Biology Reference
In-Depth Information
pronuclei and loss of paternal chromosomes at the first mitosis, resulting in the
production of haploid males derived from the maternal chromosome set.
Some insects seem to have
Wolbachia
only in their germ-line tissues (ovaries
and testes), whereas others have
Wolbachia
in somatic tissues as well (
Dobson
et al. 1999
). Large numbers of
Wolbachia
have been found in ovaries and tes-
tes of populations with cytoplasmic incompatibilities. Incompatible strains have
been converted to compatible by treating the colonies with heat or antibiotics,
which eliminates or greatly reduces the
Wolbachia
population.
Wolbachia
can be transferred to new populations experimentally by micro-
injecting infected egg cytoplasm into uninfected eggs. Transinfected strains of
D. simulans
and
D. melanogaster
with high titers of
Wolbachia
exhibited cyto-
plasmic incompatibilities at high levels, but those with low titers exhibited
low levels of incompatibility, suggesting that a threshold level of infection is
required and that host factors may determine the density of the
Wolbachia
in
the host (
Boyle et al. 1993
).
Wolbachia
have been identified in many species of parasitic Hymenoptera,
including species in the Aphelinidae, Encyrtidae, Eulophidae, Pteromalidae,
Torymidae, Trichogrammatidae, Cynipidae, Eucoilidae, Braconidae, Ichneumonidae,
and Proctotrupoidae and in three dipteran parasitoids (Tachinidae) (
Cook and
Butcher 1999
).
Wolbachia
may cause both cytoplasmic incompatibility and induction
of parthenogenesis in these parasitoids. Many hymenopteran parasitoids have both
bisexual (arrhenotokous) and unisexual strains consisting only of females (thely-
toky), probably due to the presence of
Wolbachia
.
Phylogenetic analysis suggests that the
Wolbachia
common ancestor evolved
between 80 and 100 million years ago (
O'Neill et al. 1992
), whereas the
arthropod common ancestor occurred at least 200 million years earlier. Thus,
Wolbachia
probably have invaded arthropods through
horizontal transmission
(
O'Neill et al. 1992, Heath et al. 1999, Jeyaprakash and Hoy 2000
). Some arthro-
pods have been found to have double, triple, or even greater numbers of infec-
tions with different strains of
Wolbachia
. The effects of these multiple infections
usually are unknown.
Several methods have been proposed as mechanisms for horizontal transfer,
including the movement of
Wolbachia
from host arthropods to their parasitoids.
Heath et al. (1999)
experimentally transferred
Wolbachia
from
Drosophila simu-
lans
to a novel host, its endoparasitoid
Leptopilina boulardi
, 0.711% of the time.
The
Wolbachia
infection rate diminished during subsequent vertical transmis-
sion to the F
2
and F
2
generations, perhaps due to poor maternal transmission
(unstable vertical transmission).
