ON THE BIOSYSTEMATICS OF ANOPHELES DIRUS COMPLEX (DIPTERA:CULICIDAE) IN CHINA*

Abstract Anopheles dirus complex is a major malaria vector in Southeast Asia and South China. Rut the role played by different member of the species complex in malaria transmission is not clearly known. Correct identification of the sibling species is a foundational requirement for working out a sound scheme in mosquito biosystematics. This paper reports the resuets of biosystematic studies on the chromosomal karyotype, egg microstructure, ribosomal DNA sequences of a second internal transcribed spacer region and polymerase chain reactions of the complex in China. Specimens of species A and D from Hainan and Yunnan Provinces were carefully analyzed and the importance of development aspect of the mosquito biosystematics in malaria control is discussed.     

Microplate assay of elevated esterase activity in individual pyrethroid-resistant mosquitoes

Involvement of esterase-mediated hydrolysis as a mechanism of pyrethroidresistance in three species of mosquitoes,viz., Aedes aegypti, Culex quinquefasciatus andAnopheles Stephensi was investigated by microplate assay of Β-esterases in individual larva and adult female and male mosquitoes. Assuming an absorbance value of 0.4 and above at 555 nm as the threshold level of elevated esterase activity which confers resistance, frequency distributions of such individual test mosquitoes were constructed in resistant and susceptible populations. The results indicate the involvement of ester hydrolysis of Pyrethroids as a predominant mechanism of pyrethroid-resistance in the larvae ofCulex quinquefasciatus but not inAedes aegypti. However, a marginal role of esterases is indicated in the larvae ofAnopheles stephensi.     

Location of genes on chromosome arms in the Anopheles gambiae group of species and their correlation to linkage data for other anopheline mosquitoes

The use of paracentric inversions as genetic markers in the Anopheles gambiae group of mosquitoes is described. The gene for dieldrin resistance is assigned to chromosome 2 which in turn is correlated to the previous assignment of the gene to linkage group II. The locus of the enzyme phosphoglucomutase 2 (Pgm 2) is similarly assigned to chromosome 2 and evidence is presented for possible linkage between Pgm 2 and dieldrin resistance. There was no linkage or correlation of chromosome 2 and loci of the enzymes superoxide dismutase (Sod) and octanol dehydrogenase (Odh). These genes are therefore assumed to be on chromosome 3 (linkage group III). Evidence that such gene linkage group/chromosome correlations may extend to other species for which chromosome maps and homologies have been worked out is discussed.     

Anopheles hervyi in Niger: no evidence for a role in Plasmodium falciparum transmission

Anopheles hervyi is an endemic mosquito species with a very limited spatial distribution in the south east of Niger. No new captures have been reported since the 1960s and its role in malaria transmission has not been studied. In the present study, the use of CDC light traps showed it to be much more abundant than previously found but there was no evidence to suggest it was a malaria vector in this region. The larval habitats have not been identified but the potential role of a saline lake in determining the distribution of this species is discussed.     

Permethrin-impregnated bednets are more effective than DDT house-spraying to control malaria in Solomon Islands

Abstract. A field trial compared DDT house-spraying with permethrin-impregnated bednets for malaria control in Solomon Islands from 1987 to 1991. Mortality-rates of malaria vector Anopheles farauti in exit window traps were 11.6% from an untreated hut, 10.1% from a hut sprayed with DDT 2 g/m², and 98% of those from a hut in which the occupants used bednets treated with permethrin 0.5 g/m². Since bioassays of the DDT-sprayed walls (15 min exposure in W.H.O. standard test cones) gave 77% mortality of Anfarauti , it was concluded that the insignificant impact of DDT could be explained by the exophilic behaviour of endophagic vectors, whereas the greater impact of permethrin was attributed to the more effective exposure of Anfarauti females to the impregnated bednets - attracted by the occupants. The parous rate was higher indoors, except in the area with permethrin-impregnated bednets. It was therefore concluded that permethrin-impregnated bednets reduced the mean longevity of Anfarauti and hence its vectorial capacity. The circumsporozoite (CS) antigen positivity rate of Anfarauti in the DDT area was 0.18% outdoors, significantly less than 1.42% indoors. In the comparison area CS rates were 0.65% outdoors and 0.75% indoors. CS antigen was not detected in Anfarauti from the bednet area, indicating the apparent prevention of malaria transmission. As DDT spraying was so much less effective, it was discontinued in 1993 and permethrin-impregnated bednets are now the principal malaria control method in Solomon Islands.     

Rapid isolation of anopheline mosquito eye-colour mutants, based on larval colour change

Abstract. A method is presented for the rapid isolation of eye-colour mutants in anopheline mosquitoes based on their inability to undergo a background-stimulated morphological colour change. For application of this method, larval mosquitoes, whose grandfathers had been mutagenized, were reared in black containers and examined with the naked eye en masse during the third or fourth instar. The vast majority of larvae became dark-coloured; however, rare exceptional pale larvae were observed and examined individually microscopically. Approximately half of the pale types examined were eye-colour mutants. By this method, seven sex-linked mutations in the mosquito Anopheles gambiae s.s. were easily isolated. Additional existing anopheline eye-colour mutants in An. gambiae and An. stephensi were tested and were found to be unable to undergo colour change. Several applications of this simple technique are suggested.     

Testing Anopheles albimanus for genetic linkage of insecticide resistance genes by combining insecticide bioassay and biochemical methods

A microtitre-plate assay which distinguishes propoxur-resistant from susceptibles Anopheles albimanus Weidemann was used to test for linkage between the genes for propoxur- and dieldrin-resistance. The adult progeny of a backcross between a doubly-resistant colony and a fully susceptible colony were exposed in conventional test kits to the standard discriminating dose of dieldrin, and kept in the insectary overnight. Both live and dead insects were then assayed individually for propoxur-resistance. The results showed that heterozygotes for propoxur-resistance could be reliably distinguished from susceptibles whether or not they had been killed up to 24 h previously by dieldrin treatment. In this way all the backcross progeny could be scored at both resistance loci, and all four genotypic classes identified. Resistant and susceptible alleles at the two loci were inherited independently, demonstrating the absence of linkage. The usual method of testing for linkage between resistance genes is inefficient and open to bias, because insects have to be exposed to each insecticide in turn, and only half of them can be scored at both loci. The method shown here avoids these drawbacks.     

Cuticular lipid differences between the malaria vector and non-vector forms of the Anopheles maculatus complex

Two chromosomal forms (E and F) of the Anopheles maculatus Theobald complex were distinguished by gas-liquid chromatographic (GC) analysis of cuticular lipids in association with a multivariate principal component analysis. The GC chromatogram obtained from n-hexane extracts of individual specimens showed no consistent qualitative differences in normalized peak areas between forms. Of the seventeen consistent peaks, five were found to be quantitatively different between forms at a high (99.5-99.95%) level of statistical confidence. Relative ratios of these five quantitatively different GC peaks were used as criteria to distinguish single specimens as either form E or form F. Chemical structures of the five GC peaks were assigned by both electron impact and chemical ionization gas chromatography/mass spectrometry analysis. The first three peaks, which were always doublets, were partially resolved saturated and mono-unsaturated free fatty acids; the other two peaks were n-alkanes. Principal component analysis substantiated that the vector form E has very similar cuticular lipid profiles and is well separated from the non-vector form F.