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We karyotyped and identified by polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analysis Anopheles gambiae s.s. samples collected in several African countries. The data show the existence of two non-panmictic molecular forms, named S and M, whose distribution extended from forest to savannahs. Mosquitoes of the S and M forms are homosequential standard for chromosome-2 inversions in forest areas. In dry savannahs, S is characterized mainly by inversion polymorphisms typical of Savanna and Bamako chromosomal forms, while M shows chromosome-2 arrangements typical of Mopti and/or Savanna and/or Bissau, depending on its geographical origin. Chromosome-2 inversions therefore seem to be involved in ecotypic adaptation rather than in mate-recognition systems. Strong support for the reproductive isolation of S and M in Ivory Coast comes from the observation that the kdr allele is found at high frequencies in S specimens and not at all in chromosomal identical M specimens. However, the kdr allele does not segregate with molecular forms in Benin.

Afrotropical malaria vectors of the Anopheles gambiae complex (Diptera: Culicidae), particularly An. gambiae sensu stricto, are attracted mainly to human hosts. A major source of human volatile emissions is sweat, from which key human-specific components are the carboxylic acids (E)- and (Z)-3-methyl-2-hexenoic acid and 7-octenoic acid. Electrophysiological studies on the antennae of An. gambiae s.s. showed selective sensitivity to these compounds, with a threshold at 10(-6) g comparable to that of known olfactory stimulants 1-octen-3-ol, P-cresol, isovaleric acid, and lower than threshold sensitivity to L-lactic acid and the synthetic mosquito repellent N,N-diethyltoluamide (DEET). A combination of the acids released at concentrations > 10(-5) g in wind tunnel bioassays significantly reduced the response to CO2, the major attractant released by human hosts, for strains of An. gambiae s.s. originating from East and West Africa. Field trials with odour-baited entry traps (OBETs) in Burkina Faso showed that 7-octenoic acid significantly increased (by 1.7-fold) the catch of females of An. gambiae sensu lato (comprising two sibling species: An. arabiensis Patton and An. gambiae s.s.) in OBETs baited with CO2, whereas combinations of the acids significantly reduced the catch in CO2-baited traps (by 2.1-fold) and in whole human odour-baited traps (by 1.5-fold). The pure (E) and (Z) geometric isomers of 3-methyl-2-hexenoic acid gave comparable results to the (E/Z) isomer mixture. These results provide the first experimental evidence that human-specific compounds affect the behaviour of highly anthropophilic An. gambiae s.l. mosquitoes. The compounds appear to inhibit the 'upwind flight' response to known long-range attractants, and may serve either to 'mask' the attractants present or, more probably, to 'arrest' upwind flight when mosquitoes arrive at a host under natural conditions. In the final approach to hosts, vectors are known to reduce their flight speed and increase their turning rate, to avoid overshooting the source. In our experimental apparatus, these changes in flight behaviour would reduce the number of mosquitoes entering the ports of the collection devices.

Chromosomal rearrangements abound in nature and can be studied in detail in organisms with polytene chromosomes. In Drosophila and in Anopheline mosquitoes most speciation processes seem to be associated with the establishment of chromosomal rearrangements, particularly of paracentric inversions. It is not known what triggers inversions in natural populations. In the laboratory inversions are commonly generated by X-rays, mutagens or after the activity of certain transposable elements (TEs). The Anopheles gambiae complex is comprised of six sibling species, each one characterized by the presence of fixed paracentric inversions on their chromosomes. Two of these, An. gambiae s.s. and An. arabiensis, are the most important vectors of human malaria and are structured into sub-populations, each carrying a characteristic set of polymorphic chromosomal inversions. We have cloned the breakpoints of the naturally occurring polymorphic inversion In(2R)d' of An. arabiensis. Analysis of the surrounding sequences demonstrated that adjacent to the distal breakpoint lies a transposable element that we called Odysseus. Characteristics of Odysseus' terminal region and its cytological distribution in different strains as well as within the same strain indicate that Odysseus is an actively transposing element. The presence of Odysseus at the junction of the naturally occurring inversion In(2R)d' suggests that the inversion may be the result of the TEs activity. Cytological evidence from Drosophila melanogaster has also implicated the hobo transposable element in the generation of certain Hawaiian endemic inversions. This picture supports the hypothesis of the important role of TEs in generating natural inversions.

