The worldwide migration of insect pests has only become more of a concern as plants, animals, and materials have begun being shipped or flown to places around the globe. Ways to control these pests through providing resistance to plants or by killing off the insects directly has proven effective, but only to a point. Many pests have such a wide variety of target foods that the former application proves difficult to confer onto all the necessary species and the latter method of just exterminating them has always been challenging in general.
The Creation of Gene Drives
With the advent of gene manipulating technologies, there has long been an interest among insect and pest researchers to see if a mechanism called a gene drive could prove successful. The idea of modifying insects this way is hardly anything new, it was first proposed more than half a century ago back when understanding of DNA and genetics first arose.
The most commonly implemented version of a gene drive in recent years have been what is called a “Release of Insects carrying a Dominant Lethal” (RIDL), which means creating modified male insects with a repressed lethal transgene that only activates in their subsequent offspring, making them nonviable. It has been extremely effective in our efforts against mosquitoes and, while it isn’t likely to eradicate them, it does keep their populations at low enough levels that the risk of disease transmission is greatly diminished.
Of course, there are numerous downsides to this system. In order to keep up with the ever expanding insect population, new modified males must be continuously developed and released year after year. It is a very labor-intensive process and, in the long run, quite expensive. More permanent or, at least, longer-lasting options would be preferable.
Add to this the fact that gene drives of any sort have to be carefully constructed, as it has been found that insects have a strong ability to develop resistance against them, even when the inserted genes are placed in a highly conserved genomic region. So there is a lot of careful planning, execution, and trial and error required in order to make a working gene drive system in any insect species.
The Mechanisms of Medea
Of those proposed and tested, one system that has seen effectiveness in wild type insects when tested in a lab is the Maternal Effect Dominant Embryonic Arrest (Medea) method. The mechanism by which this approach works is by combining a “toxin” gene in a maternal host and a closely linked “antidote” gene.
An example of how this was used is an experiment where a microRNA toxin gene was inserted that activates during the production of egg cells in the female’s body (oogenesis). The toxin would, if not stopped in time, kill the mother and the offspring she would be having, if it wasn’t for the “antidote” gene activating during embryogenesis. If an antidote wasn’t available, the toxin would be kept active long enough to be inherited by all the offspring insects, where the microRNA then suppresses an essential gene for embryonic development. This ultimately kills the progeny.
With the antidote gene, however, it forms an active microRNA-resistant form of the targeted gene, essentially a copy of the critical embryonic sequence, that then allows the offspring to grow normally. This means that only offspring with the antidote survive, allowing the combined Medea system to spread throughout the entire insect population.
This can then be used to piggyback any other desired genes into a population by making them linked to the antidote gene. There are a variety of options at this point, including using repressed lethal sequences that are only activated once a certain chemical or other sort of signal is applied to the insects. Once they have all obtained the Medea system and the linked gene, you could wipe them all out.
These subtle sorts of gene drive spreading have proven to be the more effective options, as it reduces the likelihood of resistance immediately forming, as what happens when directly trying to kill all the insects with a fatal gene right off. Another option would be to simply add a gene that makes the insects susceptible to a particular insecticide or other chemical solution.
Combating The Summer Fruit Fly
Scientists at UC San Diego decided to look into Medea and whether it could be repurposed for use against the pest Drosophila suzukii, a fruit fly species that consumes summer fruit and can be found in locales around the world. One problem with this target is that D. suzukii is poorly genetically characterized, making it complicated to find the right site to insert the transgene system.
They began by copying the Medea format used in labs with the model organism fly, Drosophila melanogaster. The microRNAs were set up to target the gene myd88, a maternally given gene that is needed for dorsal-ventral formation in early embryogenesis. The antidote, therefore, was the myd88 coding region with the microRNA 5’ region removed so it wouldn’t be a binding location for the microRNAs. Then, eukaryotic Green Fluorescent Protein (eGFP) and double stranded Red Fluorescent Protein (dsRFP) genes were added to act as fluorescent marker genes to confirm integration of the Medea system.
The combined Medea transgenes were then introduced into a wild type male. Due to the system being maternal, his offspring had a random chance of receiving it, resulting in 50% with and 50% without. The flies were then taken through six generations, with only the progeny containing the Medea genes being crossbred with more wild types. By then, the majority of the heterozygous females were producing offsprings with the system, at a rate of 86.4%. Concerningly, however, it does look like a minority of the insects still managed to obtain some resistance to the microRNA toxin.
The Ongoing Problem of Resistance
To check for consistency, this designed system was tested against nine different strains of D. suzukii collected in various global locations. Interestingly, three of the strains saw a 100% ultimate inheritance rate in the population, while the other strains ranged from 87.6% to 99.4%. This showed that, overall, Medea is successful at infiltrating an insect population and, depending on the strain, it can even encompass the entirety of the available insects.
The researchers hypothesized that the reason for the inheritance dropping below 100% in generations 5 and 6 in most cases is that natural genetic variation causing changes in the microRNA target sites increased embryonic resistance. This was confirmed through genetic sequencing testing. While the Medea system has a reduced incidence of resistance formation compared to other techniques, it’s still a long term concern that has to be considered.
This risk can be reduced by pre-sequencing a population to ensure the target sequence chosen does not have significant genetic variation in that population. Also, minimizing fitness costs, such as those coming from the marker genes and fluorescence, would likely help to stop natural selection randomly selecting those individuals that do not have those costs occurring. Lastly, keeping a high release threshold amount of modified flies is additionally a necessity for quickly spreading Medea throughout the population in as few generations as possible. If it can reach population saturation and fixation, that would reduce the chance of resistance happening in any one individual.
The scientists made sure to note all of this and recommend further research into how resistance forms against gene drives. No matter what setup is used, it is seemingly always going to be a problem, but it is one that can be managed. Overall, the Medea method has proven to have a high success rate with only minimal resistance forming, making it a great candidate for managing D. suzukii populations and those of other insect species
Photo CCs: Spotted-wing Drosophila (Drosophila suzukii) male (15359228246) from Wikimedia Commons