Exploring genetic variation in insecticide resistance loci in Anopheles gambiae s.l. populations from Ghana

Problem Statement
Malaria remains a critical public health burden in Sub-Saharan Africa, with Ghana contributing significantly to global case numbers. Vector control relies heavily on insecticide-treated nets (ITNs) and indoor residual spraying (IRS), but their efficacy is increasingly compromised by widespread insecticide resistance in Anopheles gambiae sensu lato (s.l.) populations. While resistance mechanisms such as target-site mutations (in vgsc, rdl, ace1) and metabolic resistance (in gste2) are known, there are significant geographical gaps in Ghana regarding the phylogeny, genetic diversity, and specific distribution of resistance alleles across different ecological zones. Furthermore, morphological identification of An. gambiae s.l. species is often inadequate, necessitating molecular tools for precise surveillance.

Methodology
The study employed a low-cost, high-throughput multiplex amplicon sequencing approach using Oxford Nanopore Technologies (ONT).

• Sampling: 330 An. gambiae s.l. mosquitoes were collected from 18 sites across Ghana's three major ecological zones: Coastal Savannah, Forest, and Guinea Savannah. A subset of these mosquitoes had been previously phenotyped for pyrethroid and organophosphate susceptibility using WHO tube tests.
• Sequencing Strategy: DNA was extracted from individual mosquitoes. Multiplex PCR assays were designed to amplify seven genomic regions: four domains of the voltage-gated sodium channel (vgsc), partial sequences of ace1, rdl, and gste2, alongside species identification markers (ITS1, ITS2, SINE200, cox1, nd4, 28S, IGS).
• Barcoding and Sequencing: Unique 8bp barcodes were attached to primers to allow pooling of samples. Libraries were prepared using the Native Barcoding Kit 24 V14 and sequenced on a MinION Mk1B device with r10.4.1 flow cells.
• Bioinformatics: Reads were basecalled, demultiplexed, and aligned to the An. gambiae reference genome (AgamP4). Variants were called using FreeBayes and GATK HaplotypeCaller, filtered for quality, and annotated. Species were assigned based on the presence/absence of the SINE200 insertion. Phylogenetic and diversity analyses were conducted on ITS2 and mitochondrial sequences.

Key Contributions and Results

• Species Distribution: Molecular identification revealed An. coluzzii as the predominant species in Coastal (77.6%) and Forest (86.5%) zones, while An. gambiae s.s. was most prevalent in the Guinea Savannah zone (50.0%). Hybrids were detected across all zones.
• Genetic Diversity and Gene Flow: Phylogenetic analysis of ITS2 and mitochondrial sequences showed low nucleotide diversity and no distinct clustering by geography or species, indicating high gene flow across An. gambiae s.l. populations in Ghana.
• Resistance Allele Detection: The study identified 202 SNPs across the target loci, including 31 non-synonymous mutations.
  • Known Mutations: 13 previously associated resistance mutations were detected: 7 in vgsc (including high-frequency L995F at 75% and L995S at 6%), 2 in rdl (A296G at 51%), 1 in ace1 (G280S at 22%), and 3 in gste2 (I114T, F120L, L119V).
  • Novel and Emerging Variants: 17 novel SNPs were identified. Notably, the gste2 mutation T154S was detected at a 24% frequency, a variant recently observed in Guinea-Bissau. Other novel variants included vgsc A1553T and L1680F.
  • Linkage Disequilibrium: Strong linkage was observed between vgsc mutations V402L and I1527T (r² = 0.90), suggesting they frequently occur on the same haplotype.
• Species-Specific Variations: Significant differences in allele frequencies were observed between species. For instance, vgsc L995F was higher in An. gambiae s.s. (90%) than An. coluzzii (72%), while vgsc V402L and I1527T were significantly higher in An. coluzzii.
• Phenotype-Genotype Association: While most SNPs showed no significant association with resistance phenotypes in the limited phenotyped subset, the heterozygous genotypes of vgsc L995S and I1527T were significantly associated with deltamethrin and permethrin resistance, respectively.

Significance
The paper demonstrates that high-throughput multiplex amplicon sequencing using nanopore technology is a scalable, cost-effective method for the genomic surveillance of malaria vectors. It successfully characterizes the genetic landscape of insecticide resistance in Ghana, revealing a complex mix of known and novel variants and highlighting the dynamic nature of resistance evolution. The findings underscore the necessity of continued genomic monitoring to inform targeted insecticide resistance management and adaptive vector control strategies, particularly given the evidence of high gene flow that could facilitate the rapid spread of resistance alleles across ecological zones.