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Ýxation in the host population. The subsequent accumulation of host mutations can result in
complete parthenogenesis that is irreversible despite removal of the
Wolbachia
infection (Zchori-
Fein
infections that feminize genetic males in some isopod species act by
suppressing an androgenic gland and result in reproductively competent females (Rigaud and
Juchault, 1993). Male-killing
et al., 1992).
Wolbachia
infections can also be selected in host populations with
strong antagonistic sibling interactions or with intraspeciÝc competition for resources. For example,
Wolbachia
Wolbachia
-infected daughters can receive an early nutritional boost by cannibalizing their unhatched
male siblings (Hurst, 1991b; Majerus
et al.,
2000, 2001). Male killing can also reduce the inbreeding rate in infected females (Werren, 1987).
Interestingly, the reproductive manipulations by
et al., 2000; Zakharov
et al., 2000; von der Schulenburg
Wolbachia
are of interest as potential mecha-
nisms of speciation. Three primary routes have been suggested.
-induced host partheno-
genesis can result in speciation via genetic drift in the asexual hosts. Genetic divergence can lead
to daughter populations that are genetically incompatible due to deleterious gene combinations
(Stouthamer
Wolbachia
et al., 1992). Similarly, the post-zygotic reproductive barrier
caused by bidirectional CI (discussed below) can facilitate host genetic divergence and speciation
(Laven, 1967b; Breeuwer and Werren, 1990; Werren, 1997; Bordenstein
et al., 1990; Zchori-Fein
et al., 2001; Wade, 2001).
In a third proposed route, unidirectional CI can contribute to the reproductive isolation of two taxa
(Werren
et al., 1995b; Perrot-Minnot
et al., 1996; Shoemaker
et al., 1999).
CYTOPLASMIC INCOMPATIBILITY
-induced reproductive manipulations described to date, the
form of reproductive parasitism known as CI has attracted a large portion of the scientiÝc
attention. This bias is historical in part, since CI was the Ýrst described
Of the multiple types of
Wolbachia
phenotype
(discussed below). The diversity of taxa in which CI occurs has also contributed to the level of
research with this reproductive manipulation. A partial list of taxa in which CI has been described
includes Coleoptera (Stanley, 1961; Hsiao and Hsiao, 1985b; Wade and Stevens, 1985), Diptera
(Laven, 1967c; Trpis
Wolbachia
et al., 1981; Hoffmann
et al., 1986; Solignac
et al., 1994; Giordano
et al.,
1995), Homoptera (Noda, 1984), Hymenoptera (Saul, 1961; Richardson
et al., 1987; Bordenstein
et al., 2001), Lepidoptera (Brower, 1976), Acari (Breeuwer, 1997; Johanowicz and Hoy, 1998),
and Isopoda (Michel-Salzat
et al., 2001).
CI results in karyogamy failure and arrested development of early embryos in diploid insects
(Callaini
et al., 1997; Tram and Sullivan, 2002). In haplodiploid insect hosts with arrhenotokus
parthenogenesis, CI-induced loss of the paternally contributed chromosomes results in all male
broods (Dobson and Tanouye, 1998). Unidirectional CI can occur in matings between
-
infected males and uninfected females (
Figure 13.1)
.
The reciprocal cross and matings between
individuals harboring similar infections are compatible. In host populations that include infected
and uninfected individuals, CI provides a reproductive advantage to infected females because they
can mate successfully with all male types. In contrast, uninfected females are incompatible with
infected males, reducing their reproductive success. The advantage afforded to females by CI comes
at the expense of infected males, which are incompatible with uninfected females.
As demonstrated both theoretically and empirically (Caspari and Watson, 1959; Fine, 1978;
Stevens and Wade, 1990; Hurst, 1991a; Turelli and Hoffmann, 1991; Turelli, 1994; Hoffmann and
Turelli, 1997), the reproductive advantage afforded by CI to infected females can result in population
replacement, with the infected cytotype invading the host population and replacing the uninfected
cytotype, known as Ñpopulation replacement.Ò The reproductive advantage afforded by CI to
infected females can result in the spread of infections despite female Ýtness costs associated with
Wolbachia
Wolbachia
et al., 2000). Factors affecting the rate
of population replacement are CI levels (egg hatch resulting from cytoplasmically incompatible
crosses),
infection (Turelli and Hoffmann, 1995; Fleury
transmission (mater-
nal transmission rate) (Prout, 1994; Turelli, 1994; Rousset and Solignac, 1995; Dobson
Wolbachia
effect on host-female Ýtness, and the Ýdelity of
Wolbachia
et al.,
