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Experimental microinjection (artificial horizontal transfer) of Wolbachia from
the parasitoid Muscidifurax uniraptor into its host D. simulans resulted in a tem-
porary infection, but no specific phenotypic effects were observed ( van Meer
and Stouthamer 1999 ). These results suggest that host-symbiont interactions are
important for successful establishment of a Wolbachia infection in a new host,
although it is clear that Wolbachia has successfully bridged large phylogenetic
distances in its horizontal movements over evolutionary time.
The availability of PCR primers for Wolbachia genes revolutionized the study
of the distribution and evolution of Wolbachia . The Wolbachia genome proj-
ect further revolutionized such studies. Based on a phylogeny developed using
the ftsZ gene, Wolbachia infecting arthropods have been divided into groups
A and B, and these groups are estimated to have diverged from each other 58
to 67 million years ago ( Werren et  al. 1995 ). Phylogenies based on wsp gene
sequences have yielded more groups, indicating considerable genetic varia-
tion exists ( Zhou et  al. 1998, van Meer and Stouthamer 1999, Jeyaprakash and
Hoy 2000 ). It is unclear whether these Wolbachia groups are strains or species
because the definition of a bacterial species is complex.
Wolbachia may have a role in speciation of arthropods by generating repro-
ductive isolation ( Rokas 2000 ), although some argue that Wolbachia 's role(s)
remain unproved ( Hurst and Schilthuizen 1998 ). Typically, Wolbachia cause uni-
directional cytoplasmic incompatibility when a Wolbachia -infected male mates
with an uninfected female ( Figure 4.2A ). The eggs or embryos of such matings
die, resulting in a fitness cost to uninfected females that, over time, results in
the infected cytotype becoming fixed in the population. A problem with the
“speciation hypothesis” is that Wolbachia are not transmitted 100% of the time
from a female to her progeny, so progeny will be produced that are compatible.
Also, incompatibility is not completely expressed (incomplete penetrance of the
trait) when infected males and uninfected females mate in natural populations
(perhaps due to differences in the titer of the Wolbachia within individuals).
Furthermore, selection on both the host and Wolbachia may favor reduced
penetrance of the incompatibility phenotype or loss of Wolbachia , leading
to a situation in which there is no gene flow to some gene flow ( Hurst and
Schilthuizen 1998 ). Thus, unidirectional incompatibility caused by Wolbachia
may be insufficient to cause the reproductive barriers that could lead to spe-
ciation. Additional factors, such as hybrid sterility (sterility of the hybrid when
crossed with either of the parental species) and hybrid breakdown (inviability or
sterility of progeny resulting from a backcross of hybrid progeny with either of
the parental species) may be necessary ( Shoemaker et al. 1999 ). Thus, Wolbachia
may enhance the speciation rate by acting in conjunction with behavioral
isolation.
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