The bacterial march toward symbiosis: on-ramps and off-ramps.

Population genetics models reveal that long-lived hosts have higher probabilities of fixing heritable symbionts, shaping the evolution of host-microbe relationships.

• Why it matters: Understanding how symbioses establish and persist is crucial for comprehending their roles across diverse life forms and ecological systems, yet the conditions promoting symbiont fixation remain unclear.
• What they did: The authors used mutation-selection models to analyze how host population size, life history, and fluctuating fitness effects influence the likelihood of symbiont fixation within host populations.
• The result: Findings suggest that long-lived hosts and variable symbiont fitness over time increase the chances of fixing even deleterious symbionts, providing a framework to predict symbiosis emergence across biological systems.