Genome Dynamics and Temperature Adaptation During Experimental Evolution of Obligate Intracellular Bacteria

Area of Science:

• Microbial Evolution
• Bacterial Adaptation
• Symbiosis Research

Background:

• Evolution experiments with free-living microbes are common, but research on host-associated bacteria is limited.
• Strictly host-associated bacteria are widespread and crucial for understanding microbial adaptation.
• Protochlamydia amoebophila, an Acanthamoeba symbiont, serves as a model for protist-associated chlamydiae.

Purpose of the Study:

• To investigate the evolution and adaptation of obligate intracellular symbionts to elevated temperatures.
• To understand the molecular basis of adaptation in Protochlamydia amoebophila under thermal stress.
• To explore the evolutionary leap from protist to endothermic animal hosts.

Main Methods:

• Established 12 replicate populations of Protochlamydia amoebophila at 20 °C and 30 °C for 510 bacterial generations.
• Utilized infectivity assays to assess evolved phenotypes.
• Employed pooled whole-genome resequencing to identify genetic mutations and adaptive changes.

Main Results:

• Symbionts evolved at 30 °C showed an overall reduction in infectivity.
• Numerous nonsynonymous mutations and small indels were identified, with some variants persisting and reaching high frequencies.
• Mutated genes within the same temperature regime exhibited higher similarity than those between regimes.

Conclusions:

• Elevated temperature adaptation in chlamydial symbionts involves a trade-off, attenuating infectivity to reduce host cell burden.
• This adaptation strategy provides insights into the molecular evolution of intracellular bacteria under host dependence.
• Findings suggest a mechanism facilitating the evolutionary transition to endothermic animal hosts.