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sexually. Many cytoplasmically inherited factors cause female-biased sex ratios (Werren et al.,
1988; Wrensch and Ebbert, 1993), but the extreme bias of 100% females and the induction of
parthenogenesis were shown for the Ýrst time by Stouthamer et al. (1990a). In many cases, microbe-
induced parthenogenesis may not be optimal for the organism (host) but is forced upon them by
the Ýtness advantages their symbionts derive from their hostÔs parthenogenetic reproduction.
These parthenogenesis-inducing (PI) symbionts inhabit the cytoplasm and can therefore be
transmitted only vertically through egg cells. Males are considered a dead end for the symbionts.
By inducing parthenogenesis the symbionts enhance their own transmission despite the cost they
may inÞict on their host. Two different microbes have been described as being associated with
parthenogenesis in insects, and a potential third case was found in nematodes.
At present the most common symbiotic microbe found to induce parthenogenesis is the
bacterium
(Chapter 13).
It was Ýrst identiÝed as the microbe inducing
parthenogenesis in parasitoid wasps of the genus
Wolbachia
pipientis
Trichogramma
(Rousset et al., 1992; Stouthamer
et al., 1993). In general,
was thought to be unique in its ability to induce partheno-
genesis, but recently Zchori-Fein et al. (2001) found an undescribed vertically transmitted bac-
terium also associated with parthenogenesis. The microbe was found in six parthenogenetic
Wolbachia
Encarsia
is a genus of parasitoids that use whiteÞies as their host. The bacterium
belongs to the Cytophaga-Flexibacter-Bacteroid (CFB) group, thus being unrelated to
species.
Encarsia
Wolbachia.
Bacteria belonging to the CFB group are also capable of inducing feminization (Weeks et al.,
2001). Vandekerckhove et al. (2000) discovered a potential third case of microbial involvement
in parthenogenesis. They detected a verrucomicrobial species that is maternally transmitted and
seems to be associated with parthenogenesis in the nematode species
Xiphinema americanum
.
These bacteria are the Ýrst endosymbionts found among the
. More experimental
evidence is needed to show that these bacteria do indeed induce parthenogenesis.
This chapter describes current knowledge of PI-
Verrucomicrobia
in insects. Because studies on the
involvement of the CFB bacterium and the verrucomicrobial species in parthenogenesis have just
started, these endosymbionts will only be mentioned brieÞy (but see
Chapter 11
of this topic).
Wolbachia
WOLBACHIA
-INDUCED PARTHENOGENESIS
IN HAPLODIPLOIDS
The main requirement for parthenogenesis induction by
seems to be a haplodiploid
mode of reproduction of the host species. Haplodiploidy is known from the arthropod groups
Hymenoptera, Acari, Thysanoptera, and a few genera in the Coleoptera (Wrensch and Ebbert,
1993). However, there is no fundamental reason why species with a diplodiploid sex-determination
system should not be vulnerable to microbial induction of parthenogenesis.
Haplodiploidy, or arrhenotoky, is a mixture of parthenogenetic and sexual reproduction; males
develop from unfertilized (haploid) eggs, whereas females develop from fertilized (diploid) eggs
(Hartl and Brown, 1970; White, 1973). A completely parthenogenetic mode of reproduction is
thelytoky. In thelytoky, all eggs, fertilized or not, develop into females. In arrhenotoky, only a single
barrier must be overcome for the induction of parthenogenesis, namely, diploidization of the
unfertilized egg. An additional barrier to complete parthenogenesis exists in diplodiploid species:
egg development must also be induced, which is normally initiated by sperm penetration in sexual
species. The terminology surrounding parthenogenesis in haplodiploids is complicated and clearly
in need of revision (Luck et al., 1992; Stouthamer, 1997); in this chapter, we call arrhenotoky and
thelytoky, respectively, sexual and parthenogenetic reproduction.
The Ýrst indications for microbial involvement in parthenogenesis were found through exposure
of the parthenogenetic parasitoid wasp
Wolbachia
to elevated rearing temperatures (Flanders,
1945). Females reared at 26.6 C or less produced only daughters, whereas females reared at 32.2 C
produced sons and daughters or only sons. Several other studies conÝrmed male production by
thelytokous wasps at higher temperatures (Wilson and Woolcock, 1960a,b; Flanders, 1965; Bowen
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