Enzyme heterogeneity and genetic variation In aedes (stegomyia) Aegypti L. of a coastal area in Kenya
Mukiama, Titus K
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Enzyme heterogeneity and genetic variation were studied in Aedes (Stegomyia) aegypti L. in the Rabai area, Coast Province Kenya. Mosquitoes were collected from inside houses (domestic habitat), from the immediate vicinity of human dwellings (peridomestic habitat), and from the woods far from human dwellings (feral habitat). These were bred in the laboratory. Morphological variants were separated by examination of anaesthetised adults under the dissecting microscope. Biochemical variation was determined by starch gel electrophoretic techniques and ultra-violet spectrophotometric techniques. The life cycle of Ae. aegypti was completed in at least 11 days. There were behavioural traits differing between - larvae "of Ae. aegypti type form and sub-species formosus Walker. There was a slight excess of males, such that the sex ratio was 1.07 males: 1 female. More eggs,were oviposited in 1 male : 1 female crosses than in 2 'male~ : 1 female crosses. 'The mean pupal expect~cy from eggs was 76%, indicating larval ~ortality an~robably infertility of eggs. Females generally l~~ed longer than males. Reciprocal crosses between type form and ssp. formosus indicated the presence of a partial sub-specific fertility barrier, operating in matings of the type spp. formosus female X type form male. With respect to the esterase loci, differences were Preliminary data on a morphological phenotype, the extent of wing cover over the terminal abdominal segments, suggested that more variation exists between ssp. fonmosus and type form than otherwise envisaged. A hypothetical explanation for the observed data was advanced. More data is required for a conclusive explanation. More than one isozyme variant was found for esterases (Est.), malate dehydrogenase QMDH), glutamate oxaloacetate transaminase (GOT), peroxidases and tetrazolium oxidase (To). The esterases were shown to exhibit some locus-enzymesubstrate specificity. Genetic polymorphism was discovered at the Est-3, Est-6, MDH-l, GOT-2, and malic enzyme OMe) loci. found between type form Ae. aegypti and ssp. formosus on the one hand and between domestic, peridomestic and feral populat1ons on the other hand. These differences were in terms of the types and frequencies of alleles present. The same differences were also found to occur on a spatial basis, i.e. between mosquito populations of the sampling localities. The domestic, peridomestic and feral populations were shown to be in Hardy-Weinberg equilibrium, even though there was poor fitness in p~ridomes~~c populations. There was very little evidence for gene flow between these populations. Gene activity during enzyme ontogenesis was studied by electrophoretic and spectrophotometric techniques. The activity of a-glycerolphosphate dehydrogenase (a-GPDH) was generally very high in pupae and adults, principally due to the temporal activation of the locus responsible for the major adult isozyme a-GPDH3. Lactate dehydrogenase (LDH) activity was rather restricted to the early larval stages, with the gene responsible either producing an inactive enzyme or probably being under the control of regular gene(s). Gene repression probably occurs after the third larval instar. MDH activity is highest in pupae with changes in activity mainly due to the MDH-2 locus, and pupal peroxidases were found to be different from adult peroxidases, suggesting that the temporal repression and activation of genes, and vice-versa, is an important mechanism during ontogenesis in insects. Ae. aegypti has been shown to be a very variable mosquito, both morphologically and biochemically. Only a fe~ ~rlzymes have been studied here, and the variation already found in adult populations and during ontogenesis suggests great potential variability. Three esterase alleles were found in Ae. aegypti ssp. formosus that were absent in type form. More enzymes must be studied so that differe~ces can b,e found which may form the basis of . biochemical taxonomy. Non-glucose..metabolising enzymes (esterases) were more polymorphic than glucose-metabolizing enzymes (e.g. LDH). Enzymes acting on multiple substrates were the most polymorphic group (esterases). This supports the theory that selective forces have a role in the maintenance of polymorphism.