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Introduction:

Reductive Evolution

  Reductive Evolution is the process of microorganisms causing a downsizing in an organism’s genome, usually found within a symbiotic relationship. It can also occur when bacteria leave a free-living state and enter a restrictive state. Altogether this results in a rearrangement causing a loss in the bacteria of regulatory elements. The parasite creates the bacteria into its own host and supplies itself nutrients by downsizing the genome. Some bacteria will adapt to survive in the restrictive state by altering and reducing its genome to get rid of the nonessential material.

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Microorganisms:

Rickettsia prowazekii

Buchnera aphidicola

History:

Article body

Reductive Evolution suggests that Eukaryotes are the ancestors of Bacteria and Archaea instead of the Endosymbiotic Theory which is said that Eukaryotes absorbed another microorganism and kept it around because it was advantageous to do so. Reductive evolution deals with reduced genome size: loss of genes because they were deemed not to be important to the cell or benefit the cell in its specific niche. When compared to fossil evidence reductive evolution can be demonstrated.

DNA found in ancient prokaryotic and mitochondria fossils have been found to have higher levels of Cytosine and Guanine compared to today's reduced genome size of the same organism. Different segments of the genome found to be unfavorable have possibly been removed over time due to deletions of repetitive DNA and shrink its genome.

There are a few general models for reductive evolution, there are phases that microorganisms can go through.

The short phase: This is generally a fast phase of reproduction which leads to a greater and more complex genome usually involving mutations. This leads to a longer reductive phase, which means loss of genetic material, or a more direct genome. Basically it means the genome can become simplified and more complex due to random mutations.

Methods:

There are many methods to help identify if genes have been deleted, two of which are maximum parsimony (MP) or maximum likelihood (ML) patterns are used to recreate the evolutionary tree of these species and their gene compositions of the ancient forms as well as the gene losses and gained along the tree branches which are then compared to each other. There are limitations however, mostly due to using different models or adding new information which can skew results. Such as using Dollo Parsimony or Weighted Parsimony,

Genome Sizes

Maximum parsimony (MP)

Maximum likelihood (ML)

Rickettsia prowazekii is an unrestricted microorganism which has been used to demonstrate genome degradation DNA and genome size is not linked to the complexity of an organism. There are some bacteria that have a lot more DNA than a human. This is not yet understood and is referred to as the C-value Enigma or C-value Paradox. To put it simply, the amount of DNA in a haploid genome doesn't relate to the complexity of an organism and can be very different.

References

https://www.biology.lu.se/research/research-groups/microbial-ecology/research-projects/reductive-evolution-of-microbial-genomes