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Short-term advantage

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BIOL 1500
Scott Brandon

o short-term advantage in survival and reproduction A Correlation between Lethality and Transmission?  Short-sighted evolution is not the only reason HIV infection is fatal o There is evidence that there are rare strains of HIV exist that kill their hosts more slowly than common strains  Important point is that, alterations to HIV’s genome can render the virus less damaging to the host, but still able to survive in the host’s body  If the host lives longer, then the HIV population persists longer too  To persist beyond the lifespan of the host, a viral population must at some point colonize new hosts o This means that there is a second level of natural selection acting on HIV o First level of selection is the one we have already explored:  There are differences among virions in their ability to survive and reproduce within a given host o Second level of selection occurs when there are differences among viral strains in their ability to move from one host to another  Strains that are good at getting transmitted to new hosts will become more common over time; strains that are bad at getting transmitted will disappear  HIV-2 is less common worldwide than HIV-1  HIV-2 is a related virus that humans acquired from a different primate species o It is less damaging to its hosts than HIV-1 o Individuals infected with HIV-2 do progress to AIDS, but they do so much more slowly than individuals infected with HIV-1  The slower course of HIV-2 infection appears to be related to the fact that viral loads are lower in hosts infected with HIV-2 than in hosts infected with HIV-2  HIV-2’s Nef protin has a function that was lost in HIV-1’s ancestors before they began infecting humans  Nef protein prevents chronic immune activation o Therefore HIV-2 would not kill its host so easily as HIV-1 does  HIV-2 is also transmitted at lower rates than HIV-1  Differences in transmission rate may explain why HIV-2 has remained largely confined to West Africa, while HIV-1 has spread across the globe  A second reason why HI
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