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and Stern, 1966; Orphanides and Gonzalez, 1970; Laraichi, 1978; Jardak et al., 1979; Cabello and
Va rgas, 1985).
To determine the genetic basis of parthenogenesis, Stouthamer et al. (1990b) backcrossed the
nuclear genome of a temperature-treated parthenogenetic line of
into a sexual line of
the same species. If the parthenogenesis trait had been inherited through genes on the chromosomes,
the expectation was that the backcrossed females, when kept at low rearing temperatures, would
reproduce by parthenogenesis. After nine generations of backcrossing, unmated
T. pretiosum
T. pretiosum
females still produced only male offspring. Therefore, they concluded that parthenogenesis was
not caused by a simple Mendelian trait and might be caused by a cytoplasmic factor. Strong evidence
for microbial involvement came from feeding antibiotics to parthenogenetic
wasps
(Stouthamer et al., 1990a). Antibiotic treatment (tetracycline hydrochloride, sulfamethoxazole, and
rifampicin) caused male offspring production and reverted females from parthenogenetic to sexual
reproduction. Temperature treatment had the same effect. Three years later, Stouthamer and Werren
(1993) showed the presence of microbes in eggs of parthenogenetic
Trichogramma
females. They
were absent in eggs from lines cured by antibiotic treatment and in Ýeld-collected sexual lines.
The microbes were identiÝed as
Trichogramma
(Rousset et al., 1992; Stouthamer et al., 1993). Grenier
et al. (1998) provided deÝnitive proof that
Wolbachia
Wolbachia
was the causal agent of parthenogenesis.
They infected eggs of the sexual species
T. dendrolimi
with PI-
Wolbachia
from a
T. pretiosum
line
through microinjection. After several generations the
,
and a low level of parthenogenesis was induced. Pintureau et al. (2000b) tested the dynamics of
infection in two transfected
Wolbachia
was still present in
T. dendrolimi
lines and found the frequency of infected females to
decrease dramatically from 52.9% in generation 44 to 3.7% in generation 60 in one line and from
75.5% in generation 32 to 4.5% in generation 48 in the other line.
Recently we found evidence for horizontal transfer of PI-
T. dendrolimi
under natural conditions,
followed by complete expression of parthenogenesis (Huigens et al., 2000). When infected and
originally uninfected
Wolbachia
larvae share the same food source, a butterÞy egg, approximately
40% of the female offspring of the uninfected line acquire the infection and produce some daughters
from unfertilized eggs. In subsequent generations, perfect (100%) transmission of, and PI by,
T. kaykai
Wolbachia
was observed. This study, together with the work of Grenier et al. (1998), showed that
Wolbachia
was the causal agent of parthenogenesis in
Trichogramma
.
induced parthenogenesis is. So far, it has been
detected mainly in Hymenoptera because the reproduction in these wasps has been studied inten-
sively for their application in biological control (Stouthamer, 1997). A total of 66 hymenopteran
species have been reported as being most likely infected with a PI microbe.
It still remains unclear how common
Wolbachia-
Wolbachia
was detected
in 46 of these species
(Table 15.1)
,
14 cases are unknown (Table 15.1), and 6
Encarsia
species are
infected with the CFB bacterium.
In some parthenogenetic species, there are strong indications of PI by microbes other than
Wolbachia
, as is the case in
Galeopsomyia fausta
, where evidence for microbial involvement was
found through antibiotic treatment, but
could not be detected (Argov et al., 2000).
Outside the Hymenoptera, Werren et al. (1995) detected
Wolbachia
Wolbachia
in a parthenogenetic beetle,
Naupactus tesselatus
, but it remains uncertain if
Wolbachia
causes parthenogenesis in this species.
Pintureau et al. (1999) found
Wolbachia
in two parthenogenetic thrips species,
Heliothrips haem-
orrhoidalis
Ôs
involvement in their parthenogenesis. Arakaki et al. (2001a) showed the Ýrst strong evidence of PI
by
and
Hercinothrips femoralis
. Antibiotic or heat treatment should show
Wolbachia
. In this case, a population Ýxed
for the infection and completely parthenogenetic is found on a Japanese island. Sexual populations
occur in Central and South America.
In Acari, the only ÑnoninsectÒ order with haplodiploidy, parthenogenesis is widely distributed.
In oribatid mites, whole families reproduce parthenogenetically (Norton et al., 1993). Perrot-
Minnot and Norton (1997) tested eight oribatid species for the presence of
Wolbachia
in the predatory thrips
Franklinothrips vespiformis
Wolbachia
but could
not Ýnd the symbiont. Weeks and Breeuwer (2001) found
Wolbachia
infection associated with
