Mapping distributions of chromosomal forms of Anopheles gambiae in West Africa using climate data M. N. BAYOH, C. J. THOMAS and S. W. LINDSAY Department of Biological Sciences, University of Durham, U.K. Abstract. The mosquito Anopheles gambiae Giles sensu stricto (Diptera: Culicidae), the principal vector of malaria in West Africa, comprises several chromosomal forms (e.g. Bissau, Forest, Mopti, Savanna) associated with climatic zones. Here we show how climate data can be used to map the geographical distribution of these chromosomal forms. The climate at 144 sites surveyed for mosquitoes in West Africa between 1971 and 92 was determined using computerized climate surfaces. Forest and Bissau forms occurred at relatively wet sites: median annual precipitation 1325 mm and 1438 mm, respectively, interquartile ranges (IQR) 1144±1858 mm and 1052±1825 mm), whilst the Mopti form was found at dry sites (annual 938 mm, IQR 713±1047 mm) and the Savanna form at sites intermediate between the wet and dry forms (annual 1067 mm, IQR 916±1279). Logistic regression analyses of the climate variables were carried out on a strati®ed random sample of half the sites. The resulting models correctly classi®ed over 80% of the sites for presence or absence of each chromosomal form. When these models were tested against excluded sites they were also correct at over 80% of sites. The combined data produced models that were correct at over 86% of sites. Mean annual precipitation, evapotranspiration, minimum temperature and maximum temperature were the most important climate variables correlated with the distribution of these forms of An. gambiae. We used the logistic models to map the distribution of each chromosomal form within the reported range for An. gambiae s.s. in West Africa employing a geographical information system. Our maps indicate that each chromosomal form favours particular climate envelopes in well-de®ned ecoclimatic zones, although these forms are sympatric at the edges of their ranges. This study demonstrates that climate can be used to map the distribution of chromosomal forms of insects across large areas. Key words. Anopheles gambiae, chromosomes, climatic zones, cytotypes, evapotranspiration, forest, GIS, incipient species, inversion polymorphism, karyotypes, malaria vectors, mapping, mosquito ecology, savanna, spatial ecology, West Africa. Introduction Mosquitoes of the Anopheles gambiae complex are amongst the most important vectors of malaria in tropical Africa (White, 1974; Coluzzi, 1984). The complex consists of at least six sibling species exhibiting varying degrees of ecological and behavioural differences and, consequently, important contrasts in their vectorial ef®ciency. Three of the sibling species are adapted to freshwater breeding sites: An. gambiae Giles sensu stricto predominates in humid areas and is generally highly anthropophilic; An. arabiensis Patton extends into drier savanna areas, is more zoophilic and exophilic; and An. quadriannulatus Theobald is zoophilic, comprising allopatric taxa in Ethiopia and southern Africa (Hunt et al., 1998). Two saltwater-tolerant species, An. melas Theobald of West Africa and An. merus Donitz of East Africa, are generally more exophagic and zoophilic and thus less ef®cient vectors than An. gambiae s.s. The sixth species, An. bwambae White occurs around hot springs in Uganda (Gillies & DeMeillon, 1968). Sympatry of two or more sibling species is a common phenomenon among members of the An. gambiae complex Correspondence: Dr S. W. Lindsay (Ecology) and Dr C. J. Thomas (GIS), School of Biological and Biomedical Sciences, University of Durham, South Road, Durham DH1 3LE, U.K. E-mail: S.W.Lindsay@durham.ac.uk and C.J.Thomas@durham.ac.uk ã 2001 Blackwell Science Ltd 267 Medical and Veterinary Entomology (2001) 15, 267±274 Medical and Veterinary Entomology (2001) 15, 267±274