Linkage studies on DDT and dieldrin resistance in species A and species B of the Anopheles gambiae complex.

Linkage studies in A. gambiae species A showed that the two autosomal markers diamond (Di) and collarless (c) belonged to different linkage groups: Di on linkage group 2 and c on linkage group 3. With the aid of the two markers it was possible to establish the linkage relationship between the genes for DDT resistance and for the two types of dieldrin resistance. The DDT resistance gene in species A assorted independently of c and was found to be linked with Di at a map distance of 26.3 units. The genes for the two types of dieldrin resistance, dominant and incompletely dominant, were found to be allelic. The major gene for dieldrin resistance was shown to be linked with the gene for DDT resistance in linkage group 2 at a map distance of 35.3 units. This suggested that Di and the dieldrin resistance gene were some 61.6 units apart.

Inheritance of DDT resistance in species A and B of the Anopheles gambiae complex.

It has been reported that Anopheles gambiae species A is resistant to DDT in Upper Volta and Togo, and DDT resistance has been found in a Sudan strain of species B. The species A strain from Upper Volta was more resistant to DDT than the Togo strain, while the Sudan strain of species B was the least resistant. The expression of resistance is genetically determined, and studies on the mode of inheritance have shown that resistance is inherited as a single dominant gene in the Togo strain of species A and the Sudan strain of species B, but that it is inherited as a single incompletely dominant gene in the Upper Volta strain of species A.

Partial exophily of Anopheles gambiae species B in the Khashm Elgirba area in eastern Sudan.

Many Anopheles gambiae were found resting outdoors in an area that had been sprayed with DDT, the numbers being particularly large 4 months after the first annual spraying cycle. A high proportion of the resting females had consumed a blood meal. Window-trap studies showed that a marked exodus from sprayed huts occurred in the morning. It is concluded that the outside resting of An. gambiae is either a natural habit or the result of the irritant action of DDT. This behaviour of the vector is regarded as one of the factors responsible for the marked increase of malaria in the area.

Cytogenetics of the Anopheles gambiae complex in Sudan, with special reference to An. arabiensis: relationships with East and West African populations.

The species composition of malaria vector mosquitoes belonging to the Anopheles gambiae complex (Diptera: Culicidae) from >40 localities in Sudan, representing most ecological situations, was determined by analysis of ovarian polytene chromosomes. Of 2162 females, 93% were identified as An. arabiensis Patton and 7% were An. gambiae Giles sensu stricto. No hybrids were found between the two species. Anopheles arabiensis occurred in all but two sites, whereas An. gambiae s.s. was effectively limited to the southernmost, more humid localities. For chromosomal paracentric inversions, the degree of polymorphism was low in An. gambiae s.s. (inversions 2La, 2Rb and 2Rd), higher in An. arabiensis (inversions Xe, 2Ra, b, bc, d1, s; 3Ra, d). Anopheles gambiae samples from Sudan were all apparently panmictic, i.e. they did not show restricted gene flow such as observed among West African populations (interpreted as incipient speciation). Chromosomal inversion patterns of An. gambiae in southern Sudan showed characteristics of intergrading Savanna/Forest populations similar to those observed in comparable eco-climatic situations of West Africa. Anopheles arabiensis was polymorphic for inversion systems recorded in West Africa (2Ra, 2Rb, 2Rdl, 3Ra) and for a novel 2Rs polymorphism, overlapping with inversion systems 2Rb and 2Rd1. Samples carrying the 2Rs inversion were mostly from Khashm-el-Girba area in central-eastern Sudan. In the great majority of the samples all polymorphic inversions were found to be in Hardy-Weinberg equilibrium. Sudan populations of An. arabiensis should therefore be considered as generally panmictic. Anopheles arabiensis shows more inversion polymorphism in west than in east African populations. Sudan populations have more evident similarities with those from westwards than those from eastwards of the Great Rift Valley. The possible influence of the Rift on evolution of An. arabiensis is discussed.