Biology Reference
In-Depth Information
for maximum efÝciency (Knipling, 1998; Krafsur, 1998), but the absolute elimination of females
from releases was not imperative to the success of the strategy. In contrast, females accidentally
released as part of a CI strategy could permit the ÑreleasedÒ
Wolbachia
infection type to become
established in the host Ýeld population. Following this establishment, the efÝcacy of continued
male releases for pest suppression would decline as the released infection spread through the Ýeld
population, resulting in compatible crosses between Ýeld females and released males. Thus, the
result would not be the eradication of the pest population but a transient suppression of the host
population followed by population replacement, in which the original cytoplasm type (i.e., cytotype)
was replaced with the cytotype of the released host strain. Although male-only releases were
possible for small, pilot experiments
(Laven, 1967a), the complete removal of females would not
be practical on a scale required for large-area-control or eradication programs.
Subsequently, interest in the applied use of
Wolbachia
-induced CI has shifted from population-
suppression to population-replacement strategies. Population-replacement strategies would employ
the reproductive advantage afforded to infected females as a vehicle for spreading desired genes
into a host population (Curtis, 1992; Turelli and Hoffmann, 1999; Sinkins and OÔNeill, 2000).
Instead of mass rearing and release of transgenic insects to replace Ýeld populations via simple
dilution, population replacement via
Wolbachia
could drive desired transgenes (e.g., genes confer-
ring refractoriness to pathogen transmission) into Ýeld populations from small release seedings.
Furthermore, additive CI observed with
Wolbachia
superinfections (Sinkins
et al., 1995; Dobson
et al., 2001) suggests that
Wolbachia
population-replacement strategies can also be employed in
host Ýeld populations that are naturally infected. The ability to generate superinfections via artiÝcial
transinfection (Braig
et al., 1994; Rousset
et al., 1999) has the potential to permit repeated popu-
lation replacements within the same host population (Sinkins
et al., 1995). However, population-
replacement strategies have not been applied to date, due to the inability to genetically transform
Wolbachia
(Sinkins and OÔNeill, 2000).
An additional strategy based on model predictions has been proposed for the suppression of
insect populations using
Wolbachia
infections (
Figure 13.2B)
.
In brief, releases of
Wolbachia
-
infected hosts can be used to artiÝcially sustain an unstable coexistence that results when
incompatible infections occur within different individuals in a single host population, allowing
the host-population size to be reduced and maintained at low levels (Dobson
et al., 2002a). Unlike
previous CI-based suppression strategies, the new strategy would permit female releases. Simu-
lations show that the vertical transmission of
Wolbachia
that occurs with infected female releases
can permit multiple generations of control resulting from a single release, with the potential to
increase cost efÝcacy. This novel suppression strategy employs the release of indigenous host
insects and does not involve transgenic organisms, reducing technical and regulatory impediments
to strategy implementation.
Additional applied strategies are based on the induction of host parthenogenesis by
Wolbachia
infections. Potential advantages of releasing parthenogenetic wasps as biological control agents are
lower production costs associated with mass rearing of females, a higher growth rate in released
populations, and improved ability to suppress the targeted pest population (Stouthamer, 1993).
CONCLUSION
Bacteria in the genus
Wolbachia
provide a useful model for examining the evolutionary interactions
between host and maternally inherited symbionts, with examples ranging across the continuum
between mutualism and parasitism. It is obvious that much work remains to be done in understand-
ing currently known examples and with the description of additional infections and new types of
reproductive manipulations. New avenues of research are possible with recently developed molec-
ular techniques and genomic analyses, aiding the study of this fastidious bacterium. The attraction
of additional researchers from other areas (e.g.,
Drosophila
developmental biologists) will also
facilitate progress.
