Biology Reference
In-Depth Information
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.