Biology Reference
In-Depth Information
the world. Traditional mosquito-control methods include the use of pesticides and habitat management,
including the use of screens on houses and elimination of breeding sites. However, mosquitoes develop
resistance to pesticides, and habitat-management methods may be difficult to implement. Dengue
potentially can affect 2.5 billion people and causes an estimated 50-100 million cases each year (
Bian
et al. 2010
).
The dengue virus has multiple serotypes and movement of people can introduce new serotypes into
an area. If a person is infected with one serotype, the result is a debilitating fever called “break bone
fever.” Unfortunately, subsequent infection with a different serotype causes an even more-serious disease,
resulting in 1 to 20% mortality in children (
Yeap et al. 2011
).
Genetic modifications of mosquitoes have been proposed as one method for managing dengue.
Approaches include developing transgenic strains of
Ae. albopictus
in which the nuclear genome has
been modified in a manner that precludes disease transmission. Another option is to release a strain of
mosquito that contains
Wolbachia
as a driver (by means of cytoplasmic incompatibility) to enhance the
spread of a transgene inserted into the nuclear genome of the mosquito. A third option is to insert a strain
of
Wolbachia
(called “popcorn” or
w
MelPop-CLA) into
Ae. aegypti
to prevent transmission of dengue (
Min
and Benzer 1997, McMeniman et al. 2009, Moreira et al. 2009a, Walker et al. 2011, Yeap et al. 2011
). The
w
MelPop strain of
Wolbachia
was found originally in
Drosophila melanogaster
, where it multiplies excessively
and reduces the life span of the fruit fly. It was called the “popcorn” strain because it produces large holes
in the brain of the flies.
The O'Neill laboratory in Australia has developed
Ae. aegypti
containing the
w
MelPop strain of
Wolbachia
.
Their research resulted in releases into the field in Australia in 2011 and 2012, and plans are underway to
conduct trials in other countries. The project progressed because
McMeniman et al. (2009)
introduced
a life-shortening
Wolbachia
strain (
w
MelPop) into
Ae. aegypti
after rearing the
Wolbachia
in cell lines of the
mosquito over a 3-year period to adapt the novel strain to the mosquito. The infection causes cytoplasmic
incompatibility between infected and uninfected lines and reduces the longevity of infected females by
half; epidemiology suggests that such a reduction in longevity could result in reduced transmission of
pathogens. Transmission of the novel
Wolbachia
strain to progeny by infected females is high (
>
99.7%).
Moreira et al. (2009a)
found that the
w
MelPop strain of
Wolbachia
limited infection with dengue,
Chikungunya virus, and
Plasmodium
in
Ae. aegypti
, and speculated that this reduced-infection status and
the life-shortening effects shown by
McMeniman et al. (2009)
could provide a method for controlling
disease transmission.
Moreira et al. (2009b)
showed that aging
Ae. aegypti
females infected with this novel
Wolbachia
strain have difficulty in blood feeding.
Turley et al. (2009)
quantified the decrease in blood-
feeding success in the infected mosquitoes. Again, this was speculated to reduce disease-transmission
capability.
Evans et al. (2009)
showed that mosquitoes infected with this
Wolbachia
strain also had
increased locomotor activity and increased metabolism, for unknown reasons. The increased activity levels
and metabolic activity of the infected mosquitoes suggests they are fit and could potentially compete with
field populations if released. At the cellular level,
Frentiu et al. (2010)
showed that the
w
MelPop strain of
Wolbachia
reduced replication of dengue virus in mosquito cells in relation to the density of the
Wolbachia
;
these results were comparable to those of
Bian et al. (2010)
who studied whole mosquitoes. Thus,
“although
Wolbachia
infection imposes a metabolic cost that can lead to lowered host fitness, in certain
circumstances, such as viral infections and nutritional stress, these bacteria also provide a fitness benefit”
(
Frentiu et al. 2010
).
Hussain et al. (2011)
discovered that
Wolbachia
uses host microRNAs to manipulate
host gene expression.
Yeap et al. (2011)
evaluated the effects of
w
MelPop on
Ae. aegypti
after the infected line was crossed
with a population from Cairns, Australia to increase the competitive ability of the infected (laboratory)
line. The outcrossed lines were evaluated for transmission of
Wolbachia
, longevity, egg viability, cytoplasmic
incompatibility, larval nutrition, and developmental time. Genetic variability was assessed using eight
molecular markers. The results indicated that the out-crossed lines had increased genetic variability, infected
