title: Landscape genetics of Anopheles gambiae in southern Ghana
Creators: name: Essandoh, John
Publisher: name: University of Liverpool Repository
Year: 2021

description: Insecticide application via indoor residual spraying (IRS) and long-lasting insecticide-treated bednets (LLIN) has been a key component in combating malaria in sub-Saharan Africa, with pyrethroids being the traditionally recommended insecticides. However, the resistance of Anopheles gambiae s.s. and An. coluzzii to pyrethroids is high and widespread, especially in West Africa, underpinned in part by high frequencies of the L1014F target-site mutation in the voltage-gated sodium channel (Vgsc). Carbamate (CB) and organophosphate (OP) resistance is generally lower but appears to be spreading via increasing frequencies of acetylcholinesterase (Ace-1) target site mutations involving the G119S polymorphism. Whilst Vgsc-1014F often nears fixation, Ace-1-119S is far more heterogeneous with complex copy number variant (CNV) dynamics. This study seeks to identify the determinants of Ace-1-119S spatial and temporal distribution and use population genetics to quantify the fine-scale population structure of An. gambiae s.s and An. coluzzii to understand how genetic barriers and gene flow may determine the current and future geographical spread. The study across southern Ghana demonstrated that ecological zones and land-use activities are important determinants of Ace-1-119S and Vgsc-1014F distributions. The Vgsc-1014F is associated with rural, mining, and cash crop plantations, while Ace-1-119S is associated with urban agricultural activities. These human-influenced environmental factors are likely to influence selection for, and the spread of the insecticide resistance mutations. Interesting, whilst almost all past studies have shown that insecticidal mortality increased with mosquito age, this was not the case for pirimiphos-methyl. Indeed, mortality was well predicted by Ace-1 mutant type but not age, suggesting that age-dependent costs of resistance may be low, and more broadly that age-specific resistance is not a universal phenomenon and may depend on insecticide class and resistance mechanisms. The study also showed that informative molecular markers such as Ace-1 can be used for sensitive temporal surveillance to track the changes in resistance in populations missed by standard diagnostic dose assays. In the focal area of Accra from which the time series of samples were collected, Ace-1-119S frequencies within An. gambiae s.s. and An. coluzzii appeared destined for fixation but deviated from this path in recent years. This resulted from the co-existence of homogeneous and heterogeneous Ace-1 duplications, with balancing of mutant polymorphisms likely a result of different costs and benefits of each. There was also evidence of shifting Ace-1 duplication type dynamics that are operating in the opposite direction in each species – presumably bringing each towards a common equilibrium. v Finally, Anopheles gambiae s.s. and An. coluzzii sampled from 34 sites spanning the four ecological zones, and land-use types were genotyped using around 10,000 markers generated by low-coverage genome sequencing. Results showed a striking lack of close relatives among mosquitoes, despite larval sampling, suggesting that the strategy of collecting from multiple sites within (on average) a 3.0-km radius successfully yielded unrelated mosquitoes. There was low genetic differentiation and no population structure within both An. coluzzii and An. gambiae s.s. across southern Ghana. However, An. coluzzii showed isolation by distance at a local sampling scale, suggesting there is distance-limited dispersal, and that patterns reflect contemporary gene flow. In An. gambiae s.s, no isolation by distance was detectable, even at a smaller geographical distance of separation suggesting the implication of historical signals for the observed no population structure. The results also showed that distance is not the primary cause of Ace-1 differentiation, but rather habitat differentiation which may correlate with distance. The findings of this study have given insight into phenotypic resistance of primary Anopheles malaria vectors to CB and particularly OP and how underlying target-site resistance mechanisms may be selected by environmental factors to modulate resistance and its spread. This information provides guidelines for vector control and resistance management.