Biology Reference
In-Depth Information
arthropods, and the mechanisms vary, indicating that
Wolbachia
can cause
parthenogenesis by several mechanisms. In the eggs of
Wolbachia
-infected
Trichogramma
females, meiosis progresses to the stage of a single haploid
pronucleus and the diploid chromosome number is restored during the first
mitotic division. Thus, during anaphase, the two identical sets of chromosomes
do not separate and the result is a single nucleus containing two copies of the
same set of chromosomes, resulting in a female that is completely homozy-
gous at all loci (
Stouthamer 1997
). A
Wolbachia
-infected strain of the parasit-
oid
Leptopilina clavipes
has a meiosis in which diploidy is restored by anaphase
restitution during the first somatic mitosis, similar to that in
Trichogramma
(
Pannebakker et al. 2004
). However, in
Diplolepis rosae
and
Muscidifurax
uniraptor
parasitoids the first mitotic division is normal and diploidization
occurs through a fusion of the two mitotic nuclei in the second prophase
(
Pannebakker et al. 2004
).
How might parasitoids (or other insects) be infected with
Wolbachia
?
Phylogenetic analysis of
Wolbachia
suggests that both horizontal and verti-
cal transfer of
Wolbachia
occurs among insects (
Jeyaprakash and Hoy 2000
).
Huigens et al. (2000)
documented horizontal transmission of thelytoky-induc-
ing
Wolbachia
from one parasitoid strain to another within a shared lepi-
dopteran host. Offspring of uninfected
Trichogramma
females can acquire
sufficient thely toky-inducing
Wolbachia
to express the trait when they share
a host egg with progeny of
Wolbachia
-infected females. The process by which
the uninfected
Trichogramma
larvae acquire the
Wolbachia
remains unclear.
However, this intraspecific horizontal transfer suggests that interspecific hori-
zontal transfers from parasitoid to parasitoid could occur by sharing a com-
mon host.
Wolbachia
infections causing thelytoky are hypothesized to be a mechanism
that contributes to the process of speciation. For example, some populations
of the parasitoid
Encarsia formosa
no longer have males, so that these popu-
lations essentially become clonal and over time could differentiate genetically.
Wolbachia
-induced incompatibility is thought to precede hybrid incompat-
ibilities in the parasitoid
Nasonia
(
Bordenstein et al. 2001
). Under this scenario,
an uninfected ancestral population gives rise to two geographically isolated
daughter populations. If each population is infected with a different strain of
Wolbachia
, the populations could become reproductively isolated due to their
infections. The role of
Wolbachia
in speciation is controversial and, according
to some, unproven (
Werren 1997, Hurst and Schilthuizen 1998, Shoemaker et al.
1999, Rokas 2000
).
