Abstract
Viruses are ubiquitous parasites that need their hosts to reproduce and survive and cause a detrimental effect to humans, domestic animals, wildlife and plants. As a consequence, viral infections have great socio-economic and ecological importance. In the last decades, numerous strategies for controlling viral epidemics have been developed. However, many of them became rapidly inefficient due to the appearance of mutations in the viral genome that allowed overcoming such control measures. It is thought that this is due to the rapid evolution of viruses. Thus, understanding the factors affecting virus evolution may contribute to design more efficient control measures of virus epidemics. Gene overlapping is a genetic event consisting in the codification of two genes in the same genomic region. It is widely employed by viruses to generate genetic novelty while retaining a small genome size. However, gene overlapping also increases the deleterious effect of mutations as they affect more than one gene, thereby reducing the evolutionary rate of viruses and hence their adaptive capacity. Such deleterious effect has reported for RNA viruses, which have an error prone polymerase, but has not being corroborated in DNA viruses, which have read-proof polymerases. Hence, in this work we have analysed the effect of gene overlapping in the rate of evolution of DNA viruses. Given that genome size has been shown to affect the rapidity of an organism´s evolution, we have focused on plant DNA viruses, which have genome sizes in the same range than RNA viruses, hence avoiding biases caused by differences in genome size. Sequences available in public databases from all plant DNA viruses harbouring overlapping genes in their genomes were selected. Genetic diversity (as a proxy of the speed of virus evolution) in the overlapping and non-overlapping regions was compared. We also analysed the relation between genetic diversity of overlapping genes and (i) the percentage of the gene overlapping or (ii) the genome size of each virus. Our results indicated that overlapping regions have lower genetic diversity than non-overlapping ones, as in RNA viruses. No association between the genetic diversity of overlapping genes and the percentage of gene overlapping or genome size was found. Thus, gene overlapping also reduces the speed of DNA virus evolution.
