The distribution of M and S molecular forms of Anopheles gambiae in Nigeria.

The distribution of M and S molecular forms of Anopheles gambiae sensu stricto across Nigeria was determined. The molecular form of 40 to 45 specimens per locality from 9 localities was determined using mostly the same specimens from our recent study of genetic differentiation of A. gambiae across Nigeria (Onyabe & Conn, 2001). These samples were previously genotyped at 10 microsatellite loci, 5 located within chromosome inversions and 5 outside inversions. Both molecular forms occurred throughout the country, with no apparent relationship to the ecological transition from dry savannah in the north to humid forest in southern Nigeria. In all localities, however, 1 form or the other occurred virtually exclusively. No hybrids between forms were found. Across all loci, F(ST) values were as high within molecular forms as between forms. Regardless of molecular form, F(ST) values calculated across loci within inversions were much higher (range 0.0016 to 0.1988) than those calculated across loci outside inversions (range -0.0035 to 0.0260). Genetic distance was not significantly correlated with geographical distance within either form (P> 0.05). These observations suggest that, in addition to partial reproductive barriers between molecular forms, selection is a major factor shaping genetic differentiation of A. gambiae across Nigeria.

Sex chromosome variation and cytotaxonomy of the onchocerciasis vector Simulium squamosum in Cameroon and Nigeria.

On the basis of sex chromosome variation, three cytotypes of Simulium squamosum (Enderlein) (Diptera: Simuliidae) are described from Cameroon and Nigeria. Simulium squamosum A is the typical form as originally described by Vajime & Dunbar (1975) with chromosome I as the sex chromosome. It occurs throughout most of Cameroon and south-east Nigeria. A second cytotype, S. squamosum B, is described from the river Sanaga (Cameroon). It also has chromosome I as the sex chromosome, but the nature of the sex differential region is different. Simulium squamosum C has no sex-linked chromosomal rearrangements. It is widespread in Nigeria and occurs near Mount Cameroon, where it seems to hybridize with S. squamosum A.

The cytotaxonomy of Simulium damnosum s.l. (Diptera: Simuliidae) from the Thyolo onchocerciasis focus in Malawi and description of a new member of the complex.

Larvae of the Simulium damnosum complex were collected in southern Malawi, in particular from the Thyolo onchocerciasis focus. Cytological identifications revealed the occurrence of S. (Edwardsellum) thyolense sp.nov., S. kilibanum Gouteux, 'Ketaketa', a new cytoform tentatively designated 'Mwamphanzi', and two unassigned specimens. Simulium thyolense sp.nov. is presumably the major vector of Onchocerca volvulus in the area, but it remains possible that the other cytospecies are secondary vectors. Simulium thyolense sp.nov. is most appropriately classified near the 'Sanje' subcomplex, whereas 'Mwamphanzi' and the unassigned specimens are members of the 'Ketaketa' subcomplex.

Cytotaxonomic identifications of the Simulium damnosum complex (Diptera:Simuliidae) from Nigeria.

Cytotaxonomic analysis of Simulium damnosum s.l. larvae collected from 23 sites across 4 bioclimatic zones in Nigeria revealed the presence of 5 cytospecies (S. damnosum s.str., S. sirbanum, S. squamosum, S. yahense and S. soubrense (including the Beffa form). The biogeographical distribution of the sibling species was mostly similar to that observed further west in the OCP area, although geographic cytotaxonomic variation was recorded in S. squamosum, S. damnosum s.str. and S. sirbanum. Some aspects of the chemical and physical environment of the breeding habitats were investigated.

Molecular identification of onchocerciasis vector sibling species in black flies (Diptera: Simuliidae).

A number of sibling species in the Simulium damnosum species complex transmit River Blindness (onchocerciasis) in tropical Africa. Black fly sibling species can be reliably recognized only through analysis of the larval polytene chromosomes. The routine, unambiguous assignment of the potentially disease-transmitting adult females to a particular species is virtually impossible. Two molecular assays to identify the sibling species of biting female flies are presented. The first relies on PCR-mediated amplification of internal transcribed spacers of the nuclear rDNA loci while the second involves immunoblotting of silk proteins which are expressed in both larvae and adult females. Both assays yield sibling-specific size markers that can be used to identify the sibling species of biting adult females. To illustrate the use of these assays, a molecular characterization is given for two S. damnosum siblings (squamosum, sirbanum) collected from cytologically verified populations, and a partial characterization of a third sibling (damnosum s. str.) found as a contaminant in these samples.

Species complex of vectors and epidemiology.

