Genetic diversity of anopheles gambiae’s immune gene and vector competence to plasmodium falciparum in western Kenya
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Date
2025-08
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Kenyatta University
Abstract
Evolutionary pressures may introduce mosquito genotypes that adapt to new breeding habitats, favoring the selection of efficient malaria vectors increasing the risk of malaria transmission. Consequently, Plasmodium falciparum has evolved to evade the vector defenses mediated by Pfs47, impeding malaria control efforts. This study evaluated the genetic diversity of TEP1 and vector competence to P. falciparum in Homa Bay, Kakamega, Bungoma, and Kisumu counties. Cross-sectional entomological assessments and blood screening were conducted from 2017-2020. Anopheles gambiae adults and larvae were collected using pyrethrum spray catches (PSC) and plastic dippers, respectively. Adults were identified morphologically and further classified to siblings using polymerase chain reaction (PCR). The TEP1 alleles were determined using restriction fragment length polymorphisms-polymerase chain reaction (RFLP-PCR) and a representative sample sequenced for confirmation. Dried blood spots and slide smears were simultaneously collected from children between 5 and 15 years and tested by PCR and microscopy respectively. Chelex method was used to extract DNA from positive samples by microscopy and then PCR and sequencing were used to genotype Pfs47. Gametocyte carriers selected for membrane feeding experiments were identified using microscopy. Two TEP1 alleles (*S1 and *R2) and three genotypes (*S1/S1, *R2/S1, and *R2/R2) were identified in Anopheles gambiae and Anopheles arabiensis. Homozygous *S1/S1 and heterozygous *R2/S1 were widely distributed across all sites in both species. No significant difference was observed among populations and between species in TEP1 allele and genotype frequencies. Low population structure (FST = 0.019) across all sites corresponded to an effective migration index (Nm = 12.571) and low Nei’s genetic distance values (<0.5) among subpopulation. Thirteen Pfs47 haplotypes were observed. Haplotype diversity ranged from 0.69 to 0.77 and nucleotide diversity; 0.10 to 0.12 across all sites. All sites displayed negative and significant Fu’s Fs statistical values. The genetic differentiation index was not significant (FST = -0.00891, P>0.05) among Plasmodium populations The malaria infection prevalence in Chulaimbo was 19.7% (95% Cl: 0.003 – 0.007), 95% of the infections were P. falciparum. 16.9% had confirmed gametocytes. Six infectious haplotypes were identified out of 24 sequenced P. falciparum gametocyte-containing samples. Haplotypes with the L240I mutation were the most prevalent, with a frequency of 29.2%, then E27D with a frequency of 20.8%. The E27D and L240I mutation-carrying haplotypes had frequencies of 16.7%, while haplotypes with mutations S98T0 and E188D had frequencies of 4.2% and 8.3%, respectively. Haplotypes associated with positive midguts exhibited increased infectivity. These genes are potential targets for transmission blocking and should be considered when designing control interventions that focus on vectors expressing the resistant allele and prevalent infectious Pfs47 haplotypes
Description
A thesis submitted in fulfilment of the requirements for the award of the degree of doctor of philosophy in medical entomology in the school of pure and applied sciences of Kenyatta University, August 2024
Supervisors:
Prof. Elizabeth kokwaro
Dr. Andrew k. Githeko