Biology Reference
In-Depth Information
embryos. However, the patterns of additive unidirectional and bidirectional CI that have been
described in
N. vitripennis
(Perrot-Minnot
et al., 1996; Bordenstein
et al., 2001) and other insects
(Rousset and de Stordeur, 1994; Guillemaud
et al., 1997; Rousset
et al., 1999; Dobson
et al., 2001)
complicate the sole use of a timing model to explain observed CI phenomena.
As illustrated in
Figure 13.1,
differing
Wolbachia
infections can result in bidirectional CI,
demonstrating that
mod
and
resc
interact in a speciÝc manner, such that different infection types
do not necessarily rescue the modiÝcation of a differing
Wolbachia
type (Bourtzis
et al., 1998;
Merot and Poinsot, 1998). In addition, CI patterns associated with superinfection demonstrate that
differing
mod
/
resc
mechanisms may act autonomously and additively, such that coinfections may
result in novel patterns of additive unidirectional CI (see Figure 13.1; Merot
et al., 1995; Sinkins
et al., 1995; Perrot-Minnot
et al., 1996; Dobson
et al., 2001; James
et al., 2002). Thus, an additional
Ñrecognition factorÒ must be incorporated into the timing model to explain the failure of the maternal
infection to inÞuence paternal pronuclei that have been modiÝed by different
Wolbachia
infection
types. The proposed recognition factor could be tested by examining the timing of nuclear-envelope
breakdown in
N. vitripennis
embryos that result from bidirectionally incompatible crosses (Perrot-
Minnot
et al., 1996). Comparison of nuclear-envelope-breakdown timing in crosses of superinfected
males with the four female infection types would further elucidate an additive timing effect as an
explanation for the pattern of CI observed with
Wolbachia
superinfection.
Following
Wolbachia
invasion, several evolutionary trajectories are possible. With infections
that have an imperfect maternal transmission rate or a high host cost associated with
Wolbachia
modiÝcation,
Wolbachia
variants may be selected that can rescue but that have a reduced ability
to induce CI or that do not cause CI (modÏ resc
+
) (Turelli, 1994; Bourtzis
et al., 1998; Merot and
Poinsot, 1998). After the modÏ resc
+
infection has become Ýxed in the host population, there is no
selection to maintain the resc
+
phenotype. Thus, the host population can be invaded by a variant
that does not rescue (modÏ rescÏ) or by the uninfected cytotype leading to a cyclical pattern of
invasion under some circumstances (Hurst and McVean, 1996; Hatcher, 2000).
An additional trajectory includes the evolution of
Wolbachia
variants with new compatibility
types (Charlat
et al., 2001).
Wolbachia
variants with a novel modiÝcation type that is incompatible
with the infection of origin will have the same pattern of incompatibility (Table 13.1). Thus, mod-
iÝcation-type variants are neutral and may spread by drift. If the infection frequency of the variant
increases sufÝciently, the conditions can permit the invasion of an additional variant with a novel
rescue type (i.e., mod
B
resc
B
). Thus, novel CI types can emerge from ancestral infection types.
WOLBACHIA
DESCRIPTION AND EARLY APPLIED
SUPPRESSION STRATEGIES
Initially described by Hertig and Wolbach (1924), the species name
Wolbachia pipientis
was not
deÝned for an additional 12 years (Hertig, 1936). The bacterium was reported as a rickettsia,
appearing as Ñminute rods and coccoidsÒ in the germ cell cytoplasm of
Culex pipiens
(common
house mosquito). The infection was described as a Ñrelatively harmless parasiteÈtransmitted via
TABLE 13.1
Pattern of Cytoplasmic Incompatibility When the Type of Modification (mod)
and Rescue (resc) Are Affected
Male
mod
A
resc
A
mod
B
resc
A
Uninfected
Female
mod
A
resc
A
+
Ï
+
mod
B
resc
A
+
Ï
+
Uninfected
Ï
Ï
+
