Biology Reference
In-Depth Information
of an aberrant anaphase but following the completion of the Ýrst mitotic division. At that time, the
products of the two mitotic nuclei fuse. These studies show that either different forms of
Wolbachia
-
induced gamete duplication exist or the observations of Stouthamer and Kazmer (1994) are incorrect.
The gamete duplication in some
Trichogramma
species appears to be far from perfect. Tagami
et al. (2001) found in lines of two infected
Trichogramma
spp. a higher prepupal mortality of
infected eggs compared to uninfected eggs. Cytological analysis of the developmental stage of 6-
to 48-h-old eggs and larvae showed that in up to 35% of unfertilized infected eggs the embryonic
development was arrested in the early mitotic stages. This arrestment was not found in eggs of the
sexual forms or in a
T. cacoeciae
line where parthenogenesis was not associated with
Wolbachia
.
The researchers concluded that the high prepupal mortality of infected eggs was a result of these
failures in gamete duplication.
Several studies on the cytogenetic process of cytoplasmic incompatibility (CI) induced by
Wolbachia
have also been carried out
(Chapter 14).
There is some similarity in this process to
gamete duplication. Incompatibility in the mosquito
Culex pipiens
showed that the paternal chro-
mosome set did not faithfully fuse with the female chromosomes after fertilization (Jost, 1970).
Reed and Werren (1995) observed the same phenomenon in the parasitoid wasp
Nasonia vitripennis
.
Both cytogenetic processes of gamete duplication and incompatibility due to
Wolbachia
show
defects in the early mitotic divisions. Lassay and Karr (1996) state that the cytogenetic process of
incompatibility is pleiotropic and can be classiÝed into several categories. One defect, in
Drosophila
simulans
for example, seems to occur in the anaphase of the Ýrst mitotic division (Callaini et al.,
1996; Callaini et al., 1997) as with gamete duplication. In this fruit Þy
,
the maternal and paternal
chromosomes did not condense synchronically when the father was infected and the mother
uninfected. The maternal chromosomes normally enter the anaphase of the Ýrst mitotic division
and migrate to the two poles. The paternal chromosomes are delayed and stay in the mid-zone of
the spindle. This results in embryos with aneuploid or haploid nuclei that eventually die (Callaini
et al., 1997). In general, we can say that
Wolbachia
acts in the early mitotic divisions. The main
difference between the cytogenetic processes of diploidization and incompatibility induced by
Wolbachia
is of course that gamete duplication is prevented by sperm, whereas incompatibility
occurs after fertilization.
From the cytogenetic studies done in Hymenoptera
,
one might conclude that the most common
diploidization process in PI-
Wolbachia
-infected eggs is gamete duplication. However, recent work
by Weeks and Breeuwer (2001) shows that another mechanism of
Wolbachia
-induced partheno-
genesis is found in some mites, i.e., a meiotic modiÝcation in eggs infected with PI-
Wolbachia
.
Microsatellite loci in six infected mite species of the genus
Bryobia
indicate the mechanism of
parthenogenesis to be most likely a meiotic modiÝcation, with progeny being identical to their
infected heterozygous mother. It is clear that
Wolbachia
has evolved different mechanisms to induce
parthenogenetic development.
DISTRIBUTION AND DENSITY OF
WOLBACHIA
IN PARTHENOGENETIC WASPS
Transmission of
Wolbachia
from mother to daughter is through the cytoplasm of the eggs. A study
on two
Aphytis
species suggested that, inside the ovaries, the microorganisms multiply inside the
nurse cells and then move, together with all other maternal substances, into the developing oocytes
through cytoplasmic bridges (Zchori-Fein et al., 1998). Then, they move to the posterior pole where
they are found in freshly laid eggs, as was also shown in
Trichogramma
spp.
(
Figure 15.1;
Stouthamer and Werren, 1993). In later stages, they migrate to the center of the eggs surrounding
the nuclei throughout the embryo, although this does not appear to be the case in
Trichogramma
spp.
eggs (Stouthamer and Werren, 1993). Nothing is known about the distribution of PI-
Wolbachia
throughout adult host tissues other than the reproductive tissue.
