Download pioneered

Genetic Strategies for Controlling
Mosquito-Borne Diseases
Engineered genes that block the transmission of malaria and dengue
can hitch a ride on selfish DNA and spread into wild populations
Fred Gould, Krisztian Magori and Yunxin Huang
M
alaria kills more than a million
people each year, primarily children under the age of six. Dengue fever
is less deadly, but an outbreak can debilitate millions of people and easily overwhelm doctors and hospitals in tropical
cities. To combat malaria and dengue,
health agencies try to get rid of mosquitoes, which transmit both diseases. But
a scarcity of resources hampers most
control programs, and the insects are
increasingly resistant to pesticides after
decades of patchwork spraying. The
disease organisms are evolving, too:
The single-celled microbes that cause
malaria are becoming resistant to widely used, inexpensive anti-malarial drugs
such as chloroquine, which have been
the first-choice treatment for malaria.
(Drug therapies for dengue virus have
never been available.) Many research
teams are trying to develop vaccines for
these diseases, but the complex biology
of the two parasites—malaria is caused
by four species of the protist Plasmodium, and four different viruses, or serotypes, can cause dengue—makes it
difficult to predict whether vaccination
will eventually confer broad immunity.
And although pesticide-treated bed
nets offer a promising low-tech means
of preventing bites from malarial mosquitoes at night, the mosquitoes that
carry dengue bite during the day.
To oppose these grim realities, several research teams (including our group
at North Carolina State University) are
now exploring a different approach
to controlling the spread of mosquitoborne diseases, one that would reduce
an insect’s ability to transmit disease or
would induce a population crash among
selected disease-carrying species. How
could either of these goals be achieved?
By creating genetic changes in wild mosquitoes. Biologists have already extinguished other insect pests with genetic
methods and in the laboratory have
blocked the transmission of dengue and
malaria in mosquitoes with engineered
fragments of DNA. If scientists could
breed some of those same genes into the
wild mosquito population, the insect’s
bite might still be a nuisance—but it
would no longer be a threat.
Fred Gould is a professor in the departments of
entomology and genetics at North Carolina State
University. For the past 25 years he has studied
how insect pests adapt to human attempts to control
them and how humans could design new ways to
stymie that adaptation. He is part of teams that
recently won funding from the National Institutes
of Health and from the Gates Foundation to examine
transgenic approaches for taming mosquitoes that
transmit dengue virus. Krisztian Magori and Yunxin Huang are postdoctoral researchers in Gould’s
laboratory. Magori received his Ph.D. in biological
physics from Eötvös Loránd University in Budapest.
Huang received his Ph.D. in applied mathematics
from the University of Utrecht. Both have been developing predictive models to guide research aimed
at alleviation of human diseases. Address for Gould:
840 Method Road, Campus Box 7634, Raleigh, NC
27695-7634. Internet: [email protected]
Transgenes and Fitness
Setting aside concerns over the release
of genetically modified mosquitoes
(more on that later), there is reason to
be optimistic that this strategy might
work. Thanks to the tools of molecular biology, geneticists can make very
specific changes in mosquito genomes.
In 2002, a team led by Marcelo JacobsLorena, then at Case Western Reserve
University, modified Anopheles stephensi
mosquitoes with genes that blocked the
development of Plasmodium berghei, a
relative of the malaria parasites that infect human beings. In March 2006, a
team led by Ken E. Olson at Colorado
State University showed that inserting
238
American Scientist, Volume 94
a specific transgene—a piece of foreign,
engineered DNA—into the genome of
Aedes aegypti mosquitoes could substantially deactivate the dengue virus
within hours of the female’s blood meal
from an infected person. Other scientists have developed genetic strategies
that simply kill the mosquitoes that
carry a specific engineered gene.
But having mosquitoes that don’t
transmit pathogens in the laboratory
doesn’t help people in the wider world.
Even if scientists bred and released
thousands of these transgenic mosquitoes, they wouldn’t have much effect on
public health unless the genetically altered insects competed with (and eventually replaced) the local strains.
If mosquitoes that carried antiparasite genes were more evolutionarily fit
(that is, if they left more offspring) than
mosquitoes without these genes, then
more and more of the wild mosquito
population would become inhospitable
to parasites with each generation. For
example, if parasite-infected mosquitoes
had shorter lives or fewer progeny, then
a parasite-killing transgene might make
its bearer more fit. Unfortunately, that
isn’t what happens: Mosquitoes susceptible to the dengue virus or Plasmodium
are almost always just as fit as those
without the susceptibility.
