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ability to modify sperm. This hypothesis suggests that paternal chromosomes are
modified during spermatogenesis by
Wolbachia
and this modification is “res-
cued” in eggs of females infected with the same strain of
Wolbachia
during fer-
tilization. If, however, the female is
not
infected with
Wolbachia
and mates with
an infected male or male infected with a different strain of
Wolbachia
, then the
embryos die (
Figure 4.2
). Some
Wolbachia
strains have been identified that fail
to modify sperm but can rescue the modification in eggs of other
Wolbachia
strains (
Bourtzis et al. 1998
).
Cytoplasmic incompatibility caused by
Wolbachia
may be partial or complete.
Sometimes incompatibility is found in both reciprocal crosses (A
×
B and B
×
A,
bidirectional incompatibility
), perhaps due to the presence of different strains of
Wolbachia
in each population. Incompatibility is more often found in one recipro-
cal cross (A
×
B or B
×
A,
unidirectional incompatibility
). Cytoplasmic incompatibility
typically is incomplete (
<
100%), perhaps due to inefficient transfer of
Wolbachia
to all progeny or to differences in the titer of
Wolbachia
. Such differences in titer
could occur naturally if the infected insects encounter antibiotics in their environ-
ment or if they experience high temperatures (typically
>
30°C) (
Snook et al. 2000
).
Tram and Sullivan (2002)
found that the cytoplasmic incompatibility in
Nasonia
vitripennis
was due to a delay in the nuclear envelope breakdown in the male
pronucleus; this delay resulted in an asynchrony between the male and female
Figure 4.2
A) Cytoplasmic incompatibility due to
Wolbachia
between different individuals or popu-
lations can result in a failure to produce progeny when a
Wolbachia
-infected male mates with an
uninfected female. B) If isolated populations of a species become infected with different types of
Wolbachia
(W
A
and W
B
), then these populations could become reproductively isolated if they later
come into contact.