We review here what is known about the population structure and evolutionary dynamics of members of the Anopheles gambiae complex with emphasis on the situation in West Africa. First, the importance of the 2nd chromosome inversion polymorphism is demonstrated especially in adaptation to levels of aridity, a major environmental variable in Africa. This affects the distribution of karyotypes on both a macro- and micro-geographic scale as well as temporally. Such differentiation leads to karyotypes being differentially effective transmitters of malaria and differentially susceptible to indoor residual spraying of insecticides. Second, we review the evidence that cryptic taxa, especially in An. gambiae s.s., exist. This observation stems from both karyotype studies and molecular studies. It is abundantly clear that West African populations of An. gambiae s.s. are often not panmictic units, with premating factors evidently acting to maintain distinct genetic forms. Third, we review phylogenetic studies that have revealed the presence of introgression between the two most important vectors, An. gambiae and An. arabiensis. This is most evident for the 2nd chromosome inversions. This interpretation of phylogenetic data is consistent with a direct laboratory study indicating inversions in this chromosome are stably maintained in back-crossed populations. All of this information has led to the view that members of the An. gambiae complex are highly variable with an abundance of adaptive genetic variation. This presents a significant challenge to vector control programs designed to reduce malaria in sub-Saharan Africa.

Les anophèles du complexe #Anopheles gambiae$ sont avec #Anopheles funestus$ les vecteurs du paludisme au Sénégal. La détermination précise des espèces vectrices et une connaissance approfondie de leur biologie sont indispensables à la mise en place de stratégies de lutte antivectorielle efficaces et adaptées à chaque contexte épidémiologique. Les espèces du complexe #An. gambiae$, indifférenciables morphologiquement, sont maintenant identifiées par PCR à partir d'une seule patte. L'espèce #An. funestus$ présente dans certaines régions du Sénégal des comportements inhabituels, en particulier au niveau du taux d'anthropophilie. Les premiers résultats de l'étude cytogénétique menée sur des populations d'origine géographique variée sont dès maintenant en faveur de l'existence d'au moins deux espèces. Une étude gène-enzyme est menée parallèlement pour confirmer ces résultats. Ces recherches visent à améliorer la détermination des vecteurs du paludisme au Sénégal et à préciser leur statut taxonomique. (Résumé d'auteur)

Une étude de la transmission du paludisme à Cotonou, ville côtière, a été réalisée au cours d'une surveillance entomologique de 12 mois dans trois quartiers représentatifs de différents aspects de la ville. La densité du vecteur (#A. gambiae$ s.l.) varie selon le degré d'urbanisation des quartiers. Le nombre moyen annuel de piqûres est égal à 1179 au centre de la ville, 3666 dans les quartiers périphériques et 3363 en zones intermédiaires. L'indice sporozoïtique moyen est de 1,7 %. Au centre de la ville, la transmission est saisonnière et dure 3 mois pendant lesquels 33 piqûres infectées sont reçues par homme. L'indice sporozoïtique maximum observé est de 12 % avec un taux quotidien d'inoculation de 1,02. En ceinture autour de la ville, la transmission est également saisonnière mais dure huit mois pendant lesquels 58 piqûres d'anophèles infectés sont reçues par homme. Contrairement à d'autres villes continentales d'Afrique intertropicale, l'intensité de la transmission à Cotonou est élevée et est de 46 piqûres infectées/homme/an. (Résumé d'auteur)

Aedes albopictus (Skuse), the important Asian vector mosquito recently introduced in United States and Brazil, is reported from Genoa, North Italy. The infestation was discovered in a kindergarten pre-school center in September 1990 just after the summer holidays. Many discarded tires, well known to provide excellent breeding places for Ae albopictus, had been left in the school playground to be used as toys by the children. After sampling a few biting mosquito specimens for identification, the local health service carried out on September 18 an extensive indoor/outdoor treatment with pyrethroid insecticide. The extent of the infestation in the city of Genoa and in other areas of the Ligurian region has not been evaluated since the identification of the species was available in October, at the end of Ae albopictus reproductive period. A general survey in various Italian regions is being planned for the 1991 spring-summer period. The present record, together with the previous report of Ae albopictus in Albania, clearly supports the hypothesis of a spreading of the species in the Mediterranean area.

To evaluate the factors determining malaria transmission level, entomological and parasitological surveys were conducted from October 1988 to February 1990 in MANARINTSOA, in the central highland plateaux of MADAGASCAR. Mosquitoes were collected during 928 man-nights of captures, in pit shelters and in indoors resting sites. The malaria vectors were An. arabiensis and An. funestus, with no evidence of the presence of An. gambiae s.s. The vectors were mainly exophilic and zoophilic. The index of stability was below 1.5. The sporozoite rate was 0.11 for An. gambiae s.l. and 0.47 for An. funestus. The transmission level was low: the inoculation rate was 0.91 infected bite per man and per year and the infection risk 0.62. Transmission occurs 7 months per year, from November to May. In the human population parasite rates fluctuated from 29% in October to 53% in May.