The distribution and inversion polymorphism of chromosomally recognized taxa of the Anopheles gambiae complex in Mali, West Africa

Data from polytene chromosome studies on the Anopheles gambiae complex in Mali were reviewed. The banding pattern was successfully scored in 17,705 specimens from 76 sampling sites representing the main ecological strata of the country. Two members of the complex, namely An. arabiensis and An. gambiae, were found widespread and frequently sympatric, with the latter prevalent in most localities. Population genetic analysis of the inversion polymorphisms indicated the existence of panmictic conditions for An. arabiensis only, whereas the parallel study of An. gambiae supported its splitting into at least three reproductive units, characterized by different 2R chromosome arrangements, designated Bamako, Mopti and Savanna. The chromosomal evidence was consistent with the hypothesis of complete reproductive isolation between Bamako and Mopti. Partial isolation between these two taxa and Savanna was suggested by the scoring of hypothetical hybrid 2R heterokaryotypes in various samples, but the actual hybrid origin of these specimens was not confirmed Different patterns of geographical and seasonal distribution were shown as follows. An. arabiensis prevails in arid savannas (Sahel and Northern Sudan savanna) out of the flooded or irrigated zones: it is able to withstand the most arid conditions of Saharan localities and its breeding might extend throughout the dry season. An. gambiae Savanna and Bamako prevail in relatively humid savannas (Southern Sudan savanna) and their breeding generally occurs only during the rainy season. The Savanna taxon was almost absent in flooded or irrigated zones and in riverine localities; the Bamako taxon is distributed along the upper river Niger and its tributaries. An. gambiae Mopti extends its range in all ecological zones present in Mali including the Sahel and predesertic areas, showing high relative frequencies up to absolute dominance in flooded or irrigated areas; its breeding is highly successful also during the dry season. Rainfall at the sampling sites was found to correlate positively with the frequency of Savanna and negatively with the frequency of Mopti. The remarkable ecological flexibility of the latter was found associated with wide seasonal and geographical variations in its 2R inversion polymorphism bc/u. Higher frequencies of the be arrangement were recorded both in the Southern localities during the dry season and in the Northern more and localities during the rainy season. The absence or scarcity of An arabiensis and An. gambiae Savanna in most flooded or irrigated zones suggests their competitive exclusion by An. gambiae Mopti.