Biology Reference
In-Depth Information
population-suppression to population-replacement strategies, which employ
Wolbachia
to spread
desired genes into a host population (e.g., harnessing
Wolbachia
-induced population replacement
to spread genes conferring resistance to disease transmission; discussed below) (Curtis, 1992).
WOLBACHIA
INCRIMINATION
Wolbachia
research was reinvigorated when Janice Yen rediscovered HertigÔs description and pro-
posed that
W. pipientis
could be the cytoplasmically inherited agent responsible for CI (Yen and
Barr, 1971). She subsequently demonstrated
Wolbachia
to be the etiological agent of CI in
C. pipiens
by removing the infection using tetracycline. Crosses of the aposymbiotic strain demonstrated the
expected pattern of unidirectional incompatibility associated with CI (Yen and Barr, 1974).
Another signiÝcant advance in
Wolbachia
research occurred with the advent of the polymerase
chain reaction (PCR) and molecular phylogenetics. Detection of
Wolbachia
infection had previously
relied on electron microscopy of tissue sections. PCR provided a relatively quick, simple, and
sensitive assay to detect infection. Sequencing and molecular phylogenetic comparison of PCR
amplicons provided a valuable complement to prior morphological characterizations using electron
microscopy (Hertig and Wolbach, 1924; Hertig, 1936) and allowed an improved taxonomic posi-
tioning of
Wolbachia
, placing
Wolbachia
in the b-subdivision of the proteobacteria (OÔNeill
et al.,
1992; Rousset
et al., 1992).
PCR and molecular phylogenetics also demonstrated that
C. pipiens
was not unique in its
Wolbachia
infection. As described above, similar patterns of CI had been described for additional
species (Rozeboom and Kitzmiller, 1958; Saul, 1961; Stanley, 1961; Ryan and Saul, 1968; Kellen
et al., 1981; Trpis
et al., 1981; Hsiao and Noda, 1984; Hsiao, 1985a; Wade and Stevens, 1985;
Hoffmann
et al., 1986; OÔNeill, 1989; Breeuwer and Werren, 1990; Hoffmann
et al., 1990). Surveys
using
Wolbachia
-speciÝc PCR primers have revealed widespread
Wolbachia
infection in inverte-
brates, with estimates of infection rates ranging from 15 to 75% (Werren
et al., 1995a; Jeyaprakash
and Hoy, 2000; Werren and Windsor, 2000; Jiggins
et al., 2001a). The many
Wolbachia
infection
types have also been shown to be responsible for multiple types of host reproductive manipulations
including CI, parthenogenesis (Stouthamer
et al., 1993), feminization (Rousset et al., 1992), and
male killing (Hurst
et al., 1999).
Wolbachia
infections have been broadly described in nematodes as
mutualistic symbionts, attracting medical attention as novel targets for the reduction of Ýlarial
pathology (Hoerauf
et al., 2000; Bandi
et al., 2001; Hoerauf
et al., 2001; Taylor
et al., 2001).
PHYLOGENETIC ANALYSES
Wolbachia
infections are widespread in insects, with estimates ranging from 16 to 76% infection
rates (Werren
et al., 1995a; Jeyaprakash and Hoy, 2000; Werren and Windsor, 2000; Jiggins
et al.,
2001a). Early phylogenetic examination of
Wolbachia
infections from different invertebrate hosts
demonstrated a monophyletic group within the b-subdivision of the proteobacteria (OÔNeill
et al.,
1992; Rousset
et al., 1992; Roux and Raoult, 1995). The initial comparisons of host and
Wolbachia
phylogeny indicated that infections had been acquired more than once by different insects and
suggested horizontal transmission of infections between host species. Subsequent studies that
employed additional
Wolbachia
gene sequences corroborated early results and allowed improved
phylogenetic resolution of
Wolbachia
infections, including the subdivision of
Wolbachia
infections
into multiple clades and subgroups (Bourtzis
et al., 1994; Werren, 1997; Bandi
et al., 1998; Zhou
et al., 1998; Bazzocchi
et al., 2000). To date, a cautious approach of designating infections as strains
rather than species has been followed (Werren
et al., 1995b).
Although it is agreed that the primary route of
Wolbachia
maintenance within the host popu-
lation is vertical transmission via maternal cytoplasm, phylogenetic comparisons of
Wolbachia
and
host demonstrate that naturally occurring, interspeciÝc, horizontal transfer of infections can also
