The Trojan Female Technique: a new approach to pest control
Spermatozoa and egg
Pest species pose serious threats to the ongoing survival of many of our native species, and incur massive economic costs on agricultural industries and the environment. While existing pest control strategies have undoubtedly made substantial headway in mitigating these costs, recent advances in the fields of genomics and ecology offer new opportunities for the development of innovative approaches to pest control, of a kind that might have been considered the stuff of science fiction just a decade or two ago.
One such innovation, which is now well on the way to development, has been dubbed the ‘Trojan Female Technique’, or TFT, a reference to the myth of the wooden horse used by the ancient Greeks to infiltrate and conquer the walled city of Troy. Instead of filling a wooden horse with warriors, this pest control technique seeks to load fertile females of a target pest species with a heritable genetic variant that knocks out the fertility of the sons but leaves the female ‘Trojans’ and their daughters unscathed.
Sound familiar? The idea of unleashing sterile males on a pest population is already an established paradigm for pest control and is commonly used to control insect pest populations. If released in large enough numbers into a pest population, the sterile males will mate with many of the females, leaving their eggs unfertilised and conferring a reduction in population numbers. This ‘Sterile Male Technique’ has proven highly effective, but it is laborious and costly. Males must be manually sterilised prior to release, and the population control is effected only in the generation of release. The TFT picks up the baton at this point, because the sterility-inducing agent will be genetically encoded and passed from mothers to offspring and then grand-offspring. This mutation will have no effect whatsoever on female fertility, and indeed might even confer a slight reproductive advantage to the females that carry it. But in males, these variants will lead to sub-fertility or even complete infertility, and in every generation a new army of sterile males will be produced from the fertile Trojan females.
The idea of releasing pests that carry genetic ‘time-bombs’ that lead to a population’s decimation from within might evoke alarm as the first reaction. However, the intention of the TFT is to harness a set of naturally occurring genetic variants in the DNA of mitochondria, the energy-producing ‘batteries’ that occur in the cells of all animals and plants. These variants reduce the energy output of mitochondria just enough to cause male sterility through effects on high-energy-demanding sperm, but not enough to affect anything else, with reduced fertility leading to smaller populations.
Population genetic theory has long predicted that a set of genes in our energy-producing mitochondria were prone to accumulate male-sterilising variants. This is because these mitochondrial genes are maternally inherited. The evolutionary implication is profound: nature’s quality control process, which Charles Darwin called ‘natural selection’, is only able to remove harmful variants from mitochondrial genes when they are also harmful to females. Mitochondrial variants that are male-specific in their sterilising effects may therefore slip under the radar of natural selection and accumulate within populations.
Recently, candidate TFT variants have been identified within the mitochondrial DNA sequences of fruit flies that depress the fertility of males. Similar variants have been identified in European hares and mice, confirming the promise that the TFT could be applied across the gamut of pest species – from mites to mammals. Indeed, such variation in the mitochondrial DNA sequence appears to be a common cause of male infertility in humans too.
A consortium comprising researchers from Landcare Research, the University of Otago and Monash University has made significant headway towards developing the TFT. The consortium uses a multi-pronged approach that incorporates (1) theoretical modelling, (2) experimental validation of the technique in an insect model (the fruit fly, Drosophila melanogaster)and in a mammalian model (the mouse Mus musculus), and (3) research into the social acceptability of the TFT, via engagement with the general public and stake-holders through focus group sessions.
In fruit flies, the consortium has targeted a candidate TF variant in the cytochrome B gene, and has extensively tested its capacity to confer male sterility across a range of mating contexts, nuclear genomic backgrounds and thermal environments. In all cases its efficacy holds true, depressing the fertility of males. The consortium has now completed a large-scale multi-generation experiment in which hundreds of populations of flies were seeded with Trojan females carrying this TFT variant to determine its capacity to suppress populations. The results were striking: suppression occurred and persisted over the trial period of 10 generations. These experiments in fruit flies had strong theoretical underpinnings, with mathematical modelling guiding the design of the experiments in terms of the number of TFT females seeded into each population and the conditions in which populations were maintained.
Proof of utility has also been achieved in mice by screening the sperm parameters of numerous genetic strains of mice, each of which shares a common set of nuclear DNA but a different mitochondrial DNA sequence, consisting of a unique set of variants. This research has verified that variants within the mitochondrial genes of mice also affect male fertility.
The research consortium is now working with AgResearch to apply the TFT to the control of a real-world pest for the first time, the clover root weevil, which affects pasture quality and causes significant losses to the dairy industry.
This research was funded by the Ministry of Business, Innovation and Employment’s Smart Ideas programme.
Damian Dowling (Monash University)
Dan Tompkins (Landcare Research) tompkinsd@landcareresearch.co.nz
Neil Gemmell (University of Otago)