Biology Reference
In-Depth Information
mobile genetic element, if used in combination with a strong promoter such as that of
wsp
(Braig
et al., 1998), could be a useful vector for transforming
Wolbachia
.
A prophage-like genetic element was also detected in the
Wolbachia
genome and named WO
(Masui et al., 2000b). All the
Wolbachia
strains examined were shown to contain the phage WO.
The phylogenetic tree based on phage WO genes of several
Wolbachia
strains was not congruent
with that based on chromosomal genes of the same strains, suggesting that phage WO is active
and horizontally transmitted among various
Wolbachia
strains. Although the phage genome contains
genes of diverse origins, their average G+C content and codon usage are quite similar to those of
chromosomal genes. These results raised the possibility that phage WO has been associated with
Wolbachia
for a very long time, conferring some beneÝt to its host bacteria. The phage WO genome
contains a gene derived from a plasmid and genes for several ankyrin-like proteins similar to those
of mammalians and plants. Since it has been suggested that ankyrin-like repeats are a motif for
proteinÏprotein interaction (Sedgwick and Smerdon, 1999), it is possible that the proteins containing
eukaryotic ankyrin-like repeats encoded by the phage WO genes play a role in the reproductive
alteration of insect hosts by
Wolbachia
through their ability to interact with other proteins. Recently,
virus-like particles that probably represent the phage WO were observed in
Wolbachia
by electron
microscopy (Masui et al., 2001).
CONCLUSION
The effects of cytoplasmically inherited microbes range from those of obligate mutualists for host
survival to those of selÝsh parasites that may cause all of a hostÔs progeny to die prematurely. The
great diversity of insects is partly due to their frequent associations with mutualistic endosymbionts,
such as
Buchnera
, which enable hosts to exploit niches that would otherwise be nutritionally
unsuitable. In other instances, bacterial associates may promote speciation through direct effects
on reproductive systems. For example, induction of incompatibility or parthenogenesis by
Wolba-
chia
could effect reproductive isolation of host population (Stouthamer et al., 1990). The extent of
mutualism vs. antagonism underlying a particular association will determine the degree of conÞict
between selection pressure on host and symbiont, with implications for the evolution of both
participating lineages (Moran and Baumann, 1994).
To date, many studies have suggested how symbiotic microbes have undergone evolutionary
changes in the course of association with insect hosts. To further understand the evolutionary
signiÝcance of interspeciÝc association in nature, more studies on the side of hosts are desirable.
A key issue is how insects have been changed, from molecular and cellular points of view, by
acquiring microbial partners.
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