Recent studies have unearthed a treasure trove of prehistoric virus ‘fossils’, viral genomes or genome segments frozen millions of years ago as integrated copies in the genomes of diverse animal hosts. The fact that these integrated viral fossils can be easily recognized as belonging to modern virus families is stunning given the fact that modern exogenous viruses have replicated and evolved for many millions of years since these viral fossils were captured. Despite high rates of mutation, the evolution of virus sequence is clearly constrained. This constraint comes partially from intrinsic selective forces that limit virus evolution, such as selection for modulation of pathogenicity to the host and the structural constraints of the virus itself. Other major constraints on virus evolution come from the diverse immune strategies imposed by hosts. Cumulatively, these constraints act together to limit all aspects of virus evolution from the swarm of variants produced in a single host to the evolution of expanded host range. These newly identified fossils indicate that constraint on virus evolution might be far greater than has previously been appreciated.
Two-stepping through time: mammals and viruses. (2011) Trends Microbiol. 19(6): 286-294
Recent studies have identified ancient virus genomes preserved as fossils within diverse animal genomes. These fossils have led to the revelation that a broad range of mammalian virus families are older and more ubiquitous than previously appreciated. Long-term interactions between viruses and their hosts often develop into genetic arms races where both parties continually jockey for evolutionary dominance. It is difficult to imagine how mammalian hosts have kept pace in the evolutionary race against rapidly evolving viruses over large expanses of time, given their much slower evolutionary rates. However, recent data has begun to reveal the evolutionary strategy of slowly-evolving hosts. We review these data and suggest a modified arms race model where the evolutionary possibilities of viruses are relatively constrained. Such a model could allow more accurate forecasting of virus evolution.