Biology Reference
In-Depth Information
that any
Wolbachia
strain is only fully compatible with itself. This means that there is a
speciÝc recognition between the different mod and resc factors (OÔNeill and Karr, 1990;
Braig et al., 1994). Crossing patterns involving multiple infections can also be explained
by speciÝc mod/resc, complete or partial, recognitions (Merot et al., 1995; Rousset and
Solignac, 1995; Sinkins et al., 1995a; Perrot-Minnot et al., 1996; Poinsot et al., 1998;
Rousset et al., 1999).
3.
Mod and resc are distinct bacterial functions. The identiÝcation of a
Wolbachia
strain that
was unable to induce CI but capable of rescuing CI induced by other strains strongly
suggests that mod and resc are separate functions (Bourtzis et al., 1998; Merot and Poinsot,
1998). Based on this Ýnding, four different CI-
Wolbachia
types (strains) can exist:
a. The mod+/resc+ type, which can induce CI and rescue its own modiÝcation. For
example, three different mod+/resc+ types have been reported in
D. simulans
,
w
Ri,
w
Ha and
w
No (Hoffmann et al., 1986; OÔNeill and Karr, 1990; Braig et al., 1994;
Merot et al., 1995), while only one has been reported in
D. melanogaster
,
w
Mel
(Hoffmann, 1988; Hoffmann et al., 1994; Bourtzis et al., 1994, 1996, 1998; Reynolds
and Hoffmann, 2002; Weeks et al., 2002).
b. The modÏ/rescÏ type, which is unable to induce CI itself or rescue the CI effect of
other mod+ strains; such a type is, for example, present in the
w
Cof (or
w
Au) strain
of
D. simulans
(Hoffmann et al., 1996).
c. The mod+/rescÏ type, which can induce CI but cannot rescue its own effect. This
type is suicidal and has never been detected in natural or laboratory populations (but
see Charlat et al., 2001 for an alternative view).
d. The modÏ/resc+ type, which was described above. Such a type is present in the
w
Ma
strain of
D. simulans
and
D. mauritiana
and can rescue the CI effect induced by the
w
No strain (Bourtzis et al., 1998; Merot and Poinsot, 1998).
4.
The mod intensity is linked to bacterial density. Breeuwer and Werren (1993) introduced
the bacterial-density model to explain the different CI levels and patterns observed.
Although the different CI patterns could not be resolved, several studies have suggested
a relationship between
Wolbachia
density and CI levels (Hoffmann et al., 1986; Binning-
ton and Hoffmann, 1989; Boyle et al., 1993; Bressac and Rousset, 1993; Rousset and
de Stordeur, 1994; Solignac et al., 1994; Merot et al., 1995; Sinkins et al., 1995a;
Bourtzis et al., 1996, 1998; Poinsot et al., 1998; Rousset et al., 1999). In a recent study,
Veneti et al. (2002) determined and compared the distribution and the density of
Wolba-
chia
during spermatogenesis in 16 different naturally infected and transinfected
Droso-
phila
hosts using a
Wolbachia
-speciÝc antibody. They investigated the relationship
between the distribution and density of
Wolbachia
within sperm cysts and the expression
levels of CI by comparing the same
Wolbachia
strain in different hosts and different
Wolbachia
strains in the same host genetic background. Their experimental data show a
positive correlation between the number of infected sperm cysts and CI levels.
5.
The mod function alters the mitotic behavior of the paternal chromosomes after fertili-
zation
.
Cytological studies have shown that
Wolbachia
disrupts the kinetics of decon-
densation/recondensation of the pronuclei during fertilization. Details about the cell
biology of
Wolbachia-
induced CI are presented below.
Several
Wolbachia
genome projects are in progress aiming to identify the complete genome
sequence of several
Wolbachia
strains (Bandi et al., 1999; OÔNeill, 1999; Slatko et al., 1999; Oehler
and Bourtzis, 2000). The genome sequence of the
w
Mel
Wolbachia
strain has been completed,
while that of
w
No is at an advanced stage (OÔNeill, 2002; Bourtzis, 2002). These two strains are
bidirectionally incompatible. Coordinated efforts using comparative genomics, proteomics, and
post-genomics approaches will ultimately lead to the identiÝcation and characterization of
Wolba-
chia
genes involved in the induction of the three phenotypes as well as host genes involved in
