Speciation in the Anopheles gambiae complex is reviewed and discussed with emphasis on the patterns of chromosomal differentiation, particularly at the intraspecific level. The significance of inversion polymorphism in gambiae and arabiensis (the two species of greatest medical importance) is evaluated with reference to recent field investigations carried out in Nigeria. In both sibling species some of the inversions show clinical geographical changes in frequencies, with evident correlations with climatic conditions and vegetation zones. Microgeographical variations in species distribution and in intraspecific inversion frequencies are also present, which appear mostly related to man-made environmental contrasts. Parallel indoor-/outdoor collections of samples from polymorphic populations of arabiensis and gambiae show that adult mosquitoes carrying certain inversion karyotypes do not distribute at random in relation to the human environment, being significantly more frequent in outdoor than in indoor samples, or vice-versa. Optimal habitat choice appears to be involved in such variations of indoor resting behaviour, since the chromosomal types carried by less endophilic individuals are those more adapted to humid climates, i.e. those which tend to avoid the higher nocturnal saturation deficit of the indoor environment. This phenomenon, producing non-uniform exposure of the vector population to residual insecticides sprayed in houses, might explain the mediocrity of the results of malaria control projects based on house-spraying against endophilic vectors in the African savannas.

Field-collected specimens of all known taxa in the Anopheles gambiae complex were analyzed on the basis of chromosome inversions with reference to a standard polytene chromosome map. The phylogenetic relationships among the seven described species in the complex could be inferred from the distribution of fixed inversions. Nonrandom patterns of inversion distribution were observed and, particularly on chromosome arm 2R, provided evidence for genetically distinct populations in A. gambiae, A. arabiensis, and A. melas. In A. gambiae from Mali, stable genetic differentiation was observed even in populations living in the same region, suggesting a process of incipient speciation which is being confirmed by studies with molecular markers. The possible role of chromosome differentiation in speciation of the A. gambiae complex and in the emergence of distinct chromosomal forms within the nominal species is discussed in relation to human malaria.

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A comparison was made between the cuticular hydrocarbons of five Palaearctic species of the Anopheles maculipennis Meigen complex; A. maculipennis sensu stricto, A. melanoon Hackett, A. messeae Falleroni, A. labranchiae Falleroni and A. atroparvus Van Thiel. Females of these species had their cuticular lipid removed and the hydrocarbons separated and quantified by gas chromatography. Discriminant analysis determined the degree of difference between the species. Wild caught adults of the complex had an average correct classification rate of 77.9%. A. atroparvus and A. labranchiae are homosequential and have no uniquely diagnostic isoenzymes, but expressed distinct hydrocarbon profiles enabling them to be separated in more than 86% of cases. Similarly, A. maculipennis sensu stricto and A. melanoon differ only by minor karyotype alterations, yet their hydrocarbon profiles could be separated with 83% correct classification. A dendrogram was drawn up, based on the hydrocarbons, using the Mahalanobis distances between species. A. maculipennis sensu stricto and A. melanoon were the two closest groups; A. messeae was next to join the cluster, followed by A. labranchiae and then A. atroparvus. These last two species were also very close to each other, but quite distant from A. maculipennis sensu stricto and A. melanoon. The species' relationships based on hydrocarbons thus reflect the tentative chromosomal phylogeny of the complex. In nature, hydrocarbon differences between species may be a device enabling the recognition of suitable mates. Studies showing that hydrocarbon dissimilarity is elevated between sympatric populations are also discussed in support of this theory.

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