In trying to understand disease-host interaction, Sharon Messenger, Ian Molineux, and J.J. Bull conducted an experiment to examine how different viral transfer mechanisms affect the relative advantage of different levels of viral virulence. The two basic types of transfer mechanisms are i Vertical: Transmission from parent to descendent (essentially, transmission through time)
i Horizontal: Transmission from one individual to the next in the current time
What they discovered is that when many hosts are available, a virus that harms its host but increases the chance that its progeny will infect new hosts is favored. But when few hosts are available (that is, a reduced chance of horizontal transfer exists), selection favors a virus that's less harmful to its host, because only through the host's survival and reproduction can the virus survive and reproduce.
¿fc-^HSJ' The experiment used bacteria and a virus that infects the bacteria. The experiment first selected for different varieties of the virus that had a range of effects on host growth rate. More-benevolent strains had a small effect on host growth rate but couldn't be transmitted horizontally, and less-benevolent strains had a larger effect on host growth rate and could be transmitted horizontally. This part of the experiment generated a variable virus population. After the experimenters had variation of exactly the sort that interested them in their virus population, they were able to set up different experimental scenarios to see when the different viral variants would have higher fitness.
Taking a 50-50 mixture of bacteria infected with the two viral types, the scientists grew them in the presence or absence of additional bacteria that either were or weren't susceptible to infection. After allowing time for viral and bacterial reproduction, they assessed the relative proportion of the two viral types at the end of the experiment. As predicted, in the absence of an opportunity for horizontal transfer, lower virulence was selectively favored. In the presence of available susceptible host bacteria, the less-benevolent viral strain was favored even though it was more harmful to its host.
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