Evidence for Recent Population Expansion in the Evolutionary History of the Malaria Vectors Anopheles arabiensis and Anopheles gambiae

Abstract

Gene flow in malaria vectors is usually estimated based on differentiation indices (e.g., F(ST)) in order to predict the contemporary spread of genes such as those conferring resistance to insecticides. This approach is reliant on a number of assumptions, the most crucial, and the one most likely to be violated in these species, being mutation-migration-drift equilibrium. Tests of this assumption for the African malaria vectors Anopheles gambiae and Anopheles arabiensis are the focus of this study. We analyzed variation at 18 microsatellite loci and the ND5 region of the mitochondrial genome in two populations of each species. Equilibrium was rejected by six of eight tests for the A. gambiae population from western Kenya and by three tests in eastern Kenya. In western Kenya, all departures from equilibrium were consistent with a recent population expansion, but in eastern Kenya, there were traces of a recent expansion and a bottleneck. Equilibrium was also rejected by two of the eight tests for both A. arabiensis populations; the departure from equilibrium was consistent with an expansion. These multiple-locus tests detected a genomewide effect and therefore a demographic event rather than a locus-specific effect, as would be caused by selection. Disequilibrium due to a recent expansion in these species implies that rates of gene flow, as inferred from differentiation indices, are overestimates as they include a historical component. We argue that the same effect applies to the majority of pest species due to the correlation of their demography with that of humans.

Ecological Zones Rather Than Molecular Forms Predict Genetic Differentiation in the Malaria VectorAnopheles gambiaes.s. in Ghana

“…gambiae (Zheng et al 1996) and, with the exception of Ag2H197 (inversion 2Ra), were not contained within polymorphic inversions. Previous studies had found these markers to be highly variable and to amplify reliably (Donnelly et al 2001;Pinto et al 2002;Wondji et al 2002). None of these markers are within the ''genomic islands of speciation'' identified by other authors (Turner et al 2005) but some of the markers show marked differences in allele frequencies between molecular forms (Pinto et al 2002;Wondji et al 2002).…”

Global population structure of the stable fly (Stomoxys calcitrans) inferred by mitochondrial and nuclear sequence data

“…Recent expansions in a variety of disease vectors, such as Anophelinae (Donnelly et al, 2001;Dusfour et al, 2007;Matthews et al, 2007), and black flies (Pramual et al, 2005) appear to be tied to human population expansions and/or to climate changes and to the rise of agriculture, providing significant new breeding sites and sources of bloodmeals (Coluzzi, 1982). Coluzzi (1982) proposed that because these species were dependent on humans for feeding and breeding sites, populations may have mirrored the growth in populations of humans and domestic animals during this period.…”

Revisiting the role of introgression vs shared ancestral polymorphisms as key processes shaping genetic diversity in the recently separated sibling species of the Anopheles gambiae complex

“…Previous studies of partial sequence of mitochondrial DNA have shown that large numbers of haplotypes are shared between A. gambiae, A. arabiensis and A. bwambae Thelwell et al, 2000;Donnelly et al, 2001), but whether this is a result of introgression or retention of ancestral polymorphisms is unresolved Thelwell et al, 2000). In this study, we examine patterns of intra and interspecific differentiation in ways that can allow us to distinguish between contemporary introgression and retention of ancestral mitochondrial sequences.…”