Genomic insights into the population structure of malaria vectors of the Anopheles gambiae complex in The Gambia reveals distinctiveness and spatio-temporal stability of BISSAU molecular form

A new putative taxon of the Anopheles gambiae complex - genetically related to the two main malaria vector species of the complex, An. gambiae and An. coluzzii - was recently described at the westernmost extremes of the two species' range based on WGS data from The MalariaGEN Vector Observatory, and named BISSAU molecular form. This was found resting in human dwellings, in sympatry with An. gambiae and An. coluzzii and to be characterized by a unique ancestral gene pool and a complex demographic history. We here present the results of population genomic analyses (PCA, ADMIXTURE, TreeMix, unrooted neighbour-joining tree, and TESS3R) based on >200 K SNPs on chromosome-3 obtained exploiting WGS data from 2283 An. coluzzii, An. gambiae, An. arabiensis, An. melas and BISSAU molecular form specimens collected between 2005 and 2021 at 35 locations across The Gambia. Results consistently identify BISSAU molecular form as a genetically discrete, temporarily stable unit within a sister clade with An. gambiae and An. coluzzii, showing no signs of admixture with any of the sympatric members of the An. gambiae complex. This strongly supports the BISSAU molecular form as an independently evolved lineage reproductively isolated from other sympatric members of the An. gambiae complex. The north bank of Gambia river, from about 10 to 120 km from the seacoast, has the higher probability of occurrence of BISSAU molecular form. In this area, breeding sites are mainly characterized by brackish and fresh water associated with large flooded swamp areas with alluvial deposits in close proximity to the river and its tributaries.