|
ISIS Report - March 20 2001
Terminator Insects – The Killing of Females
The United States Department of Agriculture (USDA) APHIS has
approved field release of GM pink bollworm this coming summer, as a
prelude to developing female-killing traits to control bollworm pests.
Dr. Mae-Wan Ho and Prof. Joe Cummins explain the genetics
and hazards of female killing systems.
The female-killing systems developed out of the sterile insect technique
(SIT), which has been used in biological pest control since the 1950s. It
involves mass rearing and release of insects made sterile with
X-irradiation and other methods. Initially, sterile insects of both sexes
were released, but sterile females were thought to be detrimental to pest
control. One main reason is that a single male can mate with a large
number of females, while each female will mate with only a few males.
Hence, genetic sorting mechanisms (GSMs) were invented to kill the
females. Until quite recently, all GSMs have involved radiation-induced
X-Y translocation, ie moving of part of a normal X chromosome to the Y
chromosome. The resultant Y chromosome then acts as a dominant selectable
marker in a population in which all the X chromosomes carry a gene that is
‘conditional lethal’ in double dose. A conditional lethal is a
gene that kills only under certain ‘non-permissive’ conditions,
as for example, exposure to heat. Thus, when the population is heat
shocked, the males (XY) survive, while the females (XX) die.
Recently, other conditional lethal systems have been considered for kill
off females. Such systems could be introduced into any insect pest species
with the help of genetic engineering. As a result, the GM insects could be
released directly without pre-sterilisation. One method involves creating
a strain that carries a conditional, sex-specific lethal gene, ie, a
lethal gene that is expressed only in one sex under ‘non-permissive’
conditions. The design is such that the non-permissive condition is one
that is normally found in nature, whereas the permissive condition (one
that permits survival) depends on certain chemicals that could be added to
the diet in the insect factory. Researchers have constructed such a system
in Drosophila.
They make use of special transcription control elements (ie promoters)
and transcription factors, proteins that bind to promoters to enhance
transcription. First, the transcription factor, tTa, a protein that
interacts with tetracycline, is placed under the control of a promoter,
Yp3, which is active in female larvae and adults, but not in males. Next,
a reporter gene lacZ, coding for b-galactosidase
is placed under another promoter, the tetracycline responsive element,
tRe. In the absence of tetracycline, tTa binds to tRe causing the reporter
gene to become expressed. In the presence of tetracycline, however, the
tetracycline binds to tTa, thereby preventing it from binding to tRe, and
the reporter gene is not expressed.
Strains of flies homozygous for the constructs, Yp3-tTa and tRe-lacZ,
respectively were crossed with each other . The resulting progeny were
raised in the presence and absence of tetracycline in the culture medium.
Adults were stained for b-galactosidase
activity. Females grown on normal diet without tetracycline stained
strongly for the enzyme, whereas females raised on tetracycline and all
males were negative.
To engineer the killing of females, a toxic gene product, Ras64B was
placed under the control of tRe; and a line with tRe-Ras64B was
constructed. (Ras1 is a gene that codes for a protein which plays a key
role in regulating transcription in the cell. Ras 64B is a defective
mutant allele of Ras1. Ras homologues are oncogenes that contribute
significantly to human cancer.) The tRe-Ras64B line was crossed with
another line in which the tTa was placed under a nonspecific
(constitutive) promoter. The progeny grown on tetracycline were viable and
fertile. On normal medium, however, no progeny survived, ie, both males
and females died. When the tRe-Ras64B line was crossed to the Yp3-tTa
line, the male progeny but not the female survived in the absence of
tetracycline.
Subsequently, the researchers constructed another line homozygous for
both Yp3-tTa and tRe-Ras64B on the same chromosome, and was maintained on
medium with tetracycline to inhibit the expression of Ras64B. When the
flies were transferred to medium without tetracycline, no female progeny
were recovered in a sample of more than 5000 males. The genetic system
also worked with gene products that are specifically toxic to females.
The males were fertile when mated to other females. This is important
for spreading the female-killing gene throughout the pest population.
However, it would also spread the gene to related species. The potential
also exists for horizontal transfer to unrelated species.
The proposal to engineer these genes into promiscuous transposon vectors
will greatly multiply the risks of horizontal transfer to unrelated
species, with potentially disastrous effects on biodiversity.
There is a compelling case for stopping these developments altogether on
the basis of hazards that can already be foreseen. In earlier reports
(2,3), we have given evidence that the transposon used in the GM pink
bollworm project is both promiscuous and unstable, and integrated vectors
are prone to secondary mobilization.
- Thomas DD, Donnelly CA, Wood RJ and Alphey LS. Insect population
control using a dominant, repressible, lethal genetic system. Science
2000: 287: 2474-6.
- "Terminator insects – a primer"
by Joe Cummins, ISIS Report, March 2001
- "Terminator insects give wings
to genome invader" by Mae-Wan Ho, ISIS Report, March 2001
| |
|
