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et al. 1999, Vavre et al. 2001
). Some
Wolbachia
improve fertility or vigor, whereas
others appear to decrease these traits in their hosts. Some species appear to
have
Wolbachia
only in their germ line (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 testes of populations with cyto-
plasmic incompatibilities. Sometimes, infection with
Wolbachia
can increase
fecundity in females, thus favoring its transmission. For example, in
Drosophila
mauritiana
infected with a native strain of
Wolbachia
, females produce four
times as many eggs compared to uninfected strains. This is apparently due to
increased activity of stem cells in the germ-line stem-cell niche (
Fast et al. 2011
).
Wolbachia
may cause thelytoky in the Hymenoptera, which typically are
arrhenotokous (
Stouthamer 1997
).
Wolbachia
-induced thelytoky (parthenogen-
esis in which only females are known) has been found in the Tenthredinoidea,
Signiforidae, and Cynipoidea (
Stouthamer 1997
), as well as at least 70 species
of parasitoids (Aphelinidae, Encyrtidae, Eulophidae, Pteromalidae, Torymidae,
Trichogrammatidae, Cynipidae, Eucoilidae, Braconidae, Ichneumonidae, and
Proctotrupoidae) (
Stouthamer 1997, Cook and Butcher 1999, Russell and
Stouthamer 2011
). Many hymenopteran parasitoid species have both arrhe-
notokous and thelytokous strains.
In thelytokous populations of parasitoids, unfertilized eggs give rise to
females. Several thelytokous parasitoids (
Ooencyrtus submetallicus
,
Pauridia
peregrina
,
Trichogramma
sp., and
Ooencyrtus fecundus
) produce a few males,
usually
<
5%, when reared at temperatures over 30 °C (
Stouthamer 1997
), sug-
gesting incomplete transmission of
Wolbachia
or a low titer of
Wolbachia
.
Theytokous populations of the braconid
Asobara japonica
produce small num-
bers of males, which
Reumer et al. (2011)
attributed to the recent invasion of
these parasitoid populations, resulting in “a maladaptive side effect of incom-
plete coevolution between symbiont and host in this relatively young infection.”
Sometimes, the rare males produced in thelytokous populations have been
shown to mate and transfer sperm to conspecific females, indicating that the male
retained normal vigor and fertility. In other cases, the rare males are infertile, sug-
gesting that the
Wolbachia
infection has existed for a long time in the population,
which could have relaxed selection for essential fertility genes over evolutionary
time (Russell and Stouthamer 2011). In addition to heat, several antibiotics (tetracy-
cline hydrochloride, sulfamethoxazole, and rifampin) can induce the production of
males in some thelytokous parasitoid populations infected with
Wolbachia
.
The cytogenetic changes that occur during meiosis to restore an unfertilized
haploid egg to diploidy (thus permitting thelytoky) has been studied in several
