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Gene drive is a natural or engineered genetic system that causes an allele to be inherited by more than the 50% of offspring expected under Mendel's laws. Because transmission rates in engineered systems often reach 90% or more, such an allele can spread through an interbreeding population within a few dozen generations, even when it reduces the fitness of the organisms carrying it.

In medical entomology and public health, engineered gene drives are being developed to control the mosquitoes that transmit human disease: Anopheles species, which carry malaria, and Aedes species, which carry dengue, Zika, chikungunya, and yellow fever. Because a drive spreads through natural mating, it could in principle overcome several limitations of conventional vector control such as treated bednets and indoor residual spraying: it does not depend on people using it correctly, it reaches mosquitoes that bite outdoors or breed in inaccessible places, it is unaffected by insecticide resistance, and it persists where supply chains are disrupted.

Engineered drives fall into two strategic categories:

1. Suppression drives disrupt genes needed for female fertility, survival, or sexual development, or bias offspring heavily toward males, with the aim of reducing the mosquito population below the density needed to sustain transmission, or eliminating it locally.
2. Modification drives leave the population in place but spread genes that make the mosquito unable to transmit the parasite or virus.

Natural Selfish Genetic Elements

Long before genome editing existed, biologists had identified various selfish genetic elements that manipulate reproduction to achieve more than their fair share of transmission.

• Meiotic drive. Some genes sabotage the gametes that do not carry them.
• Segregation Distorter in Drosophila melanogaster. This chromosome 2 haplotype encodes an altered form of a nuclear transport protein that disrupts the reorganization of chromatin in developing sperm carrying the alternative chromosome, so that males transmit the distorter to the great majority of their offspring.
• The _t_-haplotype in house mice. A set of inversions on chromosome 17 encodes factors that impair the function of all the male's developing sperm, together with a linked element that restores function specifically in sperm carrying the haplotype.
• Sex-chromosome drive. Natural drivers on the X chromosome occur in Drosophila simulans, and drivers on the Y chromosome occur in Aedes aegypti and Culex pipiens, where damage inflicted on the other sex chromosome during sperm formation can bias offspring strongly toward males unless suppressing alleles evolve.
• Transposable elements. The spread of a selfish element was observed directly during the twentieth century, when the P element transferred from one Drosophila species into D. melanogaster and swept through wild populations worldwide over a few decades, despite causing sterility in some crosses.
• Toxin–antidote systems. Medea elements, found in flour beetles, consist of a maternally supplied toxin deposited in all of a heterozygous mother's eggs, together with an antidote expressed only in embryos that inherit the element. Offspring that do not inherit it die, raising the element's frequency among survivors.
• Wolbachia and cytoplasmic incompatibility. These bacteria live inside the cells of many arthropod species, perhaps a substantial fraction of them, and are passed from mother to offspring in the egg. Genes carried on a prophage modify the sperm of infected males so that embryos die unless the egg is also infected, in which case a maternally supplied product reverses the modification. Infected females therefore out-reproduce uninfected ones.
• Homing endonuclease genes. Found in the organelle genomes of yeast and other organisms, these encode enzymes that cut a long, specific DNA sequence. In an organism with one chromosome carrying the gene and one without, the enzyme cuts the chromosome that lacks it, and the cell repairs the break by copying the sequence from the intact chromosome, inserting the gene in the process. This copying is called homing.

Historical Evolution of Vector Control Genetics

The idea of using genetics to control insect vectors developed through four stages.

1. Chromosomal rearrangements (1940s to 1960s). In 1940, the Soviet geneticist Alexander Serebrovsky proposed releasing insects carrying rearranged chromosomes, which produce partly sterile offspring when crossed with wild insects. In 1944, Frederick Vanderplank carried out early field releases based on hybrid sterility against tsetse flies, and in 1967 Hannes Laven eliminated a Culex pipiens population using _Wolbachia_-induced incompatibility.
2. Driving a refractory gene (1968). Christopher Curtis showed mathematically that if a gene preventing parasite development were linked to such a rearrangement, releasing enough modified insects would push the refractory gene to fixation. The approach required a large release, since below a threshold frequency the rearrangement is eliminated instead.
3. Transposons (1990s). Inspired by the P element, Margaret Kidwell, José Ribeiro, and Anthony James proposed linking antiparasite genes to active transposons. In practice, transposons moved too rarely in mosquitoes, lost their cargo, and were silenced by the mosquito's own defences against mobile DNA.
4. Homing endonucleases (2003). Austin Burt proposed that homing endonuclease genes avoided these problems. If such a gene were engineered to cut an essential mosquito gene and were inserted into the site it cut, carriers of a single copy would be healthy while converting their germ cells to carry two copies. Burt's models indicated that a release of well under 1% of the population could spread and collapse an Anopheles population within a few dozen generations.

