Biology Reference
In-Depth Information
the evolution of virulence are applicable to PI-
Wolbachia
. In general, the mode of parasite trans-
mission, vertical or horizontal, is considered to be the key selective force behind the evolution of
virulence (Ewald, 1994). The two modes of transmission are expected to select for opposing
virulence levels.
Vertical transmission should select for lower parasite ÑvirulenceÒ to the host. Parasites depend
on the host and beneÝt from a high reproductive success of their host. Hosts with less virulent
parasites should produce more offspring than hosts infected with more virulent forms (Lipsitch
et al., 1996). On the other hand, horizontal transmission is expected to select for higher virulence
because parasites are not dependent on the reproductive capacity of the host. When only horizontally
transmitted, parasites obtain the highest Ýtness through a tradeoff between the negative effect of
the parasitesÔ multiplication (virulence) on the longevity of the infected host. Too many parasites
could make the host less capable of transmitting parasites (Messenger et al., 1999).
Some parasites have two modes of transmission. They provide ideal models for studying the
evolution of virulence. Bull et al. (1991) studied phage virulence evolution in
Escherichia coli
.
These phages can be both vertically and horizontally transmitted. Two forms of the phage occur
in natural populations Ð a mild form and a more virulent form that decreases the growth speed
of the host. In situations where only vertical transmission was possible, the mild form prevailed.
This was not the case when horizontal transmission was also allowed.
In
T. kaykai,
PI-
Wolbachia
are also transmitted in two ways. Vertical transmission is the main
mode, but at high parasitoid densities horizontal transfer is expected to be frequent in the Ýeld
(Huigens et al., 2000). The presence of both transmission forms in the PI-
Wolbachia
of
T. kaykai
will allow us to study the tradeoffs
between virulence and transmission of
Wolbachia
.
N
UCLEAR
-C
YTOPLASMIC
C
ONFLICT
The theory of parasite virulence evolution cannot be wholly applied to PI-
Wolbachia
because they
are not parasites in the true meaning of the word. They are reproductive parasites that drive through
a host population by manipulating the sex ratio of the host. Therefore, when
Wolbachia
are only
vertically transmitted, they can induce relatively high Ýtness costs and still spread through the host
population. For PI-
Wolbachia
, several aspects of a host population have implications for the Ýtness
costs we can expect (Table 15.2).
First, Ýtness costs of carrying
Wolbachia
are expected to be different among host populations
Ýxed for the infection and populations where infected and uninfected individuals coexist
(Stouthamer, 1997). (A) Initial infections that spread fastest through a population are the ones with
TABLE 15.2
Fitness Costs for Hosts Carrying PI-
Wolbachia
, Ranked from Lowest (1) to
Highest (4), Expected in Several Situations
Infection
Status
Horizontal
Transmission
Fitness
Cost
Fixed
Ï
1
Fixed
+
2
Mixed
Ï
3
Mixed
+
4
Costs for hosts from populations Ýxed or mixed for the
infection and from populations where horizontal transmis-
sion of
Wolbachia
does or does not occur are mentioned.
