Insecticide resistance among mosquitoes and other crop-devouring insects is a major scourge in agriculture, and costs the food production industry millions of dollars each year. In a recent study in Nature Communications, a team of geneticists report a new development employing CRISPR/Cas9 – a technology that splices, or cuts, the genetic code and inserts a sequence of interest – whereby the mutation that is responsible for insecticide resistance is reversed. This, according to the study, provides an alternate solution to the standard method of developing newer and newer pest-/insecticides.
The gene in question is the voltage gated sodium channel (vgsc), wherein a mutation makes the insect resistant to the insecticide. This is so because the insecticides DDT and pyrethroids acts by binding itself to the protein coded by the vgsc gene, and the kdr alleles does not allow the insecticide to do so anymore. Here, kdr refers to ‘knockdown resistant,’ so named because ‘knocking down’ the normal, wild-type, allele renders the insect the aforementioned resistance
Allele refers to the variant(s) of a gene. Assuming that there is just one gene responsible for fruit colour (most traits in actuality are usually governed by multiple genes acting in cohesion), one ‘allele’ at that locus (the specific place on the chromosome where the gene lives) will be responsible for white colour, while another allele will be responsible for blue colour and another still will code yet another colour. The number of alleles that occur in a population at that locus is referred to as allelic diversity.
The experiment was conducted on the common fruitfly, the Drosophila melanogaster, a model species for genetic studies. It was first used as a model organism in 1910, and since then it has become a staple in genetic research for it can be bred relatively easily even in small laboratories.
The behaviour of the three insecticide-resistant (IR) mutants, commonly found in mosquito and fly species, was tested in fruitfly populations. It was found that L1014F allele was ‘highly resistant to DDT, moderately resistant to permethrin’ but ‘susceptible to deltamethrin.’ I1011M was moderately resistant to DDT and permethrin, while I1011V mutants were quite susceptible to ‘all three insecticides even at low concentrations.’
Authors argue that the different behaviour of these alleles is because ‘substitution of different amino acids even at the same position can have different impacts on DDT binding’. These alleles had been sourced from ‘different insect species in the field… displaying [insecticide resistance],’ says Professor Ethan Bier, the corresponding author of the study, in an email conversation with indianexpress.com.
‘These alleles have not previously been characterised in flies[,] so we generated them to see how they behaved in flies and some of them display different phenotypes than they did in the field caught species,’ Professor Bier added. (Phenotype refers to the outward, physical characteristics of an individual; while genotype refers to the genetic makeup.)
Next, the study took a set of Drosophila melanogaster flies that carried the L1014F allele and used CRISPR-based tools to revert the mutation to a wild-type allele that is susceptible to insecticide. In parallel, an allele drive element was inserted at another locus on the chromosome that promotes self-copying and ‘super Mendelian inheritance’ of that element. Allele/gene drive is a technology whereby genes in a subset of a population can be modified in order to ‘drive’ its spread across the population – or even the entire species – after a few generations.
This line of fruitflies was then bred twice further, and it was observed that their progeny showed very low survival in the presence of DDT. The survival in presence of DDT was much higher in the case of the control experiment that carried the unmodified insect resistant allele.
‘These results demonstrate biased inheritance of the [wild type] allele mediated by cleavage and conversion of the [insect resistant] allele to [wild type] allele,’ the paper states. When the performance of fruitflies in population ‘cages’ was examined, it was found that the gene drive properties conferred by the gene drive allele as well as the wild-type gene dramatically reversed insecticide resistance. By the time the twelfth generation was bred, DDT resistance had reduced 68.1 per cent in males and 32 per cent in females (in comparison with the first generation).
Other species have come under the gene drive scanner as well. It has been demonstrated, albeit under laboratory conditions, that gene drive systems in mosquitoes and fruit flies can effectively disseminating anti-malarial effector genes in the entire populations. It has been claimed that if applied to even 1 per cent of the population of mosquitoes, malaria could be quickly eradicated.
But, as they say, evolution is messy. The paper does acknowledge that drawing correlations between insecticide resistance and fitness can be hard to draw conclusively. When no insecticides were present, the L1014F and the I1011M alleles led to a reduced lifespan in males. In an experiment with the herbicide paraquat, L1014F mutants remained resistant but I1011M ones were susceptible.
In a 2020 study on Aedes aegypti , the primary vector of dengue, chikungunya and Zika, it was found that the larvae of wild type (i e insecticide susceptible) populations that were crossed with insect resistant mutants took longer to develop. They also had fewer individuals reaching adult stages, had smaller wing lengths and their females had shorter lifespans on an average. Bier asserts in a press release that insecticide resistance could actually impose a fitness cost, ‘making those insects less fit in a Darwinian sense.’