An international team of researchers, led by scientists at the University of Buea in Cameroon and including collaborators from the Liverpool School of Tropical Medicine, the MRC Unit The Gambia at the London School of Hygiene and Tropical Medicine, and the Ellison Institute of Technology in Oxford, analysed whole genome sequence data from 276 mosquitoes belonging to the Anopheles gambiae complex. These insects were collected between 2020 and 2021 in six communities distributed along the altitudinal gradient of Mount Cameroon, a volcanic mountain whose slopes create a patchwork of ecological conditions that can shape the biology of the insects living there.

The Anopheles gambiae complex is a group of morphologically indistinguishable mosquito species, several of which are among the most efficient malaria vectors in the world. Because the members of the complex look identical, researchers rely on genetic tools to tell them apart. In this study, the team used ancestral informative markers, which are genetic variants known to distinguish particular species, to assign each of the 276 mosquitoes to a taxon. The results showed that 93 of the mosquitoes were Anopheles coluzzii, 178 were Anopheles gambiae sensu stricto, and four were Anopheles melas, a saltwater-tolerant member of the complex, while a single mosquito could not be assigned to any of these groups.

To investigate population structure, the researchers randomly selected a set of 100,000 neutral biallelic single nucleotide polymorphisms, or SNPs, meaning variants scattered across the genome that are not under obvious selective pressure and therefore mainly reflect ancestry and gene flow. Using principal component analysis and the fixation index, a standard measure of genetic differentiation between populations, along with admixture, identity by descent and population dynamics analyses, the team reconstructed the evolutionary relationships among the mosquito populations on the mountain and compared them with reference populations from Burkina Faso.

The most striking result emerged from the Anopheles coluzzii samples. Rather than forming a single genetically uniform population, the mosquitoes from Missellele, a locality at the foot of the mountain, split cleanly into two distinct subpopulations, which the authors designated Anopheles coluzzii A and Anopheles coluzzii B. The first of these groups contained 83 of the 93 Anopheles coluzzii individuals, while the second comprised just 10. What makes this finding remarkable is that the two subpopulations were collected from the same locality, meaning they are likely to be sympatric, sharing the same breeding grounds and habitats, yet their genomes tell stories of separate ancestry and limited interbreeding.

The smaller group, Anopheles coluzzii B, was found to be ancestrally closer to Anopheles coluzzii populations from Burkina Faso than to its geographical neighbours in the larger subpopulation. This pattern suggests a connection to more distant West African populations that has somehow been maintained despite the considerable distance separating the Sahelian country from the Cameroonian coast, or alternatively points to a complex demographic history in which lineages with different origins came to coexist in the same place. Meanwhile, the dominant subpopulation, Anopheles coluzzii A, proved to be the most genetically differentiated of all the Anopheles coluzzii populations examined in the study, standing apart not only from Anopheles coluzzii B but also from the Burkina Faso reference populations.

Digging deeper into the location of the genetic differences, the researchers found that the strong differentiation of Anopheles coluzzii A was driven principally by variants located on the X chromosome and chromosome arm 2L. The X chromosome is of particular interest in studies of speciation and reproductive isolation because genes located on it often play an outsized role in hybrid incompatibilities, a pattern observed across many organisms. The concentration of highly differentiated variants on the X and 2L therefore hints at possible local reproductive isolation, meaning the mosquitoes may be beginning to diverge into reproductively separated groups, potentially accompanied by ecological adaptation to local conditions on the mountain slopes. Windowed principal component analysis further revealed signals of chromosomal inversions on chromosome arms 2L and 2R, structural rearrangements of the genome that are known to suppress recombination and can help maintain locally adapted gene combinations in the face of gene flow.

The study also detected evidence of recent population expansion in all species and populations examined, indicating that the mosquito populations on Mount Cameroon have grown substantially in the recent past. Expanding populations can spread rapidly and can dilute the effects of vector control measures if survivors of interventions repopulate breeding sites quickly. Combined with the genetic fragmentation observed among the Anopheles coluzzii, this demographic signal paints a picture of vector populations that are both dynamic and structured, a combination that complicates the assumption, common in control programmes, that mosquitoes in a given area behave and evolve as a single homogeneous population.

The practical implications are considerable. Vector control interventions such as insecticide-treated bed nets and indoor residual spraying exert strong selection on mosquitoes, and the effectiveness of these tools depends on how resistance genes move through populations. If Anopheles coluzzii populations on the slopes of Mount Cameroon are genetically isolated from one another, then insecticide resistance, behavioural changes and other adaptations that arise in one subpopulation may not spread readily to the others. Each genetically distinct population may require its own surveillance and tailored control strategy. The authors argue that the genetic divergence and recent expansion they documented call for further genomic surveillance to support locally tailored vector control and malaria elimination efforts, effectively making the case that a one-size-fits-all approach will be insufficient in this landscape.

Mount Cameroon itself provides a compelling natural laboratory for this kind of work. Rising steeply from the Atlantic coast to more than 4,000 metres, the mountain compresses a wide range of climates and habitats into a small geographic area, from humid lowland plantations and coastal flats to montane forest and alpine scrub. Communities at different altitudes experience different intensities and seasonality of malaria transmission, and the mosquitoes that transmit the parasite face different ecological pressures along the gradient. The study, which received funding from the Pan African Malaria Genetic Epidemiology Network under the Human Heredity and Health in Africa programme and from DELTAS Africa, demonstrates how high-resolution genomic data can expose hidden population structure even within a seemingly continuous landscape, information that was simply inaccessible in the era before whole genome sequencing became routine.

More broadly, the findings contribute to a growing appreciation of the fine-scale genetic heterogeneity of Africa’s major malaria vectors. Anopheles coluzzii and Anopheles gambiae sensu stricto diverged from a common ancestor relatively recently and are still in the process of reproductive separation, making them a textbook example of ongoing speciation. Documenting an additional layer of divergence within Anopheles coluzzii itself, in a population at the edge of the species’ range and on the slopes of an isolated volcano, adds an important data point to this evolutionary picture. It also underscores the value of sustained, locally grounded genomic monitoring: as malaria elimination campaigns intensify across Africa, understanding the genetic architecture of the vectors in each transmission setting will be essential for anticipating how the mosquitoes respond to interventions and for designing strategies that stay ahead of their capacity to adapt. For the communities on the slopes of Mount Cameroon, the invisible genetic walls now revealed among their mosquito populations may ultimately help determine whether and how malaria can finally be pushed out of the region.

Subject of Research: Genomic analysis of genetically isolated Anopheles coluzzii malaria vector populations on Mount Cameroon

Article Title: Genome-wide sequence analysis of malaria vectors reveals genetically isolated Anopheles coluzzii populations along the slopes of Mount Cameroon

Article References: Kwi, P. N., Dze, J. E., Brenas, J., Sadio, A., Tangi, L. N., Chi, H. F., Assogba, B. S., Milugo, T. K., Miles, A., Clarkson, C., Amambua-Ngwa, A., & Apinjoh, T. O. (2026). Genome-wide sequence analysis of malaria vectors reveals genetically isolated Anopheles coluzzii populations along the slopes of Mount Cameroon. BMC Genomics. https://doi.org/10.1186/s12864-026-13341-2

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13341-2

Keywords: Anopheles coluzzii, Anopheles gambiae complex, whole genome sequencing, population structure, malaria vectors, Mount Cameroon, SNPs, reproductive isolation, chromosomal inversions, genomic surveillance, vector control, Cameroon