Evolution__3rd_Edition

628 PART 5 / Macroevolution
Nematode parasites of fig wasps
illustrate the theory
host’s reproduction produces the resources (that is, host offspring) that the parasite’s
offspring exploit. This trade-off will place an upper limit on virulence. A horizontally
transmitted parasite experiences no such trade-off: the success of its offspring is independent
of the reproduction of its host. Virulence is therefore much less constrained. 2
In nature, single infections and vertical transmission often occur together, and both
factors may work side by side to reduce parasitic virulence. A comparative study, by
Herre (1993), of 11 species of nematode worms that parasitize fig wasps in Panama
illustrates the idea. The fig wasp life cycle is as follows. The adult fig wasp, who is carrying
pollen from the fig from which she emerged, enters one of the structures that eventually
ripen into a fig. There she pollinates the fig, lays her eggs, and dies. The eggs grow
up and emerge within the fig; after emerging, they mate and the females pick up pollen
and exit the fig in search of another to lay their eggs in. An important fact in the story is
that fig wasp species vary in the number of fig wasps that enter a fig. In some, only one
does, whereas in others several females may enter and lay their eggs in the same fig.
Nematode worms live off fig wasps, and in Panama there is a different species of
nematode living off each species of fig wasp. The immature nematodes crawl on to a fig
wasp after she emerges in the fig. “At some point [in Herre’s words], the nematodes
enter the body cavity of the wasp and begin to consume it.” The nematodes emerge as
adults from the body of the dead wasp and mate and lay their eggs in the same fig as the
wasp did; the cycle can then repeat itself. Nematodes that live off fig wasps in which
only one wasp enters a fig will tend to be vertically transmitted, and the nematodes on
any one host will be genetically related. In contrast, the nematodes that live off fig wasps
in which several females may enter the same fig can be horizontally transmitted a
nematodes from a number of different parents may crawl on to the same wasp and they
will be genetically unrelated. We can predict that the parasitic nematodes of fig wasps in
which only one female typically enters a fig will have evolved lower virulence than those
of fig wasps in which more than one female typically enters a fig. Herre’s results are
shown in Figure 22.7 and show the predicted relation. The virulence of the parasite
appears to have been tuned by natural selection to the habits of the host.
The example here illustrates only one way in which natural selection works on virulence.
In other cases virulence may not depend on the rate at which a parasite grows in,
and uses up, its host. For other kinds of virulence, other theories may be needed. Even
when virulence does depend on the parasite’s growth rate, kin selection and vertical as
opposed to horizontal transmission are just two of the evolutionary factors that have
been hypothesized to influence it. Most of the other factors, however, have not been so
well studied. We can often, if not always, expect lower virulence when the parasites on a
host are genetically related and vertically transmitted than when they are unrelated and
more horizontally transmitted.
The theory of the evolution of virulence is rich in implications for understanding
human disease. Box 22.2 looks at an example.
2 Virulence has an upper limit, even with horizontal transmission. For instance, a parasite that is spread by
biting insects requires its host to be attractive to those insects. The longer the host can stay alive, the longer it is
available to be bitten. Indeed, the reduction in the virulence of myxoma virus that we looked at earlier may well
have occurred as natural selection maximized the chance of transmission from one rabbit to the next by
mosquitoes.
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