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Monday, October 30, 2006Evolution of influenza A virus is commonly viewed as a typical Darwinian process. In this mode of evolution, the virus’ main surface protein, hemagglutinin (HA), is thought to continually change to evade human immune response, resulting in new dominant strains that eliminate all competitors in a series of rapid successions. Unexpectedly, however, the study found that the periods of intense Darwinian selection accounted for only a relatively small portion of H3N2 flu evolution during the ten-year period examined. The study found that much of the time the H3N2 virus seemed to be “in stasis”; that is, the HA gene showed no significant excess of mutations in the antigenic regions (those recognized by the immune system). During these stasis periods, none of the co-circulating strains is significantly more fit than others, apparently because multiple mutations are required to substantially improve the virus’ ability to evade the immune system. As a result, an increased variety of strains accumulates. Ultimately, however, one of the variants will come within one mutation of achieving higher fitness and becoming dominant. Once the crucial last mutation does occur, virus evolution shifts from stasis to a brief interval of rapid Darwinian evolution, where the new dominant virus rapidly sweeps through the human population and eliminates most other variants. Are these viral micro-Cambrian explosions responsibl for the periodic flu pandemics of history? posted by Sydney on 10/30/2006 08:08:00 PM 0 comments 0 Comments: |
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