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Kambhampati et al., 1993; McGraw and OÔNeill, 1999; Jamnongluk et al., 2001; Dobson et al.,
2002; Kittayapong et al., 2002), and it has since been reported in the families Drosophilidae (e.g.,
Hoffmann et al., 1986; Hoffmann, 1988; OÔNeill and Karr, 1990; Bourtzis et al., 1994, 1996;
Giordano et al., 1995; Merot et al., 1995; Werren and Jaenike, 1995; James and Ballard, 2000)
and Tephritidae (Boller et al., 1976; Boller, 1989); in Coleoptera, in the families Curculionidae
(Blickenstaff, 1965; Hsiao and Hsiao, 1985) and Tenebrionidae (Wade and Stevens, 1985); in
Hemiptera, in the families Aleurodidae (De Barro and Hart, 2000) and Delphacidae (Noda, 1984;
Rousset et al., 1992; Noda et al., 2001); in Hymenoptera, in the families Figitidae (Vavre et al.,
2000, 2001, 2002), Proctotrupoidae (A. van Alphen, personal communication cited in Werren and
OÔNeill, 1997), ants (van Borm et al., 2001), and Pteromalidae (Saul, 1961; Breeuwer and Werren,
1990; Perrot-Minnot and Werren, 1999); in Orthoptera, in the family Grylidae (Giordano et al.,
1997; Kamoda et al., 2000; Mandel et al., 2001); and in Lepidoptera, in the family Pyralidae
(Brower, 1976; Sasaki and Ishikawa, 1999). Wolbachia -induced CI has also been reported in two
families of the class Arachnida, Phytoseiidae (Johanowicz and Hoy, 1998), and Tetranychidae
(Breeuwer, 1997; Gotoh et al., 1999; Vala et al., 2000; Egas et al., 2002), as well as in the family
Porcellionidae of Crustacea (Legrand et al., 1986; Moret et al., 2001). It is evident that Wolbachia -
induced CI is the most frequent and widely distributed of the Wolbachia -induced phenotypes.
Phylogenetic analysis suggests that CI- Wolbachia do not form a monophyletic group with respect
to the Wolbachia strains that cause other phenotypes (Werren et al., 1995; Zhou et al., 1998).
Actually, the distribution of CI within the general phylogeny of Wolbachia makes it parsimonious
to assume that it was an ancestral Wolbachia property.
T HE M ECHANISM OF W OLBACHIA -I NDUCED CI
Although the mechanism of CI has not yet been elucidated on the molecular level, several lines of
evidence suggest that Wolbachia somehow modiÝes the paternal chromosomes during spermato-
genesis (mature sperm do not contain the bacteria), thereby inÞuencing their behavior during the
Ýrst mitotic divisions and resulting in loss of mitotic synchrony (Breeuwer and Werren, 1990; Reed
and Werren, 1995; Lassy and Karr, 1996; Callaini et al., 1997; Tram and Sullivan, 2002). Based
on the available genetic and cytogenetic data, Werren (1997) proposed the so-called mod/resc
model, which assumes the presence of two distinct bacterial functions. First, the modiÝcation (mod),
a kind of an ÑimprintingÒ effect, which is expressed in the male germline, probably during sper-
matogenesis, and second, the rescue (resc), which is expressed in the egg. Sperm imprinting may
be due either to secreted Wolbachia proteins that modify the paternal chromosomes or the removal
of host proteins that are necessary for proper condensation/decondensation of the paternal chro-
mosomal set before or during zygote formation. Similarly, the presence of the same Wolbachia
strain in the egg may result in the production and secretion of rescue factors or alternatively the
recruitment of host molecules that are capable of rescuing the sperm ÑimprintÒ in a Wolbachia
strain-speciÝc manner. The mod and resc functions are now characterized through a number of
properties, which must be accounted for by any hypothesis regarding their as yet unknown molecular
nature. Molecular studies are now in progress to identify the proteins involved in CI (Sasaki et al.,
1998; Braig et al., 1998; Harris and Braig, 2001).
Charlat et al. (2001, 2002) summarized Ýve properties:
1.
The mod intensity is a variable factor . Indeed, the levels of modiÝcation, expressed as
embryonic mortality in incompatible crosses, range from 0 to 100% (Poinsot et al., 1998).
Experimental evidence suggests that both bacterial and host factors are responsible for the
variation in mod intensity (Boyle et al., 1993; Rousset and de Stordeur, 1994; Giordano
et al., 1995; Sinkins et al., 1995b; Poinsot et al., 1998; Poinsot and Merot, 1999).
2.
The mod and resc factors interact in a speciÝc manner. The phenomenon of bidirectional
CI supports this conclusion. The current data obtained by crossing experiments indicate
 
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