Transition to CRISPR-Cas9 Homing Drives

Between 2008 and 2014, researchers led by Andrea Crisanti, Nikolai Windbichler, Austin Burt, and Tony Nolan tested Burt's idea in Anopheles gambiae:

• In 2011, Windbichler and colleagues built the first working homing drive in a mosquito, using the yeast enzyme I-SceI to copy itself into an artificial target site.
• In 2014, Roberto Galizi and colleagues used a modified version of the enzyme I-PpoI, which cuts a sequence in the repeated ribosomal DNA. Because these repeats lie on the X chromosome in An. gambiae, expressing the enzyme during sperm formation destroyed X-bearing sperm, producing over 95% male offspring and collapsing caged populations.

Redesigning such enzymes to recognize new sequences required difficult protein engineering, and the alternatives available at the time, zinc finger nucleases and TALENs, were unstable during the copying process.

The development of the CRISPR-Cas9 system between 2012 and 2013 changed this, because targeting a new sequence requires only changing a short stretch of a guide RNA. In 2014, Kevin Esvelt, Andrea Smidler, Flaminia Catteruccia, and George Church set out how CRISPR could be used to build gene drives. Within a year, Valentino Gantz and Ethan Bier demonstrated one in Drosophila. This was followed by the first modification drive in Anopheles stephensi, by Gantz, Anthony James, and colleagues, which carried antibody genes against the malaria parasite and was transmitted to about 99.5% of offspring, and by the first suppression drive in Anopheles gambiae, by Andrew Hammond, Crisanti, Nolan, and colleagues, targeting female fertility genes.

Invasion Threshold and Recessiveness Conditions

Consider a large, randomly mating population with two alleles at one locus: wild type (W), at frequency p, and the drive (D), at frequency q. Let the relative fitnesses be:

where s is the fitness cost to individuals carrying two copies, and h describes how much of that cost is felt by individuals carrying one copy. For a suppression drive that makes females sterile, s = 1. If the cost is fully recessive, h = 0.

Without homing, a heterozygote passes D to half its gametes, and any fitness cost causes the allele to be eliminated. In a homing drive, the enzyme cuts the wild-type chromosome in the germ line and a fraction c of those chromosomes are converted to D, so a heterozygote transmits D to (1 + c)/2 of its gametes. The frequency in the next generation is:

where w̄ = p² + 2_pq_(1 − hs) + q²(1 − s) is the mean fitness.

To find whether the drive can spread from a very small release, consider the limit as q approaches zero:

The drive spreads if this exceeds 1, that is, if:

Two consequences follow:

1. Threshold independence. When this condition holds, the drive spreads from any starting frequency, however small. No minimum release size is needed.
2. The importance of recessiveness. For a suppression drive with s = 1 and a fully recessive cost (h = 0), the right-hand side is zero and any homing at all suffices. But if Cas9 is also expressed outside the germ line, or if the mother deposits it into her eggs, heterozygous females may suffer a cost even with one working copy. The condition then fails once h exceeds _c_/(1 + c), which for perfect homing means h above 0.5.

Equilibrium Genetic Load and Population Extinction

A suppression drive that sterilizes homozygous females cannot reach a frequency of 1, since those females leave no offspring. Instead it settles at an equilibrium where homing balances the loss of sterile homozygotes. With a fully recessive cost, this equilibrium is _q_* = _c_/(1 + c).

The reduction in the population's reproductive output at equilibrium is called the genetic load, L. Its value depends on the mating system assumed and on whether homing occurs in one sex or both; published models give expressions ranging from roughly c² to 2_c_/(1 + c), and the two are not interchangeable.

Whether this load causes extinction depends on how strongly the population compensates when its numbers fall, since fewer larvae compete for the same resources. Let R_m be the number of surviving adult daughters an average female produces when the population is sparse; field estimates for An. gambiae are in the region of 6 to 12. Under a commonly used model of density dependence, extinction requires:

So for R_m = 10, the drive must reduce reproductive output by more than 90%. A smaller load suppresses the population to a lower level without eliminating it. The threshold depends on the form of density dependence assumed and should be treated as indicative.

Classification of Drive Architectures

Drive systems differ along two axes: whether they persist indefinitely or fade away, and whether they can spread from any frequency or require a substantial release.

