Abstract:
Background: The mosquito Aedes aegypti is the main vector of dengue, Zika, chikungunya and yellow fever viruses.
This major disease vector is thought to have arisen when the African subspecies Ae. aegypti formosus evolved from
being zoophilic and living in forest habitats into a form that specialises on humans and resides near human
population centres. The resulting domestic subspecies, Ae. aegypti aegypti, is found throughout the tropics and
largely blood-feeds on humans.
Results: To understand this transition, we have sequenced the exomes of mosquitoes collected from five populations
from around the world. We found that Ae. aegypti specimens from an urban population in Senegal in West Africa were
more closely related to populations in Mexico and Sri Lanka than they were to a nearby forest population. We estimate
that the populations in Senegal and Mexico split just a few hundred years ago, and we found no evidence of Ae.
aegypti aegypti mosquitoes migrating back to Africa from elsewhere in the tropics. The out-of-Africa migration was
accompanied by a dramatic reduction in effective population size, resulting in a loss of genetic diversity and rare
genetic variants.
Conclusions: We conclude that a domestic population of Ae. aegypti in Senegal and domestic populations on other
continents are more closely related to each other than to other African populations. This suggests that an ancestral
population of Ae. aegypti evolved to become a human specialist in Africa, giving rise to the subspecies Ae. aegypti
aegypti. The descendants of this population are still found in West Africa today, and the rest of the world was colonised
when mosquitoes from this population migrated out of Africa. This is the first report of an African population of Ae.
aegypti aegypti mosquitoes that is closely related to Asian and American populations. As the two subspecies differ in
their ability to vector disease, their existence side by side in West Africa may have important implications for disease
transmission.
