evolution of virulence.--I have recently become very interested in
understanding why parasites sometimes make their hosts so sick.
A recent model suggests that parasite virulence may be
associated with interactions among propagules (Lively 2001). The
model also makes direct connections with genetic load theory and
with the mutational theories for the maintenance of sex.
The basic result can be seen in the figure at the left. The thin lines give different relationship between host survivorship and the number of parasite propagules when they become transmissible. Note that adding additional propagules has an increasingly negative effect on host survivorship for the upper-most curves.
The thick line gives the
equilibrium values for the number of parasite propagules
produced by a single infection. The key result is that host
survivorship increases at the parasite equilibrium as the
interactions among propagules increases. This result is
analogous to results from theoretical population genetics, which
show that fitness at mutation-selection balance increases with
increasing synergism among deleterious mutations. These results
suggest that, all else equal, the most virulent parasites will
be the ones for which additional propagules have diminishing
effects on the host.
I am also interested in
the effects of multiple infection on selection for within-host
growth rates by parasites (Lively 2005). My results differ
from the classic result, which shows that multiple simultaneous
infections in the same host leads to selection for more
aggressive within-host growth by the parasite. Instead I
find that coinfection can lead to less aggressive within-host
growth by each of the coinfecting strains. I argue that
the difference stems directly from different assumptions in the
I am also now very
interested in the ecology
of virulence. I have found using standard assumptions
regarding the effect of density on host birth rates that
parasites can be very virulent at carrying capacity, even if
they have no effect on the intrinsic birth rate of infected
individuals. In fact, relatively small effects of
infection on the sensitivity of infected individuals to
competition can lead to situations in which the parasites
effectively sterilize the host (Lively 2006).
Lively, C.M. 2001. Propagule interactions and the evolution of virulence. Journal of Evolutionary Biology 14:317-324. (preprint)
Lively, C.M. 2005. Evolution of virulence: coinfection and propagule production in spore–producing parasites. BMC Evolutionary Biology 2005, 5:64
Lively, C. M. 2006. The ecology of virulence. Ecology Letters 9: 1089-1095.
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C. M. Lively, Dept. of Biology, Indiana University
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