibits non-monotonic behavior.

Recombination enables viruses to escape CRISPR recognition.

Reentrant phase diagram predicts multiple coexistence and extinction phases.

Abstract

CRISPR is a newly discovered prokaryotic immune system. Bacteria and archaea with this system incorporate genetic material from invading viruses into their genomes, providing protection against future infection by similar viruses. The conditions for coexistence of prokaryots and viruses is an interesting problem in evolutionary biology. In this work, we show an intriguing phase diagram of the virus extinction probability, which is more complex than that of the classical predator-prey model. As the CRISPR incorporates genetic material, viruses are under pressure to evolve to escape the recognition by CRISPR. When bacteria have a small rate of deleting spacers, a new parameter region in which bacteria and viruses can coexist arises, and it leads to a more complex coexistence patten for bacteria and viruses. For example, when the virus mutation rate is low, the virus extinction probability changes non-montonically with the bacterial exposure rate. The virus and bacteria co-evolution not only alters the virus extinction probability, but also changes the bacterial population structure. Additionally, we show that recombination is a successful strategy for viruses to escape from CRISPR recognition when viruses have multiple proto-spacers, providing support for a recombination-mediated escape mechanism suggested experimentally. Finally, we suggest that the reentrant phase diagram, in which phages can progress through three phases of extinction and two phases of abundance at low spacer deletion rates as a function of exposure rate to bacteria, is an experimentally testable phenomenon.