Cytological and Molecular Characterization of Populations of the Anopheles Gambiae Giles Complex in a Coastal Savanna Zone of Ghana

Abstract

The effective planning of vector control strategies, genetic manipulation of vector species and the management of insecticide resistance require information on the population structure and the assessment of gene flow between vector populations. Although the appetitive flight of mosquitoes is known to be less than 5 km, most studies assessing gene flow between An. gambiae populations have been conducted over geographical distances as far apart as 6000km. For vector control, operational purposes such as the use of insecticide treated materials within a particular country setting, the gene flow between populations that are some 10-100 Km is more important To assess the level of gene flow between populations over short distances, five populations of Anopheles mosquitoes from sites covering an area of about 25 km in diameter and located in the Coastal zone of Ghana were studied. One hundred and twenty-nine mosquitoes were identified by cytotaxonomy and PCR. A subset of these were fully karyotyped, but all were analyzed for three microsatellite DNA loci: AGXH7, AG2H26 and AG2H46. Karyotype analysis revealed the presence of the ‘Forest’ and ‘Savanna’ forms of An. gambiae s.s. characterized by the karyotypes 2R+, 2L+ and 2R+, 2La respectively. The frequency of inversion a on the chromosomal arm 2L varied from 0% - 80% between populations that were furthest apart and differing in ecology from coastal forest to coastal savanna. A total of 30 polymorphic microsatellite alleles were observed. Number of alleles per locus ranged from nine to twelve (Mean = 10 per locus). Inter-population differentiation was highest between populations that were furthest apart (Frt = 0.0565) and lowest in closer populations (Fa = 0.0284). Accordingly, estimates of gene flow were higher in closer populations and lower in those further apart. Heterozygotes were in excess at the centrally located site (Fi„ = 0.0435) but deficient in others suggesting the presence of a possible hybrid zone in the study area. The study established the presence of two distinct karyotypic populations of An. gambiae s.s in the coastal zone of Ghana and also revealed that levels of gene flow between populations decreased after 5km. The importance of karyotyping and microsatellite analysis in the characterization of vector populations is discussed.