Attempts to molecularly distinguish cryptic taxa in Anopheles gambiae s.s.

Analyses of inversions in polytene chromosomes indicate that, in West Afric a, Anopheles gambiae (sensu stricto) may be a complex of more than a single taxonomic unit, and these units have been called chromosomal forms. In ord er to determine whether this genetic discontinuity extends to the rest of t he genome, as would be expected if reproductive isolation exists, we have s equenced several regions of both the nuclear and mitochondrial genomes. Wit h one exception, we were unable to identify any nucleotide sites that diffe rentiate the chromosomal forms. The exception was the internal transcribed spacer (ITS) of the ribosomal DNA (rDNA). Three sites in this region distin guish Mopti chromosomal form from Savanna and Bamako in Mall and Burkina Fa so. However, outside these two countries, the association between chromosom al form and rDNA type does not always hold. Together with the variants in t he rDNA intergenic spacer (IGS) described in the accompanying papers (della Torre et al., 2001; Favia et al., 2001), we can recognize two major types of rDNA, Type I and Type II (corresponding to molecular forms S and M in de lla Torre et al,, 2001). Type I is widespread in West Africa and is the onl y type found outside of West Africa (i.e. Tanzania and Madagascar). Type II is confined to West Africa,We were unable to detect any heterozygosity for the ITS types even in five collections containing both types. A sample fro m the island of Sao Tome could not be classified into either Type I or Type II as the rDNA had characteristics of both. In general, our results confir m that An. gambiae is not a single pan-mictic unit, but exactly how to defi ne any new taxa remains problematic. Finally, we have found minor variants of the major rDNA types fixed in local populations; contrary to most previo us studies, this suggests restricted gene flow among populations of this sp ecies.