The role of mutational dynamics in genome shrinkage.

Genome shrinkage occurs after whole genome duplications (WGDs) and in the evolution of parasitic or symbiotic species. The dynamics of this process, whether it occurs by single gene deletions or also by larger deletions are however unknown. In yeast, genome shrinkage has occurred after a WGD. Using a computational model of genome evolution, we show that in a random genome single gene deletions cannot explain the observed pattern of gene loss in yeast. The distribution of genes deleted per event can be very well described by a geometric distribution, with a mean of 1.1 genes per event. In terms of deletions of a stretch of base pairs, we find that a geometric distribution with an average of 500-600 base pairs per event describes the data very well. Moreover, in the model, as in the data, gene pairs that have a small intergenic distance are more likely to be both deleted. This proves that simultaneous deletion of multiple genes causes the observed pattern of gene deletions, rather than deletion of functionally clustered genes by selection. Furthermore, we found that in the bacterium Buchnera aphidicola larger deletions than in yeast are necessary to explain the clustering of deleted genes. We show that the excess clustering of deleted genes in B. aphidicola can be explained by the clustering of genes in operons. Therefore, we show that selection has little effect on the clustering of deleted genes after the WGD in yeast, while it has during genome shrinkage in B. aphidicola.