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Coevolution of Codependent Hosts and Symbionts
Michael Lynch1, Kunaal Joshi1, Adrian González-Casanova1
1Biodesign Center for Mechanisms of Evolution, Arizona State University, Tempe, AZ 85287.
Biorxiv : the Preprint Server for Biology
|August 1, 2026
Summary
Most symbiotic relationships appear mutually beneficial, but theory suggests endosymbionts often exploit hosts. Evolutionary outcomes depend on factors like inheritance mode and population size, rarely leading to host benefit.
Area of Science:
- Evolutionary Biology
- Microbial Ecology
- Genetics
Background:
- Eukaryotic endosymbioses often appear mutually beneficial, but direct evidence is scarce.
- Symbiotic interactions can create selection pressures for resource extraction by either partner.
- Endosymbionts may evolve to conflict with or cooperate with host interests.
Purpose of the Study:
- To develop population-genetic theory clarifying conditions favoring conflict or cooperation in obligate endosymbioses.
- To identify factors influencing the evolutionary trajectory of endosymbiont-host interactions.
Main Methods:
- Theoretical modeling of population genetics in endosymbiotic systems.
- Analysis of factors such as symbiont number, transfer modes, and mutation rates.
- Examination of host population size effects on evolutionary outcomes.
Main Results:
- Exploitation by the endosymbiont is often favored, especially with high symbiont numbers, strong selection on symbionts, horizontal transfer, or rapid symbiont mutation.
- Host exploitation can be enhanced by symbiont manipulation of host cell biology, reducing within-host drift.
- Even with strict vertical inheritance, symbiont exploitation can occur if host population size is too small.
Conclusions:
- The evolution of endosymbionts benefiting hosts (evolutionary enslavement) requires a specific combination of population-biological traits in both partners.
- Most obligate endosymbioses may result in a stalemate or host exploitation rather than mutual benefit.
- Understanding these dynamics is crucial for comprehending the evolution of complex life forms.
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