Not only do anti-parasite transgenes
fail to improve fitness, they often reduce it. This penalty exists because the
transgenes in many genetically modified organisms are just inserted randomly into the genome, where they
often disrupt normal genes at the insertion site. And the transgene itself encodes RNA or protein that can change
cell function, thereby decreasing the
insect’s fitness. Transgenic strains that
bore these fitness costs would go extinct if released into the wild.
Figure 1. A century ago malaria was still prevalent in the United States: In 1914 some 600,000 Americans contracted the disease. In this photograph
taken in the 1920s, workers in Virginia are digging ditches to drain standing water, the preferred breeding habitat for the major vector of malaria,
the Anopheles mosquito. The federal Communicable Disease Center declared in 1951 that malaria had been eradicated from the United States, but
malaria, dengue and other diseases spread by infected mosquitoes are still epidemic in most parts of the world. Through the direct effect of poor
health and the indirect effects of poverty, economic stagnation and social stress, these scourges account for a significant fraction of human misery.
One conceivable solution to the problem would be to alter the DNA of mosquitoes so that they could no longer transmit the disease. The authors
discuss genetic strategies for spreading anti-parasite genes and some of the risks that might attend such a program. (Photograph courtesy of the
Centers for Disease Control and Prevention.)
Despite the challenges, the process
of engineering a disease-fighting, transgenic mosquito is not merely an academic exercise. The Bill and Melinda
Gates Foundation recently contributed
more than $35 million to the Foundation for the National Institutes of Health
for the purpose of developing transgenic mosquitoes as a weapon against
insect-borne diseases, and governmental agencies and other philanthropies in
the United States and abroad have also
funded this research.
Regardless of what the anti-pathogen
transgene turns out to be—an antiviral or
antiprotist gene, a lethal gene, or something else not yet developed—the projwww.americanscientist.org
ect will succeed or fail based on the ability to drive that transgene into the wild
population—even if it makes its bearers
less fit. A practical system to meet this
need is still far away, but it is possible.
Using the rules of population genetics, a
number of research groups are harnessing so-called selfish DNA, which spreads
without regard to the overall fitness of
its host, giving the illusion of turning
natural selection on its head.
Strain Replacement
The idea of designing a gene that actively spreads through a pest population without conveying some fitness
advantage is not new. A Soviet geneti-
cist, Alexander S. Serebrovskii of Moscow University, and a British biologist,
Frederic L. Vanderplank of the Tanganyika (Tanzania) Research Department,
sowed the intellectual seeds for this
approach in the 1940s. The two men
realized independently that in certain
circumstances, competition between
two interbreeding insect strains doesn’t
favor the fitter group. This dynamic
involves the genetic property that scientists call underdominance, which can
actually cause the strain with greater
fitness to die out.
To explain underdominance, it’s
helpful first to know the terms dominant and recessive, which describe the
2006 May–June
239
���
�����������������������������
��
��
��
��
�
�
��
�������������
�
Figure 2. Frederic L. Vanderplank (top left)
and Alexander S. Serebrovskii (lower left)
pioneered concepts of genetic control of pest
species in the 1940s, but neither received recognition for his efforts. Serebrovskii’s peers
in the Soviet Union rejected his work because of the Lysenko-era policy of dismissing Darwin’s theories as unsubstantiated
bourgeois science. Vanderplank never published his findings. The concept of underdominance, critically important to the work
of both scientists, describes the condition
when a mating between strains (labeled A
and B in the bar chart above) results in progeny (AB) that are less fit than either parent.
(Vanderplank photograph courtesy of John
Vanderplank; Serebrovskii photograph reprinted from Medvedev 1969.)
inheritance of traits. Consider a case
with two purebred parents from different, interbreeding strains: Many features of their offspring will favor one
parent or the other. For example, suppose that parent A comes from a strain
that produces 100 eggs and parent B
comes from one that produces 50 eggs.
