Ribosomal DNA-polymerase chain reaction assay discriminates between Anopheles quadriannulatus and An. merus (Diptera: Culicidae)

Distribution of the species of the Anopheles gambiae complex and first evidence of Anopheles merus as a malaria vector in Madagascar

Background

Members of the Anopheles gambiae complex are amongst the best malaria vectors in the world, but their vectorial capacities vary between species and populations. A large-scale sampling of An. gambiae sensu lato was carried out in various bioclimatic domains of Madagascar. Local abundance of an unexpected member of this complex raised questions regarding its role in malaria transmission.

Methods

Sampling took place at 38 sites and 2,067 females were collected. Species assessment was performed using a PCR targeting a sequence in the IGS of the rDNA. Analysis focused on the relative prevalence of the species per site, bioclimatic domain and altitude. Infectivity of Anopheles merus was assessed using an ELISA to detect the presence of malarial circumsporozoite protein in the head-thorax.

Results

Three species were identified: An. gambiae, Anopheles arabiensis and An. merus. The distribution of each species is mainly a function of bioclimatic domains and, to a lesser extent, altitude. An. arabiensis is present in all bioclimatic domains with highest prevalence in sub-humid, dry and sub-arid domains. An. gambiae has its highest prevalence in the humid domain, is in the minority in dry areas, rare in sub-humid and absent in sub-arid domains. An. merus is restricted to the coastal fringe in the south and west; it was in the majority in one southern village. The majority of sites were sympatric for at least two of the species (21/38) and two sites harboured all three species.

The role of An. merus as malaria vector was confirmed in the case of two human-biting females, which were ELISA-positive for Plasmodium falciparum.

Conclusion

Despite the huge environmental (mainly man-made) changes in Madagascar, the distribution of An. gambiae and An. arabiensis appears unchanged for the past 35 years. The distribution of An. merus is wider than was previously known, and its effectiveness as a malaria vector has been shown for the first time; this species is now on the list of Malagasy malaria vectors.

Assessment of rDNA IGS as a molecular marker in the Simulium damnosum complex

For five cytospecies of the Simulium damnosum Theobald complex of blackflies (Diptera: Simuliidae) from West Africa, both ends of the intergenic spacer region (IGS) of the rDNA have been sequenced with the aim of developing specific molecular markers. No specific differences in these two regions were detected between Simulium sanctipauli V. & D., Simulium sirbanum V. & D., Simulium soubrense V. & D., Simulium squamosum Enderlein and Simulium yahense V. & D., except in the number of A subrepeats at the 5' end of the IGS (two in S. squamosum and four or five in the others) and in position 310 of the 3' end (a C in S. squamosum and a G in the others). However, genetic distances within and between species overlapped. These DNA sequences had no strong phylogenetic signal, and the trees obtained were mostly unresolved. Although most sequences from S. squamosum clustered together, a few of them were more similar to those in other cytospecies. These results could be explained either by hybridization with genetic introgression or by ancestral polymorphism and recent speciation.

Systematics of malaria vectors with particular reference to the Anopheles punctulatus group

The appearance of groups and complexes of closely related cryptic or sibling species in many of the anopheline taxa has impeded studies on malaria transmission and the evaluation of control strategies which have relied on morphological characters to identify the vector species involved. The advantages of morphological identification are low cost, speed and simplicity, which allow large numbers of specimens to be processed rapidly in the field. The need for accurate identification is crucial, as time and money may be wasted in studying and controlling species of no medical importance. Various techniques such as cross-mating, chromosome studies and allozyme analysis have been developed to resolve problems of identifying sibling species, though none, as yet, can match the speed and simplicity afforded by morphology markers. The latest of these identification methods comes from advances that have been made in DNA-based technology. Although costly and requiring fairly sophisticated laboratory support, methods such as DNA probe hybridisation and PCR are the quickest and most user-friendly to date. The use of DNA has other advantages in the study of intraspecific differences and in providing characters for phylogenetic studies. This review looks at the development of DNA-based techniques for taxonomic and systematic studies of anopheline mosquitoes. The Anopheles punctulatus group of the southwest Pacific is featured as an example of how this technology has been applied and how it has progressed.