Abstract
BackgroundAnopheles gambiae sensu stricto, one of the principal vectors of malaria, has been divided into two subspecific groups, known as the M and S molecular forms. Recent studies suggest that the M form found in Cameroon is genetically distinct from the M form found in Mali and elsewhere in West Africa, suggesting further subdivision within that form.MethodsChromosomal, microsatellite and geographic/ecological evidence are synthesized to identify sources of genetic polymorphism among chromosomal and molecular forms of the malaria vector Anopheles gambiae s.s.ResultsCytogenetically the Forest M form is characterized as carrying the standard chromosome arrangement for six major chromosomal inversions, namely 2La, 2Rj, 2Rb, 2Rc, 2Rd, and 2Ru. Bayesian clustering analysis based on molecular form and chromosome inversion polymorphisms as well as microsatellites describe the Forest M form as a distinct population relative to the West African M form (Mopti-M form) and the S form. The Forest-M form was the most highly diverged of the An. gambiae s.s. groups based on microsatellite markers. The prevalence of the Forest M form was highly correlated with precipitation, suggesting that this form prefers much wetter environments than the Mopti-M form.ConclusionChromosome inversions, microsatellite allele frequencies and habitat preference all indicate that the Forest M form of An. gambiae is genetically distinct from the other recognized forms within the taxon Anopheles gambiae sensu stricto. Since this study covers limited regions of Cameroon, the possibility of gene flow between the Forest-M form and Mopti-M form cannot be rejected. However, association studies of important phenotypes, such as insecticide resistance and refractoriness against malaria parasites, should take into consideration this complex population structure.
Highlights
Anopheles gambiae sensu stricto, one of the principal vectors of malaria, has been divided into two subspecific groups, known as the M and S molecular forms
In Mali, S forms were dominant below latitude 12°N in mostly savanna regions, while M forms were predominant above 15°N where land cover types are mostly grasslands or open shrublands (Table 1, Figure 1)
Chromosome inversion polymorphism was examined on the second chromosome, 2La, 2Rj, 2Rb, 2Rc, 2Rd and 2Ru
Summary
One of the principal vectors of malaria, has been divided into two subspecific groups, known as the M and S molecular forms. An extensive literature describes a highly complex population genetic structure on a much smaller spatial scale in West and Central Africa [2,3,4,5,6,7,8] Touré and his colleagues [5] examined the distribution of five paracentric chromosome inversions on the right arm of chromosome 2 (2R j, b, c, d and u). Analysis of karyotype frequencies among An. gambiae s.s. collected from a number of villages led to the identification of discrete subpopulations within this species which can be distinguished by their karyotype These subpopulations were given non-Linnean designations and are collectively known as "chromosomal forms" [2,5]. Detailed analysis of populations where multiple forms exist sympatrically revealed a high degree of reproductive isolation between the forms, with a strong preference for mating within rather than between forms [8]
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