Patterns of DNA sequence variation in chromosomally recognized taxa of Anopheles gambiae: evidence from rDNA and single‐copy loci

Abstract

Patterns of DNA sequence variation in the ribosomal DNA (rDNA) second internal transcribed spacer (ITS2) and five unlinked single-copy nuclear loci were examined for evidence of reproductive isolation among four chromosomally recognized taxa of Anopheles gambiae from West Africa: Savanna, Bamako, Mopti and Forest, as well as sibling species An. arabiensis and An. merus. Included among the single-copy loci were three sequence-tagged random amplified polymorphic DNA (RAPD) loci, two of which (R15 and R37) had been reported as discriminating between Mopti and other chromosomal forms. Each of the five single-copy sequences were highly polymorphic in most samples. However, the R15 and R37 loci had no diagnostic value, and therefore are not recommended as tools in recognition of field-collected An. gambiae chromosomal forms. Although pairwise comparisons between species generally revealed significant levels of differentiation at all five loci, variation was not partitioned by chromosomal form within An. gambiae at any single-copy locus examined. The few exceptions to these trends appear related to a location either inside or nearby chromosomal inversions. At the tryptophan oxygenase locus inside inversion 2Rb, variation was structured only by inversion orientation and not by taxonomic designation even between An. gambiae and An. arabiensis, providing the first molecular evidence that the 2Rb inversion was transferred between species by introgressive hybridization. By contrast, the rDNA showed fixed differences between species and a difference diagnostic for Mopti, consistent with effective, if not complete, reproductive isolation. The apparent disagreement between the data from this locus and multiple single-copy loci within An. gambiae may be explained by the much lower effective population size of rDNA, owing to concerted evolution, which confers increased sensitivity at much shorter divergence times. Taken together with the accompanying reports by della Torre et al. (2001), Favia et al. (2001) and Gentile et al. (2001), our data suggest that neutral molecular markers may not have the sensitivity required to detect isolation between these recently established taxa.

Ecological Zones Rather Than Molecular Forms Predict Genetic Differentiation in the Malaria VectorAnopheles gambiaes.s. in Ghana

“…The other loci (including the rDNA) are at the proximal (centromeric) end of the X chromosome Lehmann et al 2003). However, little differentiation has been found at loci elsewhere in the genome (Gentile et al 2001;Mukabayire et al 2001;Wang et al 2001;Turner et al 2005). Debate is ongoing as to whether such generally limited differentiation primarily reflects insufficient time for differentiation to have occurred or ongoing introgression among forms.…”

Population genetic structure of the malaria vector Anopheles minimus A in Vietnam

“…Yet, the taxonomic status of these chromosomal forms is still debated (Black and Lanzaro, 2001). In an extensive study addressing this problem, Mukabayire et al (2001) suggested that neutral markers may not have the sensitivity required to detect isolation between recently established taxa. If this is true, then the Tinh Quang population might be much more isolated than we estimated.…”

Comparative genomic analysis in the region of a major Plasmodium -refractoriness locus of Anopheles gambiae

“…A. gambiae (sensu strictu) has been distinguished into several different ecotypes on the basis of floating chromosomal inversions, whose prevalence varies geographically and seasonally, suggesting that they may ''lock in'' specific allelic combinations important for malaria transmission (35,36). More recently, a small number of molecular markers have also suggested the existence of flexibility and even mosaicism in the A. gambiae genome (37,38). The high level of polymorphism that we detected between BAC 8N20 and the Consensus Sequence is reminiscent of the human MHC locus, which encompasses at least 200 genes, most of which are highly polymorphic and have roles in immune responses (39).…”