Biology Reference
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have been reported. Notwithstanding this, it is only the matings between individuals from the same
population that are always successful. Matings between individuals from distinct populations tend
to fail, even if both the males and females are infected with
Wolbachia
. These observations can be
explained by assuming that the genetic trait of
Wolbachia
is variable due to the host population
and that only the egg cytoplasm infected with the same variant is able to rescue the chromosomes
of sperm from the infected males (Ebbert, 1993). If this is the case, interpopulation incompatibility
may occur between two populations that are infected with different variants of
Wolbachia
, which
may eventually lead to sympatric speciation of species.
As in bacteriocyte symbionts,
Wolbachia
infections are maternally inherited through transmis-
sion from females to their eggs before oviposition. Regular maternal inheritance would seem to
create the potential for cospeciation of
Wolbachia
and its hosts. However, the phylogenetic results
based on the sequences of 16S rDNAs from various
Wolbachia
species provide an interesting
contrast to those from
Buchnera
. Results indicate that
Wolbachia
lineages have sometimes been
transmitted among members of different insect orders (Moran and Baumann, 1994). For this reason,
Wolbachia
are sometimes called evolutionary hitchhikers. The extent of divergence among the most
distant of the examined
Wolbachia
16S sequences is small (<3%) (OÔNeill et al., 1992) relative to
that among distantly related
Buchnera
(>8%) (Moran et al., 1993). Assuming substitution rates in
Wolbachia
are not much different from those estimated for
Buchnera
and other prokaryotes, the
common ancestor of examined
Wolbachia
lived about 25 to 100 million years ago. This is much
younger than any common ancestor of the hosts (>300 million years), lending further evidence that
the distribution of
Wolbachia
is the result, in part, of occasional horizontal transfer of bacterial
lineages among lineages of arthropods (Moran and Baumann, 1994). Evidence for more than one
Wolbachia
strain within individual insects (Rousset et al., 1992) is consistent with the view that
horizontal transfer has been an important determinant of the modern distribution of
Wolbachia
among hosts.
Phylogenetic trees of
Wolbachia
strains have been constructed based on the sequences of 16S
rDNA and parts of the protein-coding region of
ftsZ
(OÔNeill et al., 1992; Werren et al., 1995;
Tsagkarakou et al., 1996). The
ftsZ
tree had a Ýner resolution and divided
Wolbachia
into two major
groups, designated A and B. Evidence suggests that the two diverged from each other 58 to 67
million years ago (Werren et al., 1995). The tree also implied frequent horizontal transfer of
Wolbachia
, in A group in particular, between distantly related insect orders. To further deÝne the
relationship among A group
Wolbachia
strains, a phylogenetic tree was constructed based on the
sequences of parts of the
groE
operons, which suggested unexpected patterns of horizontal transfer
of these bacteria (Masui et al., 1997).
It is assumed that in CI,
Wolbachia
modify host sperm during maturation, possibly by secreting
some proteinaceous factors. Based on this assumption, attention was directed toward the macro-
molecule secretion system in
Wolbachia
. Recently, it was found that
Wolbachia
contain the genes
encoding homologs to the type IV secretion system by which many pathogenic bacteria, such as
Agrobacterium tumefaciens
(Zambryski, 1988) and
R. prowazekii
(Andersson et al., 1998), secrete
macromolecules. The genes identiÝed encode most of the essential components of the secretion
system and are cotranscibed as an operon (Masui et al., 2000a).
Recent studies have shown that
Wolbachia
are a playground for various mobile and extrachro-
mosomal genetic elements. This may be one of the reasons why an earlier completion of the
sequencing of the whole genome of
Wolbachia
has been prevented, despite its small size (1.2 Mb).
An insertion sequence, designated ISW
1
, was identiÝed in the genome of a
Wolbachia
strain,
w
Tai
infecting the Taiwan cricket,
Teleogryllus taiwanemma
(Masui et al., 1999). ISW
1
displays a
signiÝcant similarity to IS
200
, an IS element Ýrst identiÝed in
Salmonella
that lacks the terminal
invert repeat (Lam and Roth, 1986). There were at least 20 copies of ISW
1
on the chromosome of
w
Tai. Sequence analysis of 9 ISW
1
copies and their Þanking regions showed that the copies were
identical and suggested that ISW
1
had no preference for its insertion sites. It is hoped that this
