Aedes aegypti in the Black Sea: recent introduction or ancient remnant?

Background: The yellow fever mosquito Aedes aegypti transmits viral diseases that have plagued humans for centuries. Its ancestral home are forests of Africa and similar to 400-600 years ago it invaded the New World and later Europe and Asia, causing some of the largest epidemics in human history. The species was rarely detected in countries surrounding the Mediterranean Sea after the 1950s, but during the last 16 years it re-appeared in Madeira, Russia and in the eastern coast of the Black Sea. We genotyped Ae. aegypti populations from the Black Sea region to investigate whether this is a recent invasion (and if so, where it came from) or a remnant of pre-eradication populations that extended across the Mediterranean. We also use the Black Sea populations together with a world reference panel of populations to shed more light into the phylogeographical history of this species. Results: Microsatellites and similar to 19,000 genome-wide single nucleotide polymorphisms (SNPs) support the monophyletic origin of all populations outside Africa, with the New World as the site of first colonization. Considering the phylogenetic relationships, the Black Sea populations are basal to all Asian populations sampled. Bayesian analyses combined with multivariate analyses on both types of markers suggest that the Black Sea population is a remnant of an older population. Approximate Bayesian Computation Analysis indicates with equal probability, that the origin of Black Sea populations was Asia or New World and assignment tests favor the New World. Conclusions: Our results confirmed that Ae. aegypti left Africa and arrived in New World similar to 500 years ago. The lineage that returned to the Old World and gave rise to present day Asia and the Black Sea populations split from the New World approximately 100-150 years ago. Globally, the Black Sea population is genetically closer to Asia, but still highly differentiated from both New World and Asian populations. This evidence, combined with bottleneck signatures and divergence time estimates, support the hypothesis of present day Black Sea populations being remnants of older populations, likely the now extinct Mediterranean populations that, consistent with the historic epidemiological record, likely represent the original return of Ae. aegypti to the Old World.