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

Patterns of DNA sequence variation in the ribosomal DNA (rDNA) second inter nal transcribed spacer (ITS2) and five unlinked single-copy nuclear loci we re examined for evidence of reproductive isolation among four chromosomally recognized taxa of Anopheles gambiae from West Africa: Savanna, Bamako, Mo pti and Forest, as well as sibling species An. arabiensis and An. merus. In cluded among the single-copy loci were three sequence-tagged random amplifi ed polymorphic DNA (RAPD) loci, two of which (R15 and R37) had been reporte d 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 recom mended as tools in recognition of field-collected An. gambiae chromosomal f orms. Although pairwise comparisons between species generally revealed sign ificant levels of differentiation at all five loci, variation was not parti tioned by chromosomal form within An. gambiae at any single-copy locus exam ined. The few exceptions to these trends appear related to a location eithe r inside or nearby chromosomal inversions. At the tryptophan oxygenase locu s inside inversion 2Rb, variation was structured only by inversion orientat ion and not by taxonomic designation even between An. gambiae and An. arabi ensis, providing the first molecular evidence that the 2Rb inversion was tr ansferred between species by introgressive hybridization. By contrast, the rDNA showed fixed differences between species and a difference diagnostic f or Mopti, consistent with effective, if not complete, reproductive isolatio n. The apparent disagreement between the data from this locus and multiple single-copy loci within An. gambiae may be explained by the much lower effe ctive 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 rec ently established taxa.