Classes of gene drive

Homology-Directed Copying versus End-Joining Repair

A homing drive is a cassette inserted at a chosen point in the genome, containing:

1. A promoter active specifically in the germ line, such as zpg or nanos, driving the Cas9 gene.
2. A second promoter driving one or more guide RNAs, whose target sequence spans the insertion site. Because the cassette interrupts that sequence on its own chromosome, the drive does not cut itself.
3. In modification drives, the cargo genes, often a fluorescent marker for tracking, and flanking stretches of DNA matching the sequences either side of the cut site.

In a heterozygote's germ line, the Cas9–guide complex finds the matching sequence on the wild-type chromosome, checks for a short adjacent motif, and cuts both strands. What happens next depends on which repair pathway acts:

• Copying from the intact chromosome. Active mainly when the chromosomes are close together during cell division, this pathway uses the drive chromosome as a template and copies the entire cassette into the cut site, converting the cell to carry two copies. This is successful homing.
• Direct rejoining. Active throughout the cell cycle and dominant in the early embryo, these pathways ligate the broken ends together without reference to the other chromosome. Because they frequently add or remove a few bases, the guide RNA's target sequence is often destroyed, producing an allele the drive can never cut again.

Evolution of Functional Resistance and Mitigation Strategies

Rejoining produces two kinds of allele, with very different consequences for a suppression drive targeting a fertility gene:

1. Non-functional resistant alleles. Most small insertions and deletions shift the reading frame and destroy the gene. Such an allele makes females sterile just as the drive does, so it confers no advantage and is eliminated by selection. It does, however, slow the drive, particularly when Cas9 deposited by the mother cuts the paternal chromosome in the early embryo, before the chromosomes can pair and copying can occur.
2. Functional resistant alleles. A minority of changes add or remove whole codons without disrupting the protein, or exist already as natural variation in wild populations. Such an allele keeps the gene working while being immune to cutting.

These are the critical problem. As the drive spreads and sterilizes females, any functional resistant allele is strongly favoured. In the first An. gambiae fertility drives, reported by Hammond, Crisanti, and colleagues in 2017, the drive initially rose to high frequency in cages but was then overtaken by such alleles, and the populations recovered.

Four design principles have since been used to prevent functional resistance.

• Target a sequence that cannot tolerate change. If the cut site lies in a sequence where no alteration preserves function, every rejoining product is non-functional.

In 2018, Kyrou, Hammond, Nolan, Crisanti, and colleagues applied this to the sex-determination gene doublesex in An. gambiae. The gene is spliced differently in the two sexes, and one exon is included only in females. Placing the cut site across the boundary where that exon is joined means that any change there prevents the female form from being made. The sequence is identical in every wild mosquito examined in continent-wide genome surveys. Heterozygotes of both sexes are fertile and transmit the drive to about 99% of offspring, while females carrying two copies develop as sterile intersexes that cannot bite. In caged populations, including large cages designed to reproduce natural conditions, the drive spread to fixation and eliminated the population within about ten generations, with no functional resistance detected in those experiments. Resistance has since been observed in some other doublesex drive experiments, so the approach reduces rather than abolishes the risk.

• Confine Cas9 to the germ line. Early drives used a promoter that also loaded Cas9 into the egg, causing cutting in the embryo. Switching to the more tightly restricted zpg or nanos promoters raised homing rates and greatly reduced the production of resistant alleles in embryos.
• Use several guide RNAs. Targeting several sites in the same gene means that an allele can escape only if every site acquires a rare function-preserving change, without the intervening DNA being deleted.
• Carry a recoded copy of an essential gene. For modification drives, the cassette is inserted into a gene the mosquito needs, and carries a rewritten version of that gene which the enzyme cannot cut. Mosquitoes inheriting the drive are therefore unharmed, while chromosomes repaired by rejoining lose the gene and are removed by selection. This keeps the cargo and the drive permanently linked.

Anopheles Malaria Vector Control

Because Anopheles mosquitoes transmit the parasites responsible for around 250 million malaria cases and 600,000 deaths a year, they are the main focus of gene drive development, led by consortia including Target Malaria and the University of California Malaria Initiative.

Gene drives developed in Anopheles

Aedes Arbovirus Vectors and Wolbachia Deployment

Aedes aegypti and Aedes albopictus transmit dengue, which infects an estimated 390 million people a year, of whom roughly a quarter develop symptoms, as well as Zika, chikungunya, and yellow fever. They pose different problems from Anopheles:

• The Ae. aegypti genome is roughly five times larger than that of An. gambiae and largely composed of repetitive sequence, and copying rates in the germ line are lower.
• Aedes eggs survive drying for months stuck to the walls of containers, creating a reserve that buffers the population against suppression.

