Slow coevolution of a viral pathogen and its diploid host

Area of Science:

• Evolutionary biology
• Epidemiology
• Population genetics

Background:

• Lethal infectious diseases drive host-pathogen coevolutionary dynamics.
• Understanding the genetic basis of host response and pathogen variation is critical.
• Previous models often simplify the interplay between epidemic spread and genetic adaptation.

Purpose of the Study:

• To derive explicit equations for host and pathogen genotype dynamics in an SI epidemic model.
• To analyze the evolutionary trajectories of host and pathogen gene frequencies under disease pressure.
• To investigate conditions leading to stable states versus oscillations in host-pathogen systems.

Main Methods:

• Developed an SI-type epidemic model incorporating one host locus (two alleles) and two pathogen variants.
• Applied timescale separation between epidemic and evolutionary processes for simplified analysis.
• Derived explicit equations for changes in host and pathogen gene frequencies.

Main Results:

• The model exhibits complex behavior, including multiple stable states and oscillations.
• Oscillatory dynamics in the simplified model are degenerate, preventing limit cycles.
• Frequency-dependent selection in the pathogen, such as through free-living stages, removes this degeneracy.

Conclusions:

• Frequency-dependent selection is essential for resolving degenerate oscillations and achieving stable coevolutionary dynamics.
• The findings extend to SIR-type models, highlighting the broad applicability of these evolutionary principles.
• This research provides a framework for understanding genetic adaptation in host-pathogen interactions.