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.