The most widely deployed system is the Wolbachia bacterium, developed by Scott O'Neill and the World Mosquito Program. Aedes aegypti does not naturally carry it, but mosquitoes can be infected with strains taken from other insects. Two effects follow:

1. Spread through incompatibility. Infected females have an advantage over uninfected ones once the local infection frequency exceeds roughly 20 to 35%. Repeated releases push it past this point, after which the infection rises to near-universal levels and persists; in Cairns, Australia, it has remained established for over a decade without further releases.
2. Blocking of viruses. Inside mosquito cells, the bacterium competes with viruses for lipids, alters the membranes viruses use to replicate, and raises the activity of the mosquito's immune pathways, substantially reducing the mosquito's ability to transmit dengue and related viruses.

In a cluster-randomized trial in Yogyakarta, Indonesia, reported by Utarini, Indriani, Simmons, and colleagues in 2021, releasing infected mosquitoes reduced virologically confirmed dengue by 77% and dengue hospitalizations by 86% in treated areas. Deployments have followed in Brazil, Colombia, Australia, Malaysia, Vietnam, and Singapore. A related approach releases only infected males, which sterilize wild females without establishing the bacterium in the population.

Split Drives, Self-Limiting Releases, and Other Vectors

Because high temperatures can reduce the density of some Wolbachia strains in larval containers, work led by Omar Akbari, Anthony James, Zachary Adelman, and Alexander Franz has pursued engineered alternatives:

• Split drives, in which the Cas9 source is inherited normally at a separate location while the homing element carries the cargo. These reach inheritance rates of 85 to 97% while remaining self-limiting, since the Cas9 element is diluted out over time.
• Cleave-and-rescue systems, which suit Aedes because they rely on destroying a gene rather than on copying, and so are not limited by low germline copying rates.
• Antiviral cargoes, of two kinds. The first is an array of small RNA hairpins, expressed in the gut after a blood meal, that direct the mosquito's own RNA interference machinery against conserved parts of the viral genome; in challenge experiments these prevented transmission of Zika and dengue entirely. The second is an antibody fragment derived from a human antibody that recognizes all four dengue types, which likewise blocked transmission in laboratory tests.

Some genetic methods do not spread at all and act more like an insecticide requiring repeated application.

• Female-killing releases. Males carry a gene that is switched off in the factory by adding tetracycline to the water but is active in the field. When released males mate with wild females, the female offspring die while male offspring survive and pass the gene on for a few generations before it disappears.
• Precision-guided sterile insect technique. Crossing a Cas9 line with a guide RNA line targeting genes required for female development and for sperm function produces offspring that are all sterile males. The eggs can be shipped dry and hatched at the release site, avoiding both mechanical sorting of the sexes and the damage caused by sterilizing insects with radiation.

• Culex mosquitoes. Split homing drives have been developed in Culex quinquefasciatus, which transmits West Nile virus and lymphatic filariasis. A further aim is to protect native Hawaiian birds, which are threatened by avian malaria spread by this mosquito as warming allows it to reach higher elevations.
• Freshwater snails. Because Schistosoma mansoni and related parasites, which infect many millions of people, must develop inside freshwater snails, work is under way to engineer snails that cannot support the parasite, as an alternative to chemical treatment of water bodies.

Basic Reproduction Number (R₀) and Spatial Chasing Dynamics

In the standard model of vector-borne transmission, the reproduction number is:

In the most intense African malaria settings, where R₀ is very high, conventional measures face diminishing returns: insecticide resistance reduces the mortality they impose, and mosquitoes that bite outdoors or early in the evening escape them. Gene drives act on the same quantities but independently of where and when mosquitoes bite and of their insecticide resistance:

1. Suppression drives reduce the number of mosquitoes per person, in principle to zero across the whole landscape.
2. Modification drives reduce the efficiency of transmission in both directions while leaving the mosquito's ecological role intact. The two approaches can be combined.

Simple models that treat a population as well mixed predict that a sufficiently strong suppression drive eliminates it smoothly. Real populations are spread across landscapes and contract into refuges in dry seasons. Spatial models, developed by Austin Burt, Ace North, Jackson Champer, Philipp Messer, and others, reveal a more complex behaviour known as chasing:

• Released at one point, the drive spreads outward as a travelling wave whose speed depends on how far mosquitoes disperse and how fast the drive spreads.
• Behind the wave, the population is eliminated, leaving empty habitat.
• Wild-type mosquitoes from beyond the wave front can disperse back into this empty area. Finding neither competition nor drive-carrying mates, they multiply rapidly until drive-carrying mosquitoes arrive and suppress them again, producing repeating cycles of local elimination and recolonization.

