Wild aquatic birds are thought to be the natural hosts of Influenza A viruses, which typically don't cause severe disease in these species (H5N1 being a notable exception).
Previous studies have suggested that influenza A viruses are under strong "purifying" selection in wild birds and so have reached a point of evolutionary stasis in these hosts. For example, the number of genetic changes that lead to amino acid changes in the corresponding proteins (non-synonymous substitutions) has been reported to be low relative to the number of genetic changes that don't lead to an amino acid change (synonymous substitutions).
However, CIDD researchers have now found that avian influenza A viruses from wild aquatic birds actually have high rates of evolutionary change, similar to those seen in influenza A viruses in mammals and domestic poultry, and typical of RNA viruses in general.
This new analysis, of 3147 viral sequences from 10 different viral subtypes isolated from a variety of host species, allowed for the possibility that evolutionary rates and demographic histories may have differed in different viral lineages.
Ratios of non-synonymous to synonymous substitutions were found to be similar across viral genes, viral subtypes (including H5N1), and host type. This suggests that avian influenza virus moves frequently between wild aquatic birds and other species, and is not in a state of evolutionary stasis in wild aquatic birds.
The analysis also suggested that most of today's viral sequences had arisen from common ancestors within the last 100 years. This finding is indicative of a "boom and bust" pattern of evolution: the viruses mutate and reassort to generate genetic "variations on a theme"; consequently, diversity increases until one sequence confers especially high fitness and so sweeps to fixation, becoming the "theme" on which the next set of variations are generated.
Written By: Rubing Chen& Edward C Holmes
Journal: 23: 2336-2341
Journal Reference: 23: 2336-2341
Paper Id: 10.1093/molbev/msl102