Analysis of influenza A virus nucleoproteins for the assessment of molecular genetic mechanisms leading to new phylogenetic virus lineages.

The nucleoprotein (NP) gene of influenza A viruses is decisive for separating two large individually evolving reservoirs in birds and humans. A phylogenetic analysis of the NP gene revealed that all mammalian influenza viruses originated--directly or indirectly--from an avian ancestor. The stable introduction of an avian influenza A virus into a mammalian species seems to be a relatively rare event, the latest one occurred in 1979 when such an avian virus was introduced into pigs in Northern Europe which gave rise to a new lineage. At least two concomitant events are required for such a new and stable introduction: (1) The new species has to become infected, and (2) a mutation in the polymerase complex has to establish a labile variant, which is prone to provide a large number of different variants, from which some can adapt rapidly to the new host (or to any unusual environments). Since such mutator mutations might be advantageous only during stress periods, variants with a less error prone polymerase might emerge again after adaptation. Examples for such fluctuations in terms of mutational and evolutionary rates are discussed in this brief review.