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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The comparison of malaria indicators among populations that have different genetic backgrounds and are uniformly exposed to the same parasite strains is one approach to the study of human heterogeneities in the response to the infection. We report the results of comparative surveys on three sympatric West African ethnic groups, Fulani, Mossi, and Rimaibé, living in the same conditions of hyperendemic transmission in a Sudan savanna area northeast of Ouagadougou, Burkina Faso. The Mossi and Rimaibé are Sudanese negroid populations with a long tradition of sedentary farming, while the Fulani are nomadic pastoralists, partly settled and characterized by non-negroid features of possible caucasoid origin. Parasitological, clinical, and immunological investigations showed consistent interethnic differences in Plasmodium falciparum infection rates, malaria morbidity, and prevalence and levels of antibodies to various P. falciparum antigens. The data point to a remarkably similar response to malaria in the Mossi and Rimaibé, while the Fulani are clearly less parasitized, less affected by the disease, and more responsive to all antigens tested. No difference in the use of malaria protective measures was demonstrated that could account for these findings, and sociocultural or environmental factors do not seem to be involved. Known genetic factors of resistance to malaria did not show higher frequencies in the Fulani. The differences in the immune response were not explained by the entomological observations, which indicated substantially uniform exposure to infective bites. The available data support the existence of unknown genetic factors, possibly related to humoral immune responses, determining interethnic differences in the susceptibility to malaria.The ability to resist Plasmodium falciparum malaria is an important adaptive trait of human populations living in endemic areas. The detection of significant differences in the expression of this trait and the identification of the factors involved should improve the understanding of the hostparasite relationship and might lead to advances in control strategies (1). The importance of this research area is emphasized by the recent finding that particular HLA class I and class II alleles are associated with resistance to severe malaria (2) and by the complexities faced in the design of an effective vaccine against P. falciparum (3).The study of human heterogeneities in the response to malaria can be approached at the population level by comparing either groups with similar genetic backgrounds exposed to different disease selective pressures (4)-e.g., the same population distributed over endemic and non-endemic zones-or groups with different genetic backgrounds exposed to similar parasite inoculation rates-e.g., different ethnic groups living together in an endemic zone. The interethnic comparison has been only rarely pursued (5-8), presumably deterred by the high number of possible confounding factors due to sociocultural differences. Following a preliminary survey ...