Gleb G. Ebert


Last Name

Ebert

First Name

Gleb G.

ORCID

0000-0002-7734-3824

Organisational unit

03939 - Velicer, Gregory J. / Velicer, Gregory J.

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Publications 1 - 2 of 2
  • Hypermutation and Fluctuating Stress Levels Can Enable Evolutionary Rescue
    Item type: Other Conference Item
    Ebert, Gleb G.; Toll Riera, Macarena (2025)
    ESEB 2025 Abstract Book
    When an environmental change leads to a decline in population size, selection on standing or de novo genetic variation can lead to recovery of the population. This process is called evolutionary rescue and is an important mechanism preventing populations from going extinct due to abiotic stressors such as temperature. In an earlier evolution we explored the evolvability of the upper thermal limit in Pseudoalteromonas haloplanktis TAC125, a cold-adapted strain of a globally distributed marine bacterium, by slowly increasing the cultivation temperature over the course of 900 generations. While we succeeded in extending the upper thermal limit by one degree to 30°C, the experiment revealed a constrained evolutionary potential of the upper thermal limit. We hypothesised that increasing the mutational supply might lead to this hard limit being surpassed. In this follow up evolution experiment we picked evolved clones from the previous experiment with and without a hypermutator phenotype and subjected them to regularly fluctuating levels of heat stress (2.5°C apart). By gradually increasing the temperatures over 840 additional generations, we were able to reach 32°C, at which point further adaptation stalled. Every time the temperatures were increased, the evolving populations declined in optical density but slowly recovered with time, indicating repeating waves of evolutionary rescue. Furthermore, populations descending from hypermutator clones showed higher optical densities throughout the course of adaptation to increasing temperatures compared to populations descending from nonmutator clones. While over half of the hypermutator populations acquired additional mutations in genes affecting the mutation rate, only one nonmutator population evolved a hypermutator phenotype. Hypermutator populations on average accumulated 8x more mutations than nonmutator populations. Interestingly, hypermutator populations experienced 14x more mutations rising to fixation than nonmutator populations. Furthermore, we observe parallel genome evolution with genes encoding for membrane-associated proteins in particular showing the exact same mutations in up to a third of all populations. Our experiment exemplifies how a number of evolutionary processes can interact to increase the success of adaptation, a particularly relevant finding in our rapidly changing world.
  • The Upper Limits of Thermal Adaptation in an Antarctic Bacterium
    Item type: Doctoral Thesis
    Ebert, Gleb G. (2025)
    Temperature is a complex stressor with a broad genomic basis of adaptation. The ability of organisms, primarily eukaryotes, to adapt to increasing temperatures has been studied extensively. The evolution of upper thermal limits, the highest temperature an organism can reproduce at, is of particular interest as it directly influences the survival of populations and whole species. Psychrophilic, i.e. cold-adapted, organisms are particularly suitable model systems to study adaptation to heat as they usually experience temperatures far below the temperature at which they reproduce fastest. The model organism for the research described here is Pseudoalteromonas haloplanktis TAC125 (PhTAC125), an Antarctic strain of a globally distributed marine bacterium. To improve our understanding of how PhTAC125, and bacteria in general, adapt to temperatures rising above their upper thermal limits, I conducted three research projects. (1) I examined the impacts of high-temperature-adaptation and the underlying parallel genome evolution observed in a previous evolution experiment on the transcriptome. I found that while the adapted clones grow at higher temperatures than the ancestor, their transcriptomes are still in a highly stressed state, though each evolved unique adaptations. (2) Next I investigated the role of Lon, an important protein quality control protease which has been found to be under strong selection at high temperatures. Surprisingly, I observed populations which lacked parts of the lon gene to show fitness benefits at intermediate levels of heat stress over populations with a complete lon allele. In the end, I found strong selection against the wild-type allele of lon at high temperatures. (3) Finally, I aimed to surpass the previously established upper limit of thermal adaptation at 30 °C using a fluctuating temperature regime and clones with increased mutation rates (hypermutators). While the fluctuating heat stress levels enabled both hypermutator and non-mutators to adapt to growth at 32 °C, increased mutation rates themselves only resulted in a modest fitness advantage. In conclusion, these three projects have contributed to our understanding of the evolvability of organisms, particularly under severe heat stress, a high relevance topic in the context of climate change.
Publications 1 - 2 of 2