Chromosomal inversion intergradation and incipient speciation inAnopheles gambiae

Abstract

Polytene chromosome studies on the Afrotropical mosquitoes of the Anopheles gambiae complex show that the rearrangements of the banding pattern are all based on paracentric inversions. Some of these occur at the homozygous state only (fixed inversions) and are used for cytotaxonomic identification of the six sibling species. Others occur as intraspecific chromosomal polymorphisms which are frequent particularly in An. gambiae and An. arabiensis, the two taxa showing the widest distribution and the closest association with man and man-made breeding places. A particularly high level of chromosome differentiation has been recorded in Anpoheles gambiae (the most anthropophilic taxon of the complex and possibly the one most recently evolved) which can be split into forest and savanna chromosomal forms. The forest form seems chromosomally uniform, being characterized in all the rain forest areas by the chromosome-2 standard arrangement, probably one of the most primitive in the complex. The forest form intergrades with different savanna chromosomal forms, all of which share one 2L inversion (2La) and are further differentiated by one or more 2R inversions. An apparently unique savanna form, characterized by the 2Rb inversion, is found in East Africa, while in West Africa the same arrangement occurs together with many other 2R inversions or inversion associations. Intergradation is incomplete or even missing between some of these savanna karyotypes, so that different chromosomally characterized incipient species can be recognized which are indicated with a non-Linnean nomenclature as Mopti, Bamako, Bissau and Savanna. Their comparative study shows no reproductive barriers in laboratory conditions, morphological differences and genetic distances (Nei's index) nearly undetectable, and similar bionomical characteristics with patterns of spatial and seasonal distribution possibly influenced by competitive exclusion. Complete reproductive isolation has been documented between Mopti and Bamako in Mali but gene flow theoretically exists between these taxa, since both hybridize with Forest-Savanna intergrading karyotypes.