Inherited Immunity to Toxins: Locating the Genetic Basis of Insecticide Resistance in African Mosquitoes
Despite its diminutive size, the mosquito is the deadliest animal on Earth — an expert disease-spreader that kills over 700,000 people per year. Its victims die of a wide range of diseases, including malaria, yellow fever, West Nile Virus, dengue, Zika, chikungunya, Eastern Equine Encephalitis, and Rift Valley Fever. Given the ease with which these tiny insects transmit deadly pathogens to humans, it should come as no surprise that mosquito control is a worldwide public health priority. The main techniques for preventing vector-borne diseases are limiting human-mosquito contact with nets and repellants and decreasing the size of mosquito populations through widespread insecticide spraying. While these costly techniques help to reduce incidence of disease in communities, neither is a perfect barrier to vector-borne pathogen spread.
Despite the millions of dollars invested in insecticide spraying each year, populations of disease-bearing mosquitoes continue to flourish in the wild. This is partly because mosquito populations tend to develop resistance to insecticides, just as bacterial populations develop antibiotic resistance. Scientists around the world are investigating the molecular basis for this phenomenon, with the goal of finding ways to circumvent it.
Recently, a team of researchers created the Genomics for African Anopheles Resistance Diagnostics (GAARD) project to investigate the genetic basis underlying resistance to deltamethrin and pirimiphos-methyl, two common insecticides. They focused their investigation on two species of West African mosquitoes, Anopheles gambiae and Anopheles coluzzii, which are major vectors of malaria.
The researchers collected hundreds of mosquito larvae from aquatic areas in six West African sampling sites. These specimens were raised to adulthood in a lab and used for a genome-wide association study. In this study, each mosquito was randomly selected for treatment with either deltamethrin or pirimiphos-methyl. If a mosquito survived the exposure, it was considered resistant to that insecticide. After this test was performed, all surviving mosquitoes were killed. DNA was extracted from each mosquito, and the entire genome of each specimen was sequenced.
To ensure that their sample was truly random, the researchers used statistical tests to determine if any of the sequenced genomes came from closely related mosquitoes. When groups of siblings were found, all but one of them were excluded from further analysis. This minimized the chances that the results were influenced by disproportionate sampling of a single group of siblings.
When analyzing the results of the whole-genome sequencing, the researchers searched for the presence of copy number variants (CNVs) in genes associated with metabolism. CNVs are mutations that increase the number of copies of a gene, thus increasing the number of protein products expressed by the organism. Unsurprisingly, the researchers found that certain CNVs were associated with heightened insecticide resistance, likely because they increased the mosquitoes’ ability to safely metabolize insecticides.
The researchers also searched for the presence of single-nucleotide polymorphisms (SNPs), which are variations in a single base-pair in the genome. They hoped to discover a set of SNPs in the mosquitoes’ genomes that could be considered markers of insecticide resistance. Although they found some SNPs that were positively associated with insecticide resistance, these results did not exceed the false discovery rate and thus could not be considered definitive. The researchers concluded that their sample size (only 1258 mosquito genomes were sequenced) was too small to accurately establish which SNPs are markers of resistance.
After analyzing all the genetic data, the researchers concluded that numerous alleles throughout the mosquitoes’ genomes were positively associated with resistance to each insecticide. These results contradict the theory that insecticide resistance results primarily from mutation of a single gene. In reality, insecticide resistance seems to arise from a combination of many different mutations throughout the genome, each of which slightly increases survival rates.
One of the key findings of this study is that genetic markers associated with resistance vary between populations. This could make it difficult in the future to develop genetic tests for insecticide resistance or predict which populations are more likely to develop resistance. Despite this difficulty, further research in the field of insecticide resistance is vital to the improvement of mosquito control initiatives worldwide. The ultimate goal is to promote human and animal health by maximizing the number of mosquitoes killed by spraying campaigns and promote environmental health by minimizing the damage caused by over-spraying.