If the offspring from the mating of A
and B each generate 95 eggs, a biologist would typically say that the highegg-production trait was dominant. If,
instead, the offspring laid only 55 eggs,
then a biologist would classify the trait
as recessive. But if offspring from the A
× B mating produced fewer eggs than
either parent (here, fewer than 50),
then egg production would be considered underdominant. In most crosses
between strains, traits do not show underdominance, but sometimes a mating between distantly related strains
yields offspring that don’t survive or
reproduce as well as either parent. (In
240
American Scientist, Volume 94
other words, those offspring have a
lower evolutionary fitness.)
Still, the idea that the less-fit strain
B could outcompete the more-fit strain
A doesn’t seem to make sense according to basic Darwinian theory. But
Vanderplank, Serebrovskii and others
realized this is exactly what happens
when two conditions are met: when
the offspring of a mating are less fit
than either parent (underdominance),
and when the less-fit parental strain
is more abundant. Under these conditions, adults of the less common strain
A are more likely to find and mate with
adults from the more common strain
B, thereby producing less-fit offspring.
For example, if strain A makes up 20
percent of the insects in a certain habitat and strain B makes up 80 percent,
then (all else being equal) four out of
five individuals from strain A will encounter a mate from strain B, but only
one out of five individuals from strain
B will make a match with a strain A
mate. Plugging some numbers into
this example: If an A × A cross results
in offspring that produce 100 eggs, a B
× B cross yields offspring that produce
50 eggs, and (bringing in underdominance) an A × B cross has offspring
that produce 20 eggs, then the average
egg production by female offspring
from the A strain will be (0.80 × 20) +
(0.20 × 100) = 36, and the average for
strain B offspring would be (0.20 × 20)
+ (0.80 × 50) = 44. Even though strain A
is more fit, strain B produces offspring
with higher average fitness. Over time,
strain B would replace strain A.
Vanderplank did exactly this experiment in the late 1940s with two sexually compatible species of tsetse flies,
the insects that transmit the parasite
that causes sleeping sickness. Mating
the two species yielded offspring with
low fitness. Working in an area that had
been abandoned because of disease risk,
Vanderplank released high numbers of
one species into the habitat where the
second was more fit. Over time, the first
species outcompeted the second, sending its numbers plummeting. Within
two years or so, the introduced species (which was not well adapted to the
habitat) had largely died off, leaving the
area free of sleeping sickness and enabling local people and cattle to inhabit
the region.
Serebrovskii worked out the theory
for a type of mutation called a balanced
chromosomal translocation, in which a
piece of one chromosome breaks off and
becomes attached to another chromosome. Serebrovskii calculated that even
less-fit insects with the translocation
could replace the wild-type—the normal,
nonmutant strain with higher fitness—
because some of the grandchildren of
the cross between mutant and wild-type
parents don’t inherit a complete set of
chromosomes (a lethal condition). Christopher F. Curtis at the London School
of Hygiene & Tropical Medicine later
pointed out that if the strain with the
translocation carried a desirable gene
(such as one that conferred malaria resistance) on the translocated chromosome, then the translocation would also
sweep the desirable gene into the population. The scientists needed only to inundate the natural pest population with
such a translocation.
Several research teams conducted
lab and field-cage experiments in the
1970s to test Serebrovskii’s and Curtis’s
theories. The teams made some prog-
�����������������������
���
�
www.americanscientist.org
���
�
�
�
�
�
�
�����
�����
�
�����
�����
��������������������
�
��
��
��
����
��
�
��
�
�
�
��
���������
�������������������
�������������������
��
����
�
�
��
�
�
�
�
�����������������������
��������������������
���
�����������������������
��
�������������������
�������������������
��������������������
���
�
�
�
�����
�����
�
�
�
�
��
��
�
�
�
�
��
����
��
��
�
�
�
�
�
��
���������
�
����
�
���������
A Two-Transgene Technique
The disinterest didn’t last long. Soon after the first successful addition of foreign
DNA to the fruit fly Drosophila melanogaster, biologists began to explore the
potential for genetic manipulation of
pest species. The growing sophistication
of molecular biology has enabled them
to make genetic changes with much
greater precision than before. For example, Stephen Davis and his colleagues
at the University of New South Wales
in Australia developed a novel idea for
a two-transgene system that uses underdominance to spread new genes into
a population. They envisioned the creation of two distinct pieces of DNA, or
constructs, that were spliced into different
chromosomes. Each construct contained
an on/off switch and a gene that encodes a biological toxin. The switch was
“on” by default. Construct I also carried
a gene that turns off the toxin production in construct II, and construct II had
a gene that turns off toxin production
from construct I. Thus, individuals with
both constructs (or neither) survived.
