Biology Reference
In-Depth Information
the releases of
Wolbachia
-infected mosquitoes under the Cartagena Protocol on Biosafety with regard to
concerns about the international movement of genetically modified organisms, noting that this release
uncovers a gap in that regulatory machinery. The mosquito naturally occurs in Australia and the mosquito
genome itself is not genetically modified (although it has a novel strain of
Wolbachia
artificially inserted into
it, so is genetically modified) (
De Barro et al. 2011
). Existing regulation in Australia was used to evaluate
possible risk although the case, “is an example of how science is leading to advances that outstrip existing
regulatory frameworks” (
De Barro et al. 2011
).
Lehane and Aksoy (2012)
point out that international
regulation of transgenic and paratransgenic insects is needed, because insects can disperse from the
original release sites (see
Table 14.8
).
Box 14.4 Field Releases of Transgenic Mosquitoes Containing a RIDL Construct
The sterile insect technique (SIT) or sterile insect release method (SIRM) was described in Chapter 10.
This genetic-control method involves mass rearing, sterilization by chemicals or irradiation, and release of
large numbers of males that mate with wild females (
Knipling 1955, Pal and Whitten 1974, Whitten 1985,
Tan 2000
). Sterile males must compete with wild males and reduce reproduction by wild females; in some
cases the goal is
eradication
and in other programs it is to
suppress
the pest population. Ultimately, the goal
is to reduce crop damage or transmission of insect-vectored diseases. SIRM programs have been highly
effective in the eradication of the screwworm in North and Central America, and of the Mediterranean fruit
fly in Florida and other locations (
Wyss 2000
). However, genetic-control projects are expensive because a
large number of sterile males need to be released to compete with wild males (up to 100 sterile males to 1
wild male) (
LaChance 1979
). This release ratio is needed because sterilization typically is accomplished by
whole-body irradiation and sterile males are not as competitive as wild males. It is undesirable to release
females (because they can bite, transmit disease, or cause damage to crops), so a method of eliminating
females early in the rearing program would reduce costs and eliminate these concerns. Classical genetic
methods using mutations and translocations have resulted in sexing strains that produce mostly males
(
LaChance 1979
, Wyss et al. 2000,
Marec et al. 2005, Papathanos et al. 2009, Black et al. 2011
), but
these strains can revert to wild type under factory rearing conditions if strict quality-control methods
are not maintained. The colonization and mass rearing of insects, and especially mosquitoes, is difficult,
expensive, and must preserve fitness in released males that are essential for competitive mating with wild
females (
Benedict et al. 2009
).
The development of transgenic strains of insects has been proposed to overcome these difficulties:
strains that have a conditional female-specific lethality, or strains that have males that are sterile without
undergoing irradiation could provide improved cost-effectiveness for SIRM programs.
Thomas et al. (2000)
proposed using a dominant, repressible, lethal genetic system as a genetic-control method called
RIDL
, or
R
elease of
I
nsects carrying a
D
ominant
L
ethal. Oxitec, a company producing and selling RIDL mosquitoes,
has called RIDL “biological control,” but biological control commonly is defined as the use of predators,
parasitoids or pathogens to control a pest. Oxitec has called the RIDL mosquitoes “sterile insects” (
Phuc
et al. 2007, Fu et al. 2010, Harris et al. 2011
), but sterile insects should be unable to produce viable
larvae. In fact, RIDL mosquitoes are transgenic, and the mosquitoes released produce progeny by matings
between wild females and transgenic males, with the majority of larvae or pupae dying because there is
no tetracycline in the local environment (
Phuc et al. 2007
). Due to “leakage,” a few (up to 15%) of the
progeny do survive.
Black et al. (2011)
discussed “Why RIDL is not SIT” and pointed out that RIDL actually
“refers to a whole suite of different genes and strategies including bisex lethals, flightless females and non-
sex specific late-acting lethal systems.” RIDL requires transgenesis and is a genetic-control method.
Transgenic
Aedes aegypti
were transformed using a TE vector (
piggyBac
) and contain a red fluorescent
marker gene (DsRed2), as well as a repressible dominant lethal gene (tetracycline-repressible
