Procaryots are better protected from transposable elements

Although individual are likely to die due to DNA damage, The early death of cells "infected" with transposable elements prevents these elements from spreading

„Transposable elements" are small DNA segments found in nearly all genomes across the tree of life. Their function is not fully understood, but their properties are remarkable: they can integrate themselves into DNA and also replicate there on their own, independent of their host. Therefore, it is assumed that the proliferation of transposable elements has contributed decisively to the genome size of  eukaryotes. However, transposable elements are also found in prokaryotes, which have much smaller genomes. Moreover, their genomes are significantly streamlined, meaning a much smaller proportion is non-coding.
With the help of computer modeling, a team of researchers at the Max Planck Institute for Evolutionary Biology in Plön, Germany, has now succeeded in shedding some light on the matter.
 

Evolutionary cycle of a so-called "rock-paper-scissors" equilibrium between streamlined populations, non-streamlined populations, and the transposable elements (coding sequences are depicted in blue, transposons in red).

The scientists found that populations of streamlined genomes are better protected against transposable elements. Although individual cells "infected" with transposable elements are likely to die due to DNA damage, this early death prevents those from spreading in the population. This protective mechanism only works for asexual prokaryotes, and not for eukaryotes who reproduce sexually.

In the simulation, a three-way "rock-paper-scissors" equilibrium was established between streamlined populations, non-streamlined populations, and the transposable elements: First, the streamlined population prevails over the elements, but those do not die out. This is because with fewer transposable elements, selection in favor of the streamlined populations decreases. The non-streamlined populations have an advantage over the former because they are more robust to mutations. As the population of non-streamlined genomes increases, transposable elements once again have an opportunity to thrive, etc.

The modeling shows that the "transposable elements" appear to be an important factor in the evolution of the size of a genome; they can cause both streamlining and enlargement.

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