Biology Reference
In-Depth Information
accumulation of random deleterious mutations in their genetic determinants, leading to their loss
in the course of evolution.
Wolbachia
Dependence for Oogenesis
Another case of obligatory
Wolbachia
infection in insects was reported in the wasp
Asobara tabida
(Dedeine et al., 2001). However, involvement of
Wolbachia
in this obligatory association suggests
different types of interaction that could follow different evolutionary origins. For this reason, we
will review what is known about this biological model.
A. tabida
(Braconidae) is a common solitary larval endoparasitoid of
Drosophila
species, which
live in fermenting fruits and sap Þuxes. Females oviposit in
Drosophila
larvae in which parasitoid
feed and develop. Its large distribution in Europe and its easy rearing make
A. tabida
an appropriate
model to study several aspects of hostÏparasitoid interactions (Kraaijeveld and Godfray, 1997).
Wolbachia
in
A. tabida
was Ýrst detected by Werren et al. (1995) and conÝrmed by Vavre et al.
(1999b). In the latter study, the authors described a case of triple
Wolbachia
infection in which
individuals can be simultaneously infected by three different
Wolbachia
strains, each characterized
by its partial sequence of the bacterial
wsp
gene. Based on sequences of the same gene, phylogenetic
reconstruction showed that these
Wolbachia
strains were well differentiated even if they all belonged
to the A
Wolbachia
supergroup, which has been detected only in arthropods. Using diagnostic PCR,
it was shown that all individuals (males and females) from 13 European strains were all simulta-
neously infected by the three
Wolbachia
strains (F. Dedeine, unpublished data). This total infection
in laboratory strains suggests a high triple-infection prevalence in populations and a very strong
efÝcient transmission of bacteria through the germ line, at least under laboratory conditions. This
high transmission rate could be explained in part by the particular localization of
Wolbachia
in
germ cells. Indeed, microscopy has localized
Wolbachia
in the posterior pole of mature oocytes.
Because this region of the cytoplasm contains germ-cell determinants in insects, the presence of
Wolbachia
in this region could be interpreted as an adaptation to enhance bacterial transmission to
host progeny, as already suggested in other cases of intracellular symbiosis (Breeuwer and Werren,
1990; Stouthamer and Werren, 1993). However,
Wolbachia
were also detected in isolated thoraxes,
demonstrating the presence of bacteria in tissues other than the germline (Dedeine et al., 2001).
A study on the impact of
Wolbachia
infection on
A. tabida
reproduction and performance had an
astonishingly unexpected result. Treated with antibiotics, aposymbiotic individuals (cured of the infec-
tion) had a normal overall physiological state and normal length; males were fertile and females had
an apparently normal oviposition behavior. However, females were completely incapable of producing
offspring, while each control (untreated) female produced around 200 offspring (Dedeine et al., 2001).
Why does antibiotic treatment completely and speciÝcally sterilize females? The explanation
is very simple: aposymbiotic females do not produce any eggs and consequently cannot have any
offspring. In this species, vitellogenesis is almost completed at the time of emergence, taking place
during the pupal stage at the expense of larval reserves (proovogenic oogenesis). Thus, the mature
oocytes, which are localized at the basal region of ovarioles, can be easily counted in each female.
After curative antibiotic treatments, results obtained were strongly demonstrative: while symbiotic
females produce more than approximately 240 mature oocytes 5 days after emergence, ovarioles
from aposymbiotic females are simply empty of oocytes. Emptiness of ovaries means that preimag-
inal oogenesis is inhibited, and no further oogenesis occurs in the early adult stage after treatment
has ceased. Moderate antibiotic doses that do not totally eliminate
Wolbachia
are compatible with
limited egg production, which shows a clear relationship with bacterial density. The possibility that
inhibition of oocyte production is caused directly by antibiotics, or indirectly through the release
of toxins from decaying
Wolbachia
bacteria, can be discarded (Dedeine et al., 2001). There is thus
overall evidence that egg production in
A. tabida
is wholly dependent on
Wolbachia
infection.
Moreover, a recent unpublished study demonstrated that the possible involvement of bacteria
other than
Wolbachia
is also highly unlikely. The three
Wolbachia
strains in
A. tabida
have been