Having just one of them was lethal.
In this model, a cross between a wildtype strain and a strain that was homozygous for both constructs (meaning
that each construct was present on both
halves of a chromosome pair) would
yield progeny heterozygous for both
constructs. (In other words, they would
carry only one copy of construct I and
one copy of construct II.) This generation would survive. But many of the
second generation would die because
they inherited only one of the two constructs (similar to the effect seen in Serebrovskii’s translocation model).
This engineered form of underdominance is superior to a translocation
because it can enable the transgenes
to spread even if the number of mutants released is less than 30 percent
of the population (the exact proportion depends on how much of a fitness
cost is associated with the transgene).
Furthermore, because the engineered
strains only differ from native strains
by two inserted genes (instead of a
full chromosome rearrangement), the
transgenics should be more fit under
�
�����������������������
�
�
���������
ress with the translocation studies, but
this line of investigation ultimately
failed: Rearranging a mosquito’s chromosomes left it unfit to survive in the
field. The products of classical genetics
proved too crude for the job, and interest in this approach waned.
��������������������
� frequenFigure
3. In a mating between two insect strains, underdominance
can change the
�����
��
cies
of the strains; under some� conditions, it can cause the less fit strain to outcompete the
�����
more fit strain, in seeming opposition to the laws of natural selection. This example shows a
�
hypothetical interaction between a strain producing on average 100 offspring (A) and another
�
yielding 50 (B). When a strain A (blue) male mates with a strain A female their offspring
(top right, bar 1) are more fit (that is, they leave more progeny) than those from a mating of a
strain B (orange) male and female (lower right, bars 1−4). However, in this example, strain A
makes up only 20 percent of the population (left). Thus, four out of five of A’s matings occur
with individuals of strain B. These hybrid matings produce progeny with low fitness (striped
bars). In contrast, even though strain B is itself less fit than strain A, its high abundance in the
population results in rare mating encounters (only one out of five) with strain A, so few of its
offspring are low-fitness hybrids. On average, strain A offspring in this situation leave 36 of
their own progeny compared to an average of 44 from strain B. Thus, after one cycle of mating
the frequency of strain B increases.
field conditions. And as Curtis noted
in the similar case of translocations,
an anti-pathogen gene included in the
constructs would also spread through
the population. Having the anti-pathogen gene on both constructs provides
a backup in the event that a random
mutation disables one copy.
Theoretically, the chance of success would be even higher—and the
number of engineered insects needed
even lower—if the mutant strain were
homozygous for two independent insertions of construct I and two independent insertions of construct II. Our
research team has modeled the effects
of different fitness costs and different
numbers of transgenes and found that
multiple insertions of a construct can be
more efficient than a single insertion as
long as the cost per insertion is below
10 percent. These models can predict
2006 May–June
241
Figure 4. These transgenic mosquitoes (green)
are inefficient malaria vectors. At top, a wildtype larva (middle) is flanked by transgenics
shown in dorsal (top) and ventral (bottom)
views. The adult on the right is also transgenic. (Photographs from Ito et al. 2002, courtesy
of Nature Publishing Group.)
how many lab-bred insects would be
needed to eventually saturate a wild
population, a number called the critical
release size. It’s about 15 percent when
there are no fitness costs associated
with multiple insertions—a tall challenge for molecular biologists. Even releasing enough engineered mosquitoes
to make up 15 percent of a local population is a daunting task. When entomologists first used genetics to control pest
insects in the late 1950s, they reared the
insects in giant factories that could produce millions of bugs per week. These
were costly operations. A transgenic
mosquito program might be able to use
fewer insects by releasing them during
a seasonal dip in the population or after
spraying pesticide to shrink the mosquito population temporarily.
The Uses of Selfish DNA
Although the strategy of engineered
underdominance might work in some
cases, the true Holy Grail of genetic insect control would be a transgene that
spreads through a population from
only a few individuals. The first indication of the possibility of reaching this
goal came from a transposon, or “jumping gene,” a type of selfish DNA. The
transposon encodes a protein that cuts
the transposon DNA free of its place
in a chromosome and then reinserts it
in another random part of the genome.
