Genetic differentiation of the molecular forms of Anopheles gambiae sensu stricto.

Approximately 90% of the world’s malaria-specific mortality occurs in Sub-Saharan Africa and it is primarily transmitted by Anopheles gambiae s.s., the most anthropophylic member of the gambiae complex. Chromosomal studies (based on polymorphic paracentric inversions on chromosome-2) and molecular studies (based on transcribed and non-transcribed spacers in the rDNA repeat unit on chromosome-X) led to the definition of 5 chromosomal and 2 molecular forms, respectively. In contrast to results obtained from the analysis of rapidly evolving DNA (e.g. rDNA and microsatellites), the analyses of coding and non-coding DNA regions scattered throughout the genome has revealed low differentiation between the forms so far, suggesting an ongoing speciation process, with the persistence of common variation shared because of recent ancestry, and low levels of gene flow continuing to homogenize regions of the genome not directly involved in the speciation process. Whether this picture justifies raising the forms to the rank of species is still a matter of discussion, but the most epidemiologically relevant issue is that the observed restrictions to gene flow seem to affect their bionomics and to hinder the circulation of genes associated with insecticide resistance or vector capacity. We present further data in support of the genetic differentiation of molecular forms M and S. The first set of data comes from the sequence analysis of the Intron I of the voltage-gated sodium channel gene in An.gambiae samples from several African Countries. We found several Intron I haplotypes; two of these occurred at very high frequencies over the entire geographical area studied and are separated by a single mutational step, which distinguish two main clusters almost completely corresponding to the S and M molecular forms. The other haplotypes are geographical variants of either the S- or the M-form, mostly linked to limited geografical areas. The second set of data refers to the analysis by TE-differential display of TEs insertion polymorphisms at 20 loci in the two forms. We have compared M and S sympatric populations from 2 villages from Burkina Faso and revealed high level of differentiation among forms. Moreover, TE insertion frequencies at this loci are similar between M populations in Congo and Burkina Faso . These results are particularly relevant for taxonomic inferences, since the above markers are completely independent from chromosomal inversions on chromosomal arm 2R and rDNA on chromosome-X, providing further support towards the specific status of the two molecular forms.