Biology Reference
In-Depth Information
occur (OÔNeill
et al., 1992; Rousset
et al., 1992; Werren
et al., 1995b). Examples of naturally
occurring intraspeciÝc transfer include the proposed movement of
Wolbachia
between
hymenopteran parasitoids and their hosts (Werren
et al., 1995b; Jochemsen
et al., 1998; van Meer
et al., 1999; Vavre
et al., 1999). An interesting example of a natural route of intraspeciÝc horizontal
transmission has been infected and uninfected
Trichogramma
wasps that superparasitize the same
host egg (Huigens
et al., 2000).
The interpretation of
Wolbachia
phylogeny can be confounded by recombination (Werren
et al.,
1995b; Jiggins
et al., 2001b; Werren and Bartos, 2001). Evidence for
Wolbachia
recombination is
provided by comparisons of different regions of the
Wolbachia
genome, examining for congruency.
Phylogenetic comparisons of the genomic regions are mutually incompatible. Therefore, the most
parsimonious interpretation is genetic recombination between strains.
Wolbachia
genomic sequenc-
ing (Bandi
et al., 1999; OÔNeill, 1999; Slatko
et al., 1999) will help to clarify the frequency and
evolutionary signiÝcance of recombination among
Wolbachia
strains.
WOLBACHIA
MORPHOLOGY AND INTERACTION
WITH HOST CELLS
Earlier ultrastructural observations describe
Wolbachia
as small (0.5 to 1.5 nm) and pleomorphic,
ranging from spherical to elongate (Wright and Barr, 1980; Kellen
et al., 1981; Trpis
et al., 1981;
Larsson, 1983; Louis and Nigro, 1989; Ndiaye and Mattei, 1993; OÔNeill
et al., 1997; Popov
et al.,
1998; Taylor
et al., 1999).
Wolbachia
cells are commonly observed to contain ribosomes and
nucleic-acid Ýbrils (Yen and Barr, 1974; Beckett
et al., 1978; Wright and Barr, 1980; Louis and
Nigro, 1989; Ndiaye and Mattei, 1993). Although
Wolbachia
have been rarely observed without a
host vacuole (i.e., exposed in the host-cell cytoplasm),
Wolbachia
are commonly observed within
a three-layer membrane consisting of the bacterial plasma membrane, bacterial cell wall, and host
vacuole membrane (Yen and Barr, 1974; Yen, 1975; Barr, 1982; Beckett
et al., 1978; Kellen
et al.,
1981; Trpis
et al., 1981; Larsson, 1983; Louis and Nigro, 1989; Ndiaye and Mattei, 1993; OÔNeill
et al., 1997; Popov
et al., 1998; Taylor
et al., 1999). During binary Ýssion,
Wolbachia
appears
elongate, presumably in prelude to cell division, which appears as a ÑdumbbellÒ shape (Wright
et al., 1978; Wright and Barr, 1980; Larsson, 1983; Ndiaye and Mattei, 1993). During division, the
vacuolar membrane follows the contours of the dividing cells, occasionally resulting in multiple
Wolbachia
contained within a single host vacuole (OÔNeill
et al., 1997).
Wolbachia
do not appear
to be attacked by the host lysosome (Beckett
et al., 1978). Both intracellular and extracellular
morphs have been reported
in vitro
and
in vivo
(Ndiaye and Mattei, 1993; Ndiaye
et al., 1995;
Popov
et al., 1998). With
in vitro
infections, intracellular
Wolbachia
were observed as round retic-
ulate cells and extracellular bacteria as dense-cored cells, the latter being engulfed by a host cell
in one observation (Popov
et al., 1998). Intracellular bridges may also permit the Þow of
Wolbachia
from one cell to another (Wright and Barr, 1980).
During mitosis, electron (Wright and Barr, 1980; Callaini
et al., 1994; Kose and Karr, 1995;
Ndiaye
et al., 1995) and confocal (OÔNeill and Karr, 1990; Braig
et al., 1994; Callaini
et al., 1994;
Kose and Karr, 1995) microscopy reveal a close association between
Wolbachia
and host-cen-
trosome-organized microtubules, suggesting that
Wolbachia
infections may employ the host spindle
apparatus for segregation to daughter cells. At prophase and centrosome duplication,
Wolbachia
is
loosely associated with centrosomes (Kose and Karr, 1995). During prometaphase/metaphase,
however,
Wolbachia
become tightly clustered near the centrosome and spindle-pole asters. At the
metaphase/anaphase transition,
Wolbachia
continue to associate with astral microtubules and redis-
tribute away from the centrosomes, coincident with microtubule growth outward from spindle poles.
By anaphase/telophase,
Wolbachia
is even more dispersed as the spindle elongates, forcing nuclei
and associated spindle poles further apart.
Wolbachia
are not observed to associate with kinetochore-
to-pole microtubules. By virtue of their association with the centrosome,
Wolbachia
infections