The cell’s own DNA-repair machinery
usually mends the hole at the original
position by recreating the transposon
sequence. At the end of the process, the
host cell has two copies of the transposon instead of one. If this genehopping occurs in a cell that makes
eggs or sperm, then the transposon
stacks the odds that it will be passed
on to future offspring. If the transposon
doubles each generation (assuming for
now that it doesn’t harm the organism),
then it will become increasingly com-
������������������������������������
�����������
������
�������
����������������������������������
���������
���������
������
�������
����
��������
�������
����
���������
�����������
�������
����
���������
���������
����������
Figure 5. The random insertion of transgenes can have different effects on evolutionary fitness (as measured by the number of offspring) depending on the genomic insertion site. A normal gene appears at the top, with regions of DNA that encode protein (purple bars) separated by noncoding
DNA (thin lines). Insertions into the noncoding portions usually cause less disruption of gene function than insertions into the coding regions.
The four lower rows show hypothetical examples of various insertions in the same gene (orange dot) on the same part of the insect’s three chromosomes (upright gray bars). In addition to the disruption caused by random insertion, transgenes can also decrease fitness by coding for novel
proteins or altering the timing or abundance of other native proteins, thereby compromising cellular and physiological functions.
242
American Scientist, Volume 94
mon until all individuals in the population carry one or more copies.
This basic scenario occurred “naturally” in Drosophila melanogaster populations throughout most of the world
in the past 60 years. Today, almost any
fruit fly collected from an overripe banana, whether in New York City or Nairobi, contains a transposon called the Pelement. Yet the descendants of fruit flies
isolated in laboratories before 1950 lack
this transposon. Scientists do not know
the origin of the P-element but do know
that it spreads rapidly when introduced
into a naive laboratory population.
Several entomologists have noted
that “loading” an anti-pathogen gene
into a mosquito transposon could
eventually confer disease resistance to
an entire mosquito population as the
transposon jumped from site to site
through successive generations—even
if the mosquitoes that bore these genes
had lower fitness. However, this is not
a sure-fire strategy (use of the word
“could” instead of “would” in the previous sentence was deliberate). In one
experiment with D. melanogaster in the
laboratory of Margaret G. Kidwell at the
University of Arizona, the transposon
spread as predicted through the naive
flies, but the loaded marker gene was
lost. The investigators concluded that
at some point during the experiment,
one or more copies of the transposon
must have “unloaded” the added gene.
The cargo-free transposons seemed to
replicate faster than their loaded counterparts and eventually displaced them.
Clearly, scientists will need to stabilize
the genetic composition of any transposon used for practical goals.
In addition to transposons, molecular biologists are studying several
other types of selfish DNA that could
spread a desired gene into a mosquito
population. One Gates-funded project
led by Austin Burt at Imperial College
London focuses on a curious DNA
sequence called a homing endonuclease
gene (HEG). The HEG has the unique
ability to copy itself from one chromosome to the identical site on the other
chromosome in the pair. It accomplishes this feat by encoding a protein
that recognizes the DNA sequences on
either side of the HEG. When the protein, called a homing endonuclease,
spots the same DNA pattern on the
twin, or homologous, chromosome, it
snips that sequence in two. The cell
mends this double-strand break by
using the HEG-containing chromowww.americanscientist.org
���������
������
�������
������������
������������
Figure 6. A mosquito strain with a balanced chromosomal translocation would affect the first
generation’s progeny—the grandchildren of the original genetically altered insect, or F2 generation. The top row depicts the initial mating between the wild-type mosquito and one with a
translocation. In the latter, a piece of chromosome 2 (blue) and a piece of chromosome 3 (green) are
interchanged. The middle row shows that the F1 progeny of a cross between these two mosquitoes
have an equal amount of each chromosomal segment. The bottom panel shows that in the F2 generation, because of random shuffling of chromosomes during the formation of sperm and eggs,
some individuals lack the normal number of chromosomal segments, resulting in their death.
some as a template for patching the
cut DNA. This repair incorporates the
entire HEG sequence, and the cell becomes homozygous for the HEG. If the
process of DNA cutting and repair happens in a cell that later forms sperm or
eggs, then it’s possible that nearly 100
percent of the offspring could inherit
the HEG. And if the homing endonuclease genes are neither beneficial nor
detrimental to the fitness of the organism, then they will eventually spread
to the entire population even if they
start at a very low frequency. HEGs
exist naturally in fungi, plants, bacteria
and bacteriophages, but not insects. A
major challenge for Burt and his colleagues is to design an HEG that will
function in an animal, a goal shared by
biomedical scientists looking for a new
tool for gene therapy.
