Chromosomal study of Anopheles gambiae and Anopheles arabiensis in Ouagadougou (Burkina Faso) and various neighboring villages

Adult females of Anopheles gambiae s.1. were collected by pyrethrum spray catch in Ouagadougou (Burkina Faso, formerly Upper Volta) and in four neighbouring villages. The collections have been carried out mostly during the 1984 rainy season. Monthly collections in some sampling sites allowed a preliminary longitudinal study. By analysis of nurse cell polytene chromosomes in adult females, An. gambiae s.str. and An. arabiensis were identified in the study area. Both species showed polymorphisms for various paracentric inversions. In all samples of An. arabiensis the frequencies of the alternative karyotypes were in Hardy-Weinberg equilibrium, suggesting panmictic conditions. Conversely, An. gambiae s.str. showed a different situation, since most of its samples had strong deficiency of certain expected heterokaryotypes. This same phenomenon was already observed in Mali, leading to the splitting of gambiae s.str. into different chromosomal forms, partially or totally reproductively isolated from each other. Each chromosomal form is characterized by different chromosomal polymorphisms. Two of these forms, Mopti and Savanna, were detected in the study area. Mopti chromosomal form is apparently associated with the presence of permanent waters (i.e. the "barrages" north of the town), while Savanna is usually found in situations where breeding places are mainly dependent from rain (e.g. in villages far from "barrages" or at the town's center).