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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.
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