Dual African origins of global Aedes aegypti s.l. populations revealed by mitochondrial DNA

Abstract

Background: Aedes aegypti is the primary global vector to humans of yellow fever and dengue flaviviruses. Over the past 50 years, many population genetic studies have documented large genetic differences among global populations of this species. These studies initially used morphological polymorphisms, followed later by allozymes, and most recently various molecular genetic markers including microsatellites and mitochondrial markers. In particular, since 2000, fourteen publications and four unpublished datasets have used sequence data from the NADH dehydrogenase subunit 4 mitochondrial gene to compare Ae. aegypti collections and collectively 95 unique mtDNA haplotypes have been found. Phylogenetic analyses in these many studies consistently resolved two clades but no comprehensive study of mtDNA haplotypes have been made in Africa, the continent in which the species originated.

Methods and findings: ND4 haplotypes were sequenced in 426 Ae. aegypti s.l. from Senegal, West Africa and Kenya, East Africa. In Senegal 15 and in Kenya 7 new haplotypes were discovered. When added to the 95 published haplotypes and including 6 African Aedes species as outgroups, phylogenetic analyses showed that all but one Senegal haplotype occurred in a basal clade while most East African haplotypes occurred in a second clade arising from the basal clade. Globally distributed haplotypes occurred in both clades demonstrating that populations outside Africa consist of mixtures of mosquitoes from both clades.

Conclusions: Populations of Ae. aegypti outside Africa consist of mosquitoes arising from one of two ancestral clades. One clade is basal and primarily associated with West Africa while the second arises from the first and contains primarily mosquitoes from East Africa.

Figure 1. Maximum likelihood tree of the 34 mtDNA ND4 Ae. aegypti haplotypes found to date in Africa and outgroups.

These were comprised of the 15 new unique Senegal haplotypes from the present study and one Senegal haplotype collected in Dakar in a previous study (labeled in red). Seven novel haplotypes from Kenya and one from Uganda (in blue), three from Cameroon (in black) and seven haplotypes (in large green font) that appeared in collections from Africa and other global locations in various other studies (Table S1). Branches with bootstrap support values >50% are labeled with % support. These support values are followed by clade credibility values in parentheses from MrBayes analysis.