Demographic History, Genetic Load, and the Efficacy of Selection in the Globally Invasive Mosquito Aedes aegypti.

Aedes aegypti is the main vector species of yellow fever, dengue, Zika, and chikungunya. The species is originally from Africa but has experienced a spectacular expansion in its geographic range to a large swath of the world, and the demographic effects of which have remained largely understudied. In this report, we examine whole-genome sequences from six countries in Africa, North America, and South America to investigate the demographic history of the spread of A. aegypti into the Americas and its impact on genomic diversity and deleterious genetic load. In the Americas, we observe patterns of strong population structure consistent with relatively low (but probably nonzero) levels of gene flow but occasional long-range dispersal and/or recolonization events. We also find evidence that the colonization of the Americas has resulted in introduction bottlenecks. However, while each sampling location shows evidence of a past population contraction and subsequent recovery, our results suggest that the bottlenecks in America have led to a reduction in genetic diversity of only ∼35% relative to African populations, and the American samples have retained high levels of genetic diversity (expected heterozygosity of ∼0.02 at synonymous sites). We additionally find that American populations of aegypti have experienced only a minor reduction in the efficacy of selection, with evidence for both an accumulation of deleterious alleles and some purging of strongly deleterious alleles. These results exemplify how an invasive species can expand its range with remarkable genetic resilience in the face of strong eradication pressure.