This work collates previous and recent cytotaxonomically defined segregates of the Simulium damnosum complex from western and eastern Africa. Standard cytotaxonomic procedures were used on new samples from Nigeria, Cameroon and Malawi. The main onchocerciasis vectors comprising cytotypes, cytoforms and cytospecies are highlighted and indications of host preferences are given. Thyolo form, the vector implicated in the Thyolo Highlands of Malawi, is reported for the first time. Also reported are new foci of two genetically distinct savanna taxa: Volta form and S. damnosum s.s.. The associations between forest taxa and onchocerciasis in the Forest Zone of west Africa together with the interplay of vector distributions at the interface of this and the Savanna Zone in relation to the epidemiology of onchocerciasis are discussed. The role of individual members of the species complex in epidemiology is less understood in east Africa and the Yemen but is briefly discussed.

Cytotaxonomy of Sirba form populations of the Simulium damnosum complex in west Africa: amendments to sex chromosomes and sibling status.

The banding pattern of larval salivary polytene chromosomes was analysed to re-examine the distribution, by sex and geography, of the sex linked inversion (IS-3) in "Sirba" form which was previously described as consisting of Simulium sirbanum and S. sudanense (Vajime and Dunbar, 1975). West African larval samples from seven countries within the Onchocerciasis Control Programme area and some adjacent locations were used. It was concluded that "Sirba" consists of two indisputable cytoforms, one distinguished by X0Y0 males and X0X0 females, the other by X1Y0 males and X1X1 females; and that the cytoforms are cytologically variable populations of a single cytospecies, S. sirbanum. It is argued that the species is in the process of incipient speciation.

The laboratory maintenance and rearing of Simulium damnosum complex species as a research tool for the Onchocerciasis Control Programme in the Vola River Basin.

The laboratory maintenance and rearing of Stimulium damnosum complex species and the cytotaxonomic determination of reared larvae, has been useful to OCP in the following fields of investigation: 1. Species identification of reinvading females. 2. Distribution of biting populations of S. damnosum complex species. 3. Identification of eggs and small larvae found within the OCP area. 4. Host preferences of species within the complex. 5. Vector potential of S. damnosum complex species for Onchocerca volvulus and O. ochengi. 6. Provision of identified material for morphological investigation. The technique of indirect adult identification using reared larvae, may eventually by largely superseded by progress in morphological taxonomy.

On the ecology and distribution of the species of the simulium damnosum complex in different bioclimatic zones of Liberia and Guinea.

A chromosomal identification of Simulium damnosum material collected in Liberia and Guinea revealed five species of this complex. S. yahense Vajime and Dunbar: Widely distributed in the rain forest zone (Liberia, Southern Guinea) and in the Fouta Djallon mountains. Breeding places typically in small watercourses, shaded streams and narrow rivers. S. sanctipauli Vajime and Dunbar: Restricted to the rain forest zone. Breeding confined to large rivers. S. soubrense Vajime and Dunbar: Breeding established in large and medium rivers under high forest (Liberia) and in the Guinea savanna (head waters of the Niger and its southern tributaries). S. damnosum Theobald: Distributed in the forest zone and Guinea savanna. Breeding in rivers and irrigation systems. S. sirbanum Vajime and Dunbar: Confined to the Northern Guinea savanna. Breeding in rivers and irrigation plants, occasionally in streams arising from lakes. Larvae of S. yahense and S. sanctipauli could be separated by morphological characters. The associations with other blackfly species are discussed.

Chromosomal identification of eight species of the subgenus Edwardsellum near and including Simulium (Edwardsellum) damnosum Theobald (Deptera: Simuliidae).

Examinations of the banding pattern of polytene chromsomes from larval salivary glands of the population in west Africa once thought to be a single species, Simulium (Edwardsellum) damnosum, revealed eight species all within the Nile subgroup of the complex. The following names were proposed: S. (E.) squamosum Enderlein, S. (E.) yahense n.sp., S. (E.) soubrense n.sp., S. (E.) sanctipauli n.sp., S. (E.) dieguerense n.sp., S. (E.) damnosum Theobald, S. (E.) sirbanum n.sp. and S. (E.) sudanense n.sp. Micromorphological characters by which there species are distinguished include interspecific inversions, different sets of intraspecific inversions, different sets of sex chromosomes and a few micromorphological features other than the banding pattern. By following the logical sequential development of interspecific inversions both a key to the species and a phylogenetic chart have been drawn up. Analysis of the proportions of alternate sequences for random distribution or not with ther Hardy-Weinberg equation have confirmed the existance of distinct populations, uncovered an example of incipient speciation and showed the existance of a genetic variant of S. damnosum which could possible colonize below dams.