Section: New Results

Spontaneous dynamics of genome size

Even though numerous genome sequences are now available, evolutionary mechanisms that determine genome size, notably their fraction of non-coding DNA, are still debated. In particular, although several mechanisms responsible for genome growth (proliferation of transposable elements, gene duplication and divergence, etc.) were clearly identified, mechanisms limiting the overall genome size remain unclear. By using a matrix population model, we showed that genome size can be simply limited by the spontaneous dynamics of duplications and large deletions, which tends to make genomes shrink even if the two types of rearrangements occur at the same rate. In the absence of Darwinian selection, we proved the existence of a stationary distribution of genome size even if duplications are twice as frequent as large deletions. To test whether selection can overcome this spontaneous dynamics, we also simulated our model numerically and chose a fitness function that directly favors genomes containing more genes, while keeping duplications twice as frequent as large deletions. In this scenario where, at first sight, everything seems to favor infinite genome growth, we showed that genome size remains nonetheless bounded. As a result, our study reveals a new pressure that could help limiting genome growth.

This work was part of Stephan Fischer's PhD thesis, which was defended in December 2013. A manuscript is currently under review. Stephan's PhD was co-supervised by Samuel Bernard (Inria Dracula team and Institut Camille Jordan, UMR CNRS 5208, Lyon).