ECOLOGICAL GENETIC-STUDIES IN THE CHROMOSOMAL FORM MOPTI OF ANOPHELES-GAMBIAE S-STR IN MALI, WEST-AFRICA

Among the sibling species of the Afrotropical Anopheles gambiae comple x, the nominal taxon (An. gambiae s.str.) is the major malaria vector. Its bionomics suggest a man-dependent speciation process which involv es, in West Africa, various inci pient species chromosomally recognize d by different combinations of 2R paracentric inversions. One of the m ost recent evolutionary steps of such a speciation process appears to be the chromosomal form Mopti, which is associated with dry season irr igation in arid zones, and is characterized by a remarkable ecological flexibility related to three 2R alternative arrangements, namely be, u and +, whose expected karyotypes are found in Hardy-Weinberg equilib rium. The study of this chromosomal polymorphism in samples from a 16- locality transect in Mall shows wide variations and highly significant correlation with both temporal and spatial climatic differences. Mosq uitoes homokaryotypic for 2Rbc are the actual dry season and arid area s breeders. The regular rise of 2Rbc frequency, up to fixation, during each dry season, corresponds to the South-North clinal increase of th e same arrangement along the transect, from about 30% in the humid sav anna to near fixation in the South-Saharan zone. This coherent ecologi cal genetics case provides full support to the hypothesis of the adapt ive nature of paracentric inversions. Moreover, the very peculiar syst em of combinations of contiguous 2R inversions, utilized by Mopti as w ell as by other chromosomal forms of An. gambiae, suggests a process o f polygenic reorganization based on linkage disequilibria and involvin g the inversions as driving selection units.