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the contrary, mutualism and reproductive parasitism can be retained in arthropods. Since reproduc-
tive parasitism does not require coadaptation, it should spread more easily into new host species
through horizontal transmission and thus be overrepresented. Following this hypothesis, the few
cases of mutualistic Wolbachia associations in arthropods could be explained by a sampling bias.
W HY H AS THE A SSOCIATION A RTHROPOD / W OLBACHIA N OT L ED M ORE
F REQUENTLY TO O BLIGATORY S YMBIOSIS ?
First, it is possible that instability of Wolbachia associations in arthropods, either through replace-
ment by other CI-inducing variants or through infection loss, does not allow long coevolutionary
times and consequently species-speciÝc coadaptations.
Another possibility is related to vertical transmission, which requires that a compromise be
reached between maximal transmission efÝcacy to the eggs and minimal attrition on the host (i.e.,
minimal effect on host Ýtness). Transmission efÝcacy is obviously related to Wolbachia replication
rate and Wolbachia density in the eggs. Replication rate of a microorganism is, however, usually
related to its virulence. In addition, high density of microorganisms in eggs could result in devel-
opmental arrest. It is thus generally thought that the control of the replication rate of microorganisms
is crucial for establishing a well-integrated symbiotic system, with reduction of replication rate
leading to virulence reduction (in some cases, there might be a decoupling in the replication rate
of a symbiont in different host tissues, with a delay of the pathogenetic effects; see McGraw et al.,
2002). It has recently been shown that signiÝcant virulence attenuation can actually occur in
Wolbachia in an insect host, in terms of a reduction of bacterial replication and density in the
ovaries, with reduction of detrimental effects on host fecundity (McGraw et al., 2002).
In the presence of strict vertical transmission and clonality, reduction of Wolbachia replication
rate during generations could possibly occur through kin-selection phenomena. In the presence of
a high degree of horizontal transmission (and, thus, of infections with more than one strain),
competition among unrelated bacteria could possibly hamper selection for reduced rate of replica-
tion. The presence of microorganisms that are unable to minimize their effects on host Ýtness
through the optimization of their replication rate will probably urge the host to develop counter-
measures to control the microorganism population. In these conditions, the arms race between host
and symbionts is not likely to allow a deÝnitive virulence reduction and the development of a
stable, well-regulated, and highly reproducible system of transmission. However, let us assume that
the degree of horizontal transmission is not high and that virulence reduction occurs in strains of
CI-inducing Wolbachia through reduction of replication rate. Infection with these strains could
become unsuccessful in the long term: slow-replicating strains could be overtaken by invading fast-
replicating strains. Even a relatively limited degree of horizontal transmission of CI Wolbachia
could thus hamper the establishment of obligatory symbiosis, by reducing the duration of associ-
ations with low-virulence, slow-replicating strains.
In summary, we have offered two hypotheses that could explain the dichotomous situation between
arthropod and nematode Wolbachia . In nematodes (1) the lack of horizontal transmission does not
permit the maintenance of a CI-inducing Wolbachia , thus making obligatory mutualism the only strategy
for nematode Wolbachia , while in arthropods (2) horizontal transmission of CI-inducing Wolbachia
could hamper the development of obligatory symbiosis in two ways, depending on its rate: by making
unlikely evolution of strains toward reduced virulence/slow replication rate or by rendering strains with
reduced virulence/slow replication rate (compared to the virulent, fast-replicating ones) unsuccessful.
E VOLUTIONARY T RANSITIONS BETWEEN S YMBIOTIC L IFESTYLES
Coadaptations Require (Quite) Long Evolutionary Times
We will now consider a hypothetical system where transmission of a CI-inducing Wolbachia is strictly
vertical. If we assume the existence of polymorphism for the presence of Wolbachia in the host
 
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