Dear Colleagues,

Viruses are in a perpetual arm race with their hosts. Camouflage is a common strategy viruses use to escape to the immune system (either innate or adaptive) of their hosts. This generally translates in a propensity to develop replication strategies that are, at different extents, prone to the insertion of mutations in their genome. Accumulation of mutations is nevertheless limited by the need to maintain viability and its own genetic identity. Keeping the subtle equilibrium between these two contrasting forces is vital for viruses, it often influences their pathogenic potential, and can be at the origin of outbreaks of infection of relevance for public health.

Recombination is an important source of genetic variability in viruses, particularly for viruses possessing an RNA genome. The remarkable power of recombination resides in its ability, in a single infectious cycle, to generate new combinations of mutations. This is important at two regards: one is that recombination does not generate new mutations but reshuffles pre-existing ones, whose compatibility with viral survival has already been established. This is expected to increase the probability of having a viable recombinant progeny. On the other hand, the fact that, in general, several mutations are simultaneously introduced through the recombination process, is expected to favour the opposite outcome: that a high proportion of recombinant products will not be viable. Finally, recombination in concert with natural selection, can be responsible of combining advantageous mutations, as well as removing deleterious ones, by far the most abundant type of mutations found in nature.

For many viruses the generation of recombinant variants has been associated to important moments in the processes of adaptation, gain of pathogenic potential or increased spreading. Here we intend to present several of these cases and to provide an overlook of the implications of recombination for viral evolution from the theoretical standpoint. A life style involving increasing travelling with the result of an extensive intermingling of viruses, the making a reality of intervention strategies (as transgenic crops and gene therapy) often based on the use of viral vectors, all are issues for which an improved understanding of the role of recombination in viral evolution becomes increasingly urgent.

Dr. Matteo Negroni
Guest Editor

• recombination
• viruses
• evolution
• genetic variability
• natural selection