Introgression between genetically distinct populations generates variation that may promote adaptation to diverse environmental conditions. In the mosquito Aedes aegypti, the forest-associated subspecies Aedes aegypti formosus (Aaf) underwent a domestication process approximately 5000 years ago, giving rise to the globally invasive, human-associated arbovirus vector Aedes aegypti aegypti (Aaa). Despite extensive human-mediated transport, the two subspecies remain largely distinct, with admixture documented mainly in coastal Africa and Saudi Arabia. We examined Saudi Arabian Ae. aegypti across climatically contrasting regions - the humid south and arid west- using a Saudi Arabian dataset (11,939 SNPs) to examine within-country genetic and environmental variation and a global dataset (13,455 SNPs) to assess ancestry and admixture in a worldwide context. Genomic analyses revealed strong west-south differentiation, with Jazan Highland showing admixture between subspecies and the highest proportion of Aaf ancestry. Multiple pulses of gene flow into and within southern Saudi Arabia suggest Aaf populations act as a reservoir of genetic variation. Genome-wide scans identified a candidate region associated with the humidity gradient, harbouring an elongase of very long-chain fatty acids (AAEL024147), essential for cuticle production and minimizing water loss. Because the Saudi Arabian genetic background is predominantly the arid-adapted subspecies Aaa, the presence of Aaf ancestry at this locus may reflect localized introgression with adaptive relevance under an increasingly humid environment. Our findings highlight how secondary contact between divergent lineages may contribute to local adaptation in Ae. aegypti, with implications for predicting responses to environmental change.
The Aedes aegypti (Linnaeus, 1762) mosquito is the main vector of dengue, chikungunya and Zika and is well established today all over the world. The species comprises two forms: the ancestral form found throughout Africa and a global domestic form that spread to the rest of the tropics and subtropics. In Saudi Arabia, A. aegypti has been known in the southwest since 1956, and previous genetic studies clustered A. aegypti from Saudi Arabia with the global domestic form. The purpose of this study was to assess the genetic structure of A. aegypti in Saudi Arabia and determine their geographic origin. Genetic data for 17 microsatellites were collected for A. aegypti ranging from the southwestern highlands of Saudi Arabia on the border of Yemen to the north-west in Madinah region as well as from Thailand and Uganda populations (as representatives of the ancestral African and global domestic forms, respectively). The low but significant level of genetic structuring in Saudi Arabia was consistent with long-distance dispersal capability possibly through road connectivity and human activities, that is, passive dispersal. There are two main genetic groupings in Saudi Arabia, one of which clusters with the Ugandan population and the other with the Thailand population with many Saudi Arabian individuals having mixed ancestry. The hypothesis of genetic admixture of the ancestral African and global domestic forms in Saudi Arabia was supported by approximate Bayesian computational analyses. The extent of admixture varied across Saudi Arabia. African ancestry was highest in the highland area of the Jazan region followed by the lowland Jazan and Sahil regions. Conversely, the western (Makkah, Jeddah and Madinah) and Najran populations corresponded to the global domesticated form. Given potential differences between the forms in transmission capability, ecology and behaviour, the findings here should be taken into account in vector control efforts in Saudi Arabia.