Whether chasing prevents permanent elimination depends on three things: whether very sparse recolonizing populations fail to find mates and die out; whether dry seasons compress the population into isolated refuges; and whether the drive is released simultaneously at many points rather than one.

Climate Resilience of Drive Systems

Gene drives have been argued to offer some resilience to climate change:

• As warming extends the range of malaria vectors upslope and toward the poles, a drive carried within the mosquitoes themselves moves with them.
• Floods and cyclones destroy roads, wash away larvicides, and interrupt the distribution of nets, but a drive encoded in the mosquito genome persists regardless.
• Temperature affects the machinery itself. Cas9 works across the range of temperatures mosquitoes experience, while some Wolbachia strains lose density in larval water above roughly 35 °C, which has prompted the use of more heat-tolerant strains in hot climates.

Molecular Reversal Systems and Biosafety Pathways

Because a self-sustaining drive could cross national borders, several methods of halting or reversing one have been developed:

1. Overwriting drives. One design carries guide RNAs targeting the junctions of an existing drive but no Cas9 of its own. Crossed with drive-carrying mosquitoes, it borrows their Cas9 to excise the original cassette and replace itself in its place. A related design targets the Cas9 gene itself, disabling it.
2. Anti-CRISPR proteins. Bacteriophages encode proteins that block Cas9 by mimicking DNA and occupying its binding sites. Work by Taxiarchis, Crisanti, Nolan, and colleagues showed that releasing mosquitoes carrying one such gene, inherited normally, halted the spread of a doublesex suppression drive in cages and prevented the population from collapsing.
3. Chemically controlled Cas9. Cas9 can be fused to domains that render it active only in the presence of a particular compound.
4. Ecological assessment. Studies of African savanna food webs suggest that An. gambiae forms a small fraction of insect biomass, that no predator appears to depend on it specifically, and that it contributes little to pollination. This is the current assessment rather than a settled conclusion, since the relevant ecological work remains limited.

The World Health Organization, the US National Academies, and the parties to the Cartagena Protocol on Biosafety have set out a staged pathway with explicit decision points before each step:

• Laboratory work in secure insectaries with negative air pressure, airlocks, and treated waste. Guidance requires at least two independent barriers: physical containment, ecological containment by working where the species cannot survive outdoors, and molecular containment using split drives or artificial target sequences absent from wild populations.
• Confined field trials, progressing from large environmental chambers that reproduce natural conditions to enclosed outdoor cages in the countries concerned, using non-driving strains to build local capacity, and then to releases on isolated islands.
• Open releases, combined with randomized trials measuring both entomological outcomes, such as mosquito density and the proportion carrying parasites, and clinical outcomes, such as malaria or dengue incidence in children.
• Long-term monitoring, integrating the approach into national programmes while sequencing wild mosquitoes and parasites to detect resistance, loss of the cargo, or evolutionary change in the pathogen, with continuing community consultation and consent.

How does a CRISPR homing gene drive convert heterozygous germ cells to homozygous?

In a heterozygous germline cell carrying the drive cassette on one chromosome and the wild-type allele on the homologous chromosome, germline-expressed Cas9 and its guide RNA introduce a targeted double-strand break in the wild-type allele. The cell repairs this break via homology-directed repair (HDR) using the intact drive-bearing chromosome as the repair template, copying the entire CRISPR drive cassette into the cleaved locus and converting the cell to homozygosity prior to gamete formation.

Why does targeting the female-specific splice junction of doublesex prevent functional resistance in Anopheles gambiae?

Non-homologous end joining (NHEJ) frequently introduces small insertions or deletions at a Cas9 cleavage site; if an in-frame indel preserves protein function while destroying the guide RNA target sequence, natural selection rapidly fixes this resistant allele. However, exon 5 of the doublesex (dsx) gene in Anopheles gambiae is strictly required for female sexual differentiation and is ultraconserved across wild populations, so virtually any NHEJ mutation across its splice junction produces a non-functional allele that is purged by selection.

How does Wolbachia cytoplasmic incompatibility spread through Aedes aegypti populations without genome editing?

Wolbachia pipientis is a maternally inherited intracellular bacterium whose prophage genes (cifA and cifB) modify the sperm chromatin of infected male mosquitoes. If an infected male mates with an uninfected female, embryonic mitosis fails and the eggs die; if the female is also infected, maternally deposited cifA rescues embryonic viability. Once local Wolbachia prevalence exceeds an invasion threshold of ≈20–35%, infected females have a reproductive advantage that drives the endosymbiont to near-fixation while blocking dengue viral replication.

• Genetic engineering
• Sterile insect technique
• Vector control
• CRISPR
• Wolbachia