Photo: Evolved macroscopic “snowflake” yeast from the MuLTEE experiment. The large size of the nuclei (yellow) and cells (cyan) are results of whole-genome duplication and aneuploidy. Credit: Georgia Institute of Technology
The specific chromosomal configurations and changes in their numbers may persist or disappear depending on population size and environmental conditions.
Can we live outside of Earth while considering the impact of population size on our general condition? A recent experiment from the laboratory at the Georgia Institute of Technology, published in ,,Nature”, suggests that it may be possible. How? Through the duplication of selected parts of the genome, specifically chromosomes. This isn’t just science fiction; researchers in the U.S. have demonstrated that cells with four sets of chromosomes (tetraploids) can maintain a specific number of chromosomes that differs from the standard (aneuploidy) under the right selective conditions. Although these newly created polyploid genomes are highly unstable, there are ways for the new cells, built according to the “updated” instructions, to persist in the genome for a certain amount of time and across a limited number of generations, as seen in the snowflake yeast used in the experiment.
There may be challenges in applying the study results to humans, as yeast has only 16 chromosomes, while humans typically have 46. However, the research indicated evolutionary pathways along with genetic mechanisms, specifically during the phases of mitosis when chromosomes condense and duplicate before being divided into a specific set within a single cell. In normal conditions, the standard chromosomal set is diploid, meaning two sets of chromosomes align at the edge of a dividing nucleus or cell during mitosis. Notably, the experiment demonstrated that under selection for larger multicellular size, yeast can rapidly evolve to become tetraploid. This means that the four sets of chromosomes evolved because they offer immediate fitness benefits, contributing to larger cell sizes and the ability to form more complex structures. However, it is important to remember that evolution not only produces new traits but also leads to the loss of others. The question then arises: When does this loss occur?
In our experiment, we observed that certain chromosomes were consistently gained or lost in specific lineages, particularly in the anaerobic populations that evolved macroscopic size. For example, chromosomes VIII and XIV were frequently lost in strains that reverted from macroscopic to microscopic size, suggesting these specific chromosomal configurations can be selectively maintained when they provide adaptive benefits in particular environments,
says William C. Ratcliff in a special comment for evolutionandsecurity.com.
Chromosome VIII in yeast plays a vital role in biosynthesis, which is essential for anabolism—the process of building up the body and supporting its vital functions. Additionally, this chromosome triggers adaptive mechanisms. In humans, chromosome VIII contains approximately 700 genes that provide instructions for producing proteins crucial for developmental health, particularly for bones and the brain. This chromosome also transmits genetic information from one generation to the next and is responsible for cell replication. The deletion of chromosome XIV in humans may trigger epilepsy and other diseases associated with tubulin disruption, which is a critical structure for signaling pathways in the nervous system.
The study results clearly demonstrate the evolutionary mechanisms that contribute to an increase in population size. However, a less optimistic aspect is that this growth may be accompanied by a rise in the number of diseases affecting humans. On the other hand, scientists believe that polyploidy, despite its instability and limitations, can be preserved through molecular mechanisms that stabilize duplicated genomes, thus enhancing adaptive traits. What’s more, they write that ,,the instability of nascent polyploid genomes may provide an evolutionary advantage under novel environments by rapidly generating genetic variation, especially via aneuploidy. This has been shown to facilitate the rapid evolution of microorganisms and cancer”.
Yeast is are single-celled eukaryote, whereas humans are multicellular eukaryotes. Yeast can reproduce both asexually and sexually. Asexual reproduction occurs through a process called budding, where mitosis creates similar cells or cells with variations in phenotype, allowing them to adapt better to their environment. Sexual reproduction, on the other hand, involves meiosis, which promotes genetic recombination, specifically among eukaryotes.
Both processes serve essential but different evolutionary functions. Mitosis enables growth and asexual reproduction across all domains of life, while meiosis facilitates genetic recombination and sexual reproduction specifically in eukaryotes. If I were to address which might be more fundamental evolutionarily, I would suggest mitosis, as it’s universal across all cellular life and enables basic cellular proliferation. However, meiosis has been crucial for creating the genetic diversity that has allowed for the evolution of complex multicellular organisms,
emphasizes William C. Ratcliff in a special comment for evolutionandsecurity.com.
The scientists concluded that they demonstrated how even complex aneuploid karyotypes (the number of chromosome sets) can be maintained over thousands of generations, despite the inherent instability of the genomes. They achieved this by better tolerating the potential costs associated with reduced gene dosage effects.
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