The theory of evolution in all its elegant explanations for the ongoing development of different species has left aside a significant aspect: identifying the factors determining the fitness of a species.
Organisms have adapted to environmental variability throughout evolution. Factors that determine evolutionary fitness are reproduction-related quantities like birth rate, number of offspring, the span of fertility, and viability. All those factors depend on the specific environment in a species’ habitat.
The fluctuating environment has a significant effect on the genetic structure of populations. A study found that changes make a population more able to adapt to initial environmental conditions and make them more adaptable in entirely new environments.
Fluctuating environments make populations capable of adaptive evolution and thus more able to adapt to new environmental change.
Environmental factors influence epigenetic switches and mutations. Epigenetic switches are molecular systems that can spontaneously switch between heritable phenotypes without DNA mutation. Heritable gene regulation, DNA methylation, chromatin modification, non-coding RNAs, and prions are some examples of epigenetic switching.
It has been hypothesized that epigenetic switches first evolved as a mechanism of bet-hedging and adaptation, but the evolutionary trajectories and conditions by which an epigenetic switch can outcompete adaptation through genetic mutation remain unknown.
Read the original publication of this study here: [ Epigenetic switching outcompetes genetic mutations during adaptation to fluctuating environments ]
This study aimed to determine whether a strain capable of epigenetic phenotypic switching dominates over a strain that cannot switch and how the rate of epigenetic switching influences the adaptation dynamics in fluctuating environments.
Epigenetic switching outcompetes genetic mutations during adaptation to fluctuating environments
Scientists predicted clones with an epigenetic control of genes targeted by selection should be fitter than clones that rely solely on genetic mutation.
To test this, an experimental setup that compares two strains was designed. The first is of a yeast strain containing epigenetic machinery with different rates of epigenetic switching. The second strain is characterized by the ability to only adapt through genetic mutation created through epigenetic silencing components.
Using previously constructed Saccharomyces cerevisiae (Brewer’s yeast) strains in which a URA3 reporter gene was inserted into a subtelomeric region, researchers were able to achieve differential epigenetic silencing of the gene.
At the beginning of the experiment, the cells were preselected by growing the strain cultures in complete synthetic media (CSM) lacking uracil. To distinguish the two strains, each was marked with a different fluorescent marker.
Co-cultures were exposed to fluctuating environments with regular intervals equal to or higher than the epigenetic switching rate, with alternating selection pressures for ON or OFF state of the URA3 gene.
An epigenetic system of inheritance established phenotypic heterogeneity in the population due to the rapid switching rate between different selectable phenotypes. This system produces phenotypes that might be maladaptive in one environment but beneficial upon environmental fluctuations.
Researchers observed an initial increase in the frequency of epigenetic switcher in a constant environment selecting for URA3 inactivation (5-FOA stable environment) (up to 98% at 72h). This was followed by the acquisition of mutations by the end of the experiment. Epigenetic switcher strains were still maintained under stable environmental conditions, even after 200 generations, thereby maintaining phenotypic and genetic diversity in the population.
Therefore, the results of this study indicate that epigenetic switching, the strength of gene activation or inactivation, and the resulting phenotypic variation are dependent on the environment. Additionally, the correlation between particular epigenetic gene expression patterns and the habitat seems to be species-specific.
In conclusion, an epigenetic form of inheritance dominates over genetic mutations when yeast is propagated in fluctuating environments. Most importantly, epigenetic switching can provide an additional layer for the maintenance of phenotypic and genotypic heterogeneity in the population, which represents the basis for evolution.
Takeaways:
- Epigenetic form of inheritance dominates over genetic mutations when yeast is propagated in fluctuating environments
- Epigenetic switcher strains were still maintained in the population under stable environmental conditions, even after 200 generations, thereby maintaining phenotypic and genetic diversity in the population.
- In stable environments, where populations are under constant uniform selective pressure, epigenetic systems offer a buffering mechanism until adaptive mutations are acquired.
You can read the original publication of this study here: [ Epigenetic switching outcompetes genetic mutations during adaptation to fluctuating environments ]