Mitochondrial DNA sequence divergence and diversity of Glossina fuscipes fuscipes in the Lake Victoria basin of Uganda: implications for control

Population Structure and Migration Patterns of the Tsetse Fly Glossina fuscipes in Congo-Brazzaville

Tsetse flies of the palpalis group, particularly Glossina fuscipes, are the main vectors of human African trypanosomiasis or sleeping sickness in Congo-Brazzaville. They transmit the deadly human parasite, Trypanosoma brucei gambiense and other trypanosomes that cause animal trypanosomiasis. Knowledge on diversity, population structure, population size, and gene flow is a prerequisite for designing effective tsetse control strategies. There is limited published information on these parameters including migration patterns of G. fuscipes in Congo-Brazzaville. We genotyped 288 samples of G. fuscipes from Bomassa (BMSA), Bouemba (BEMB), and Talangai (TLG) locations at 10 microsatellite loci and determined levels of genetic diversity, differentiation, structuring, and gene flow among populations. We observed high genetic diversity in all three localities. Mean expected heterozygosity was 0.77 ± 0.04, and mean allelic richness was 11.2 ± 1.35. Deficiency of heterozygosity was observed in all populations with positive and significant F IS values (0.077-0.149). Structure analysis revealed three clusters with genetic admixtures, evidence of closely related but potentially different taxa within G. fuscipes. Genetic differentiation indices were low but significant (F ST = 0.049, P < 0.05), indicating ongoing gene flow countered with a stronger force of drift. We recorded significant migration from all the three populations, suggesting exchange of genetic information between and among locations. Ne estimates revealed high and infinite population sizes in BEMB and TLG. These critical factors should be considered when planning area-wide tsetse control interventions in the country to prevent resurgence of tsetse from relict populations and/or reinvasion of cleared habitats.

Single-strand conformation polymorphism (SSCP) of mitochondrial genes helps to estimate genetic differentiation, demographic parameters and phylogeny of Glossina palpalis palpalis populations from West and Central Africa

A good understanding of tsetse fly population structure and migration is essential to optimize the control of sleeping sickness. This can be done by studying the genetics of tsetse fly populations. In this work, we estimated the genetic differentiation within and among geographically separated Glossina palpalis palpalis populations from Cameroon, the Democratic Republic of the Congo and Ivory Coast. We determined the demographic history of these populations and assessed phylogenetic relationships among individuals of this sub-species. A total of 418 tsetse flies were analysed: 258 were collected in four locations in Cameroon (Bipindi, Campo, Fontem and Bafia), 100 from Azaguié and Nagadoua in Ivory Coast and 60 from Malanga in the Democratic Republic of the Congo. We examined genetic variation at three mitochondrial loci: COI, COII-TLII, and 16S2. 34 haplotypes were found, of which 30 were rare, since each was present in <5% of the total number of individuals. No haplotype was shared among Cameroon, Ivory Coast and the Democratic Republic of the Congo populations. The fixation index F_ST of 0.88 showed a high genetic distance between Glossina palpalis palpalis populations from the three countries. That genetic distance was correlated to the geographic distance between populations. We also found that there is substantial gene flow between flies from locations separated by over 100 km in Cameroon and between flies from locations separated by over 200 km in Ivory Coast. Demographic parameters suggest that the tsetse flies from Fontem (Cameroon) had reduced in population size in the recent past. Phylogenetic analysis confirms that Glossina palpalis palpalis originating from the Democratic Republic of the Congo are genetically divergent from the two other countries as already published in previous studies.

Genetic diversity and population structure of Glossina morsitans morsitans in the active foci of human African trypanosomiasis in Zambia and Malawi

The tsetse fly, Glossina morsitans morsitans, is a significant problem in Zambia and Malawi. It is the vector for the human infective parasite Trypanosoma brucei rhodesiense, which causes human African trypanosomiasis, and various Trypanosoma species, which cause African animal trypanosomiasis. Understanding the genetic diversity and population structure of G. m. morsitans is the basis of elucidating the connectivity of the tsetse fly populations, information that is essential in implementing successful tsetse fly control activities. This study conducted a population genetic study using partial mitochondrial cytochrome oxidase gene 1 (CO1) and 10 microsatellite loci to investigate the genetic diversity and population structure of G. m. morsitans captured in the major HAT foci in Zambia and Malawi. We have included 108 and 99 G. m. morsitans samples for CO1 and microsatellite analyses respectively. Our results suggest the presence of two different genetic clusters of G. m. morsitans, existing East and West of the escarpment of the Great Rift Valley. We have also revealed genetic similarity between the G. m. morsitans in Kasungu National Park and those in the Luangwa river basin in Zambia, indicating that this population should also be included in this historical tsetse belt. Although further investigation is necessary to illustrate the whole picture in East and Southern Africa, this study has extended our knowledge of the population structure of