plex provide an ideal system to test this prediction. Hybrid females are fertile between all of the species pairs of the complex. Therefore, the potential for gene flow persists up to the present, although hybrids are rarely detected in nature (0.02–0.76%) (Temu, 1997; Touré et al., 1998). In any case, the increased genic differentiation between the rearranged chromosomes that would have taken place during the speciation process should be detectable, given that it has occurred within the last few thousand years, rather than hundreds of thousands (as between D. pseudoobscura and D. persimilis) or millions of years (as between humans and chimpanzees) ago.
The currently available evidence is limited but consistent with the model. An investigation of DNA sequence variation in four gene loci (one from each of chromosomes X and 2 and two from chromosome 3) sampled from multiple specimens shows considerable similarity of gene polymorphisms and even haplotypes between the two autosomal chromosomes of A. gambiae and A. arabiensis (Besansky et al., 2003), which share polymorphic inversions. At the X chromosome locus, however, there are fixed nucleotide differences and greater overall nucleotide differentiation between A. gambiae and A. arabiensis. These results are consistent with the suppressed-recombination model because the X chromosomes of the two species are fixed for different inversions, where reproductive factors are located.
The shared nucleotide polymorphisms in the autosomal chromosomes may, however, be ancestral rather than originated by recent gene flow between the two species. An investigation of the ND5 locus of mitochondrial DNA shows that ancestral haplotypes persist in the two species, A. gambiae and A. arabiensis (Donnelly et al., 2004). Nevertheless, comparison of allopatric and sympatric populations suggests locale-specific unidirectional introgression from A. arabiensis into A. gambiae. Indeed, the acquisition by A. gambiae of alleles from the more arid-adapted A. arabiensis may have contributed to its spread and ecological dominance (Besansky et al., 2003). A more definitive test of the suppressed-recombination model of chromosomal speciation would call for estimation of the KA/KS ratio in 20 or more genes distributed among the three chromosomes of these two species, as well as among the other five species and additional incipient species.
It may be worth adding that among the nearly 500 known members of the genus Anopheles, there are no fewer than 170 cryptic taxa belonging to 30 complexes of closely related species (Harbach, 2004). Most siblings are outcomes of recent speciation processes detected by paracentric inversions, mostly involving the X chromosome, as well as ribosomal DNA sequences. The chromosome rearrangements act as mechanisms of crossover suppression for reorganized regulatory units of gene expression.