Wolbachia
All these types of selfish DNA involve
genes on chromosomes in the nuclei
of each cell in a genetically altered
pest. An alternative approach uses a
selfish genetic element carried in the
cytoplasm of the cell. The tool that allows this unorthodox inheritance is
2006 May–June
243
����������������������
�������
���������
���������
�������
�����
������
�������
�
a
��������
�����
����������
�������������
����
�������������������������������
����
�
������������
b
�������������������������������
����
�
c
������������������������������
������������
�����
�
d
e
Figure 7. One of the strategies for driving anti-pathogen genes into a population uses a pair of transgenes (a). Both carry a gene that encodes a
lethal toxin (red), and each transgene also codes for a unique repressor that acts on the other (purple and green). The desired anti-pathogen gene
(orange) has it’s own promoter that isn’t affected by these repressors. If a mosquito inherits either transgene singly, then the toxin gene remains
active and kills the individual (b and c). But when both transgenes are present, the reciprocal block of toxin production allows the insect to live (d).
The pattern of inheritance and mortality with this arrangement (e) resembles that of Alexander S. Serebrovskii’s chromosomal translocation.
a kind of intracellular bacteria called
Wolbachia, which is passed through the
female line only (similar to mitochondria) and can manipulate the reproductive success of its insect hosts. One
type of Wolbachia causes cytoplasmic
incompatibility, in which the progeny of
Wolbachia-infected males and uninfected females are nonviable. However,
infected males breed normally with infected females. This situation provides
a reproductive advantage to infected
females, which can mate successfully with either infected or uninfected
244
American Scientist, Volume 94
males. And because infected females
pass on the Wolbachia infection to male
and female offspring (regardless of the
infection status of their mate), the frequency of Wolbachia in the population
increases over time.
A group led by Stephen L. Dobson at
the University of Kentucky transferred
this type of Wolbachia into Aedes aegypti
mosquitoes and found that the parasite spread from 20 percent to 100 percent of a laboratory population within
eight generations. A strain of Wolbachia
carrying an anti-pathogen gene is tan-
talizing to consider. But as with the
other approaches, several challenges
must be met before this system could
be deployed. Most fundamentally, molecular biologists need to learn how
to genetically manipulate Wolbachia, a
process that is more difficult because
the bacteria live within the cells of another organism.
Simple Eradication
These strategies are designed to spread
an anti-parasite gene that would interrupt disease transmission but leave
the mosquito population otherwise intact. Although entomologists view this
approach as the best, most efficient
means of genetic control, the earliest
implementation is at least 10 years
away. An alternative strategy is to use
existing genetic tools to temporarily
eradicate local mosquito populations.
The U.S. Department of Agriculture
had a similar objective in 1958 when it
began eradicating the wound-infesting
screwworm fly in Florida by releasing
millions of radiation-sterilized adult
males. The project achieved ratios of
up to 100:1 sterile to normal males,
so most native females mated with
irradiated males to produce embryos
that quickly died. Unlike the tactics
above, the sterile males never passed
any genes into the natural population, so factories had to produce more
males for irradiation in each generation. Although it was labor-intensive,
the strategy worked: Over the past
40 years, entomologists have pushed
the flies to extinction from the United
States to the Panama Canal.
One of the problems with sterilization is that radiation often weakens
the flies to the point that they are not
very competent mating partners. Fur-
thermore, the strategy is typically only
practical with pest species that allow
entomologists to separate easily males
from females.
Molecular geneticists have overcome
these obstacles by engineering an insect
that lived and mated normally, but produced progeny that could only survive
on a special laboratory diet that contained the antibiotic tetracycline. The
investigators use the tetracycline not
to help the insect fend off bacteria, but
to control the on/off switch on a lethal
transgene carried by the insect.
In 1998, Walter J. Gehring and colleagues at the Universität Basel in
Switzerland created the first Drosophila
strains whose progeny survived only
if their mothers were fed on a diet
containing tetracycline. In 2005, a research team at the University of Oxford led by Luke S. Alphey reported
on the first success in applying the
so-called Tet-Off technique to a pest
species, the Mediterranean fruit fly. If
entomologists could rear such a transgenic pest strain in factories on a tetracycline-laced diet, they could release
the mutants to mate with native pests
and produce young that would not
live without the drug. This strategy, in
which both the males and females die
when deprived of tetracycline, has the
potential to be much more efficient
than the old irradiation method.
A twist on this offspring-killing strategy was introduced in 2000 in a pair
of papers from two independent laboratories, one headed by Alphey and
the other by Maxwell J. Scott at Massey
University in New Zealand. Both teams
developed transgenic Drosophila in
which only the female was dependent
on tetracycline—a feature that would
allow scientists to rear high numbers
in a factory and then remove tetracycline from the diet in the last generation, leaving only adult males ready for
release. A further advantage to this
approach is that all female offspring
of these males would die in the field,
but the male offspring would survive
and transmit the female-killing genes
to some of their offspring, which would
repeat the pattern.
Paul Schliekelman, a former graduate student in our laboratory who is
now at the University of Georgia, developed models to examine how specific strains of mosquitoes with female
killing genes are likely to affect the
native populations. His work showed
Reuters/Corbis
Figure 8. Society faces a difficult decision in deploying genetics in the fight
against mosquito-carried diseases: Is the fear of unintended consequences from
genetically modified organisms greater than the hope that a genetically altered
mosquito could alleviate the suffering of millions of people? Presumably, the
public would be concerned about transgenic mosquitoes for one of the same
reasons that they have protested transgenic crops: A worry that engineered genes
will spread to other species. Ironically, in the case of genetically altered mosquitoes, scientists and public-health experts fret about the possibility that the antipathogen genes might not spread widely enough among the mosquito population. Above, protesters march at a meeting of the World Trade Organization in
Montreal, Canada in 2003; at left, a mother comforts her child who is suffering
from malaria. (Photograph at left courtesy of the World Health Organization.)
www.americanscientist.org
2006 May–June
245
that a strain containing multiple copies could be 10 to 100 times more efficient at reducing the native population than a strain that killed males
and females. As with the other genetic
control methods discussed here, minimizing the impact of the inserted constructs on mosquito fitness is critical
to success. When there are no fitness
costs, more insertions result in higher efficiency because more progeny
will carry at least one female-killing
gene. However, as the fitness cost increases, the optimal number of insertions goes down. One nonintuitive
prediction from modeling fitness costs
was that the first transgenic strains
released should have few copies of the
engineered construct, but strains with
more constructs should be released as
the population declines. A Gates-funded research team led by Anthony A.
James at the University of California,
Irvine, is examining the possibility of
using the female-killing approach for
control of Aedes aegypti, the mosquito
vector of dengue. But James and other
investigators realize that in addition to
solving the challenging technical problems involved in developing these and
other types of engineered mosquitoes,
they must address a host of social and
ethical concerns about release of such
transgenic organisms.
Social Context and Risk
When we mention genetically engineered mosquitoes in conversations
with our friends and colleagues, a sizeable fraction of them seem to cringe.
Just the idea of such an insect scares
them. Given the public discomfort with
genetically engineered crops (which
cannot perpetuate themselves), we expect to meet significant anxiety when
people realize that our goal is to build a
mosquito that outcompetes the natives.
The Pew Charitable Fund and other
groups have begun to examine the social, ecological and public-health issues that would accompany the release
of an engineered mosquito. Scientists
and health agencies need to educate
the public about the true biological
properties of genetically modified organisms. Research and regulation in
this area will need to be wide open
to public scrutiny. The prospective
release of these mosquitoes in developing countries presents unique challenges as well. We hope that people
will be more likely to accept this type
of genetic engineering because of the
246
American Scientist, Volume 94
public benefit and the fact that nonprofit groups are in charge, but to earn
this endorsement, scientists need to
talk about the real risk involved. The
nature of that risk depends in part on
the type of genetic-drive system and
the disease target. For example, an engineered transposon would be unlikely
to stop at national borders, so a release
in one country would eventually spill
into bordering countries and beyond.
Alternatively, if the anti-pathogen gene
were driven by an underdominance
construct, then the spread would probably be much more local. (Transgenic
mosquitoes are unlikely to reach high
enough numbers in new areas to replace the local strain.)
Too much success could also cause
trouble in the future. For example, if all
the mosquitoes in a certain region were
dengue-free, then a growing fraction of
the population would never have been
exposed to the virus. If dengue then
evolved so that it could hitch a ride
even on a transgene-carrying mosquito,
then the human population could be
vulnerable to an epidemic.
Fortunately, the increase in pesticide resistance (which would have a
similar effect) has not caused such rebound epidemics, but epidemiologists
do not have enough information to
dismiss the possibility of future breakouts. The release of engineered mosquitoes would have to include careful
monitoring and contingency plans—
insecticides, different transgenic strains
or vaccines (if available)—for dealing
with the risk of newly evolved strains
of malaria or dengue. Geneticists are
already looking for ways to engineer
mosquitoes that have multiple means
of blocking the pathogen. (Physicians
prescribe a “cocktail” of drugs to combat the AIDS virus for the same reason:
A virus that mutates to overcome one
drug still gets knocked down by others, preventing the spread of the drugresistance mutation.) Given the evolutionary plasticity of microbes there is
no room for complacency.
The uncertainty, effort and expense
have led some scientists on the front
lines in the fight against mosquito-borne
diseases to oppose this line of high-tech
research. Current disease-control programs are severely underfunded, and
they worry that the excitement over genetic engineering will pull more money
away from proven technologies such as
bed nets and pesticides. This critique
is valid, and funding agencies need to
preserve a balanced approach. Hightech projects should not grow at the
expense of these other initiatives.
You can hear both optimism and
frustration at meetings where entomologists get together to talk over genetic
control strategies. Scientists in this field
have made great progress in the past
10 years, but major technical and social
hurdles remain. In the end there will
be poetic justice if biologists are able to
use selfish DNA to serve the altruistic
goal of improving world health.
Bibliography
Bello, B., D. Resendez-Perez and W. J. Gehring.
1998. Spatial and temporal targeting of gene
expression in Drosophila by means of a tetracycline-dependent transactivator system.
Development 125:2193–2202.
Curtis, C. F. 2000. Infectious disease: The case
for deemphasizing genomics in malaria control. Science 290:1508.
Franz, A. W. E., I. Sanchez-Vargas, Z. N. Adelman, C. D. Blair, B. J. Beaty, A. A. James and
K. E. Olson. 2006. Engineering RNA interference-based resistance to dengue virus type
2 in genetically-modified Aedes aegypti. Proceedings of the National Academy of Sciences of
the U.S.A. 103:4198–4203.
Gong, P., M. J. Epton, G. L. Fu, S. Scaife, A.
Hiscox, K. C. Condon, G. C. Condon, N.
I. Morrison, D. W. Kelly, T. Dafa’alla, P. G.
Coleman and L. Alphey. 2005. A dominant
lethal genetic system for autocidal control of
the Mediterranean fruitfly. Nature Biotechnology 23:453–456.
Gould, F., and P. Schliekelman. 2004. Population genetics of autocidal control and strain
replacement. Annual Review of Entomology
49:193–217.
Ito, J., A. Ghosh, L. A. Moreira, E. A. Wimmer and M. Jacobs-Lorena. 2002. Transgenic anopheline mosquitoes impaired in
transmission of a malaria parasite. Nature
417:452–455.
James, A. A. 2005. Gene drive systems in mosquitoes: Rules of the road. Trends in Parasitology 21:64–67.
Magori, K., and F. Gould. 2006. Genetically
engineered underdominance for manipulation of pest populations: A determinisitic
model. Genetics. (Published online ahead
of print January 16. doi:10.1534/genetics.105.051789.)
Medvedev, Z. A. 1969. The Rise and Fall of T.
D. Lysenko. translated by I. M. Lerner. New
York: Columbia University Press.
Scott, T. W., W. Takken, B. G. J. Knols and C.
Boete. 2002. The ecology of genetically modified mosquitoes. Science 298:117-119.
For relevant Web links, consult this
issue of American Scientist Online:
http://www.americanscientist.org/
IssueTOC/issue/841