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the endosymbiotic association between the intracellular x-bacteria and the unicellular eukaryote
Amoeba proteus
. In this case, the initially harmful hostÏbacteria association evolved to a beneÝcial
state after 200 generations in culture, and experiments demonstrated that the host nucleus had become
dependent on the infective organisms for its own functioning (Jeon, 1972).
WOLBACHIA
SPECIALIZATIONS
AND EVOLUTIONARY TRANSITIONS
Vertical transmission can lead to the establishment of different kinds of hostÏsymbiont relationships
or specializations (Werren and OÔNeill, 1997; Bandi et al., 2001a). An obvious pathway is toward
mutualistic symbiosis: the symbiont will increase its own Ýtness by increasing the Ýtness of the host
that is involved in its transmission. Another possible outcome is to become a reproductive parasite:
by manipulating host reproduction, the symbiont can reduce the Ýtness of those members of the host
species that are not involved in its transmission. There is, however, no intrinsic conÞict between
mutualistic symbiosis and reproductive parasitism. For example, a maternally inherited microorganism
could be beneÝcial toward females (the host sex responsible for transmission to the offspring) while
being detrimental toward males (which are not involved in transmission). In
Wolbachia
symbiosis,
the only example of reproductive parasitism and mutualism coexisting is observed in
A. albopictus
,
where a CI-inducing
Wolbachia
strain also increases female fecundity (Dobson et al., 2002). Indeed,
in all other cases,
Wolbachia
seems to be specialized either in obligatory mutualism in Ýlarial
nematodes or reproductive parasitism in the majority of arthropod species.
R
EASONS
FOR
S
PECIALIZATION
IN
W
OLBACHIA
S
YMBIOSIS
We believe that four main evolutionary keys may explain specialization in the two host phyla: (1) the
different rate of horizontal transmission, (2) the evolutionary stability of association, (3) the efÝcacy
of vertical transmission, and (4) multiple infection. In Ýlarial nematodes, we assume that horizontal
transmission and multiple infection do not occur (or occur involving closely related
Wolbachia
strains).
Moreover, the prevalence of infection appears complete. Conversely, as stated previously,
Wolbachia
in arthropods likely has chances for horizontal transmission and multiple infections. In addition,
Wolbachia
is haphazardly distributed within arthropod taxa and populations: not all populations within
a species appear to harbor
Wolbachia
and not all species within a genus appear to harbor it (and so
on along the taxonomic hierarchy). This pattern is consistent with theoretical models on the population
biology of bacteria that induce CI: both loss of infection and invasion by new compatibility types are
expected to occur in the host populations. Once
Wolbachia
has become Ýxed within a population,
selective pressures for maintaining the ability to modify sperm should decrease.
This could be followed
by the loss of the sterilizing trait and, Ýnally, by the loss of the infection in the population (Hurst and
McVean, 1996; Vavre et al., in press). Both empirical and recent experimental studies are consistent
with this conclusion (Werren and Windsor, 2000; Vavre et al., 2002). From these differences, we will
now speculate on two general questions that address the difference between the general picture of
Wolbachia
associations in arthropods and nematodes.
W
HY
H
AS
THE
A
SSOCIATION
F
ILARIAL
N
EMATODE
/
W
OLBACHIA
L
ED
TO
O
BLIGATORY
S
YMBIOSIS
I
NSTEAD
OF
R
EPRODUCTIVE
P
ARASITISM
?
If we assume that CI-inducing
Wolbachia
are systematically lost in the course of the hostÏsymbiont
evolution, the ability to be horizontally transferred appears necessary for a CI-
Wolbachia
strain to
perpetuate through evolutionary times. In absence of horizontal transfer in nematodes, CI-inducing
Wolbachia
certainly cannot be retained in these species, even though we hypothesize that a CI
phenotype could be expressed in these hosts. Therefore, the remaining strategy for a maternally
transmitted bacterium such as
Wolbachia
is to become beneÝcial and obligatory for the host. On
