Population Structure of Anopheles Arabiensis Breeding in transient and , Permanent in land Habitats at two Sites in Western Kenya

Abstract

Malaria is one of the main diseases that cause great morbidity and mortality in: sub-Saharan Africa region. The prevalence and incidence of malaria in this region is determined mostly by the mosquito vector species. Several measures 'andprogrammes have been undertaken to control malaria vectors and reduce malaria incidence in Africa. However, most of these measures have not been very successful. This has led to proposals to use mosquitoes that are refractory to malaria parasite infection as one of the options in malaria control programmes. Such a control strategy requires that population genetic structure of the vector species is understood. The understanding of population genetic structure of vector species will enable malaria entomologists to predict how genes associated with refractoriness will spread through the vector populations. Knowledge of vector population genetic structure is also useful for predicting the spread of insecticide-resistance genes in a given vector population. Studies on the population genetic structure of vector species have been carried out using various techniques and markers. These include restriction fragment length polymorphism (RFLP), randomly amplified polymorphic DNA (RAPD), chromosomal inversions, and microsatellite DNA. In this study,Anopheles arabiensis populations from two different types of breeding habitats in Western Kenya were analysed using a total of eight microsatellite loci. The aim of this study was to determine whether Anopheles arabiensis species partitioned larval breeding habitats based on the habitat type. Two habitat types namely, transient small size larval breeding habitats and permanent/semi-permanent large size larval breeding habitats were considered and sampling done in three sites, two in Ahero and x one in Nyakach. Specimen collection was done by pipetting and dipping using standard World Health Organisation (WHO) procedures. A total of 500 mosquito larvae were. collected. The specimens were processed by DNA extraction and fhen analysed using polymerase chain reaction (Pf.R) and polyacrylamide gel electrophoresis (PAGE). The data were analysed using GENEPOP version 3.2a software programme, which evaluated Hardy- Weinberg equilibrium, allele frequencies, and genotypic frequencies. Levels of genetic differentiation was analysed using G-like test available in GENEPOP. Significance of multiple tests was evaluated using sequential rejective Bonferroni test and binomial test. There was polymorphism at SIX out of eight studied loci. Overall allelic and genotypic frequency distribution at all loci deviated significantly from Hardy-Weinberg equilibrium and pooled data from similar habitats and those from all breeding habitats at Ahero also deviated from Hardy-Weinberg equilibrium (P < 0.05). The deviation from Hardy- Weinberg equilibrium suggested lack of random mating and presence of population sub-divisions. This meant that there was restricted gene flow between An. arabiensis populations from similar habitat types in the study sites. The G-like test showed genotypic differentiation at all loci across all populations. The observed genotype distribution frequency was' not in agre.ement with the expected frequency suggesting the presence of non-random mating and population sub-division. The presence of sub-populations and non-random mating suggested that Anopheles arabiensis did not partition their larval breeding habitats based on habitat type. The non random mating and presence of sub-population may suggest presence of genetic variability within Anopheles arabiensis populations in the study area. This may XI contribute to the diversity and stability of malaria transmission in study area. The variation may also cause the proposed control measure based on vector replacement to· be less effective. Therefore studies such as this, which, apply markers that can show '-' association with breeding habitats, can be useful in understanding whether the type of breeding habitat may have effect on genetic variability of Anopheles arabiensis populations at a local scale.