Originally published as Genetics Published Articles Ahead of Print on November 16, 2006.

Genetics, Vol. 175, 751-761, February 2007, Copyright © 2007
doi:10.1534/genetics.106.065888

Ecological Zones Rather Than Molecular Forms Predict Genetic Differentiation in the Malaria Vector Anopheles gambiae s.s. in Ghana

Alexander E. Yawson*,{dagger},{ddagger}, David Weetman*, Michael D. Wilson{dagger} and Martin J. Donnelly*,1

* Vector Group, Liverpool School of Tropical Medicine, Liverpool L3 5QA, United Kingdom, {dagger} Noguchi Memorial Institute for Medical Research, University of Ghana, Legon, Accra, Ghana and {ddagger} Biotechnology and Nuclear Agriculture Research Institute, Ghana Atomic Energy Commission, Kwabenya, Accra, Ghana

1 Corresponding author: Vector Group, Liverpool School of Tropical Medicine, Pembroke Place, Liverpool L3 5QA, United Kingdom.
E-mail: m.j.donnelly{at}liv.ac.uk

The malaria mosquito Anopheles gambiae s.s. is rapidly becoming a model for studies on the evolution of reproductive isolation. Debate has centered on the taxonomic status of two forms (denoted M and S) within the nominal taxon identified by point mutations in the X-linked rDNA region. Evidence is accumulating that there are significant barriers to gene flow between these forms, but that the barriers are not complete throughout the entire range of their distribution. We sampled populations from across Ghana and southern Burkina Faso, West Africa, from areas where the molecular forms occurred in both sympatry and allopatry. Neither Bayesian clustering methods nor FST-based analysis of microsatellite data found differentiation between the M and S molecular forms, but revealed strong differentiation among different ecological zones, irrespective of M/S status and with no detectable effect of geographical distance. Although no M/S hybrids were found in the samples, admixture analysis detected evidence of contemporary interform gene flow, arguably most pronounced in southern Ghana where forms occur sympatrically. Thus, in the sampled area of West Africa, lack of differentiation between M and S forms likely reflects substantial introgression, and ecological barriers appear to be of greater importance in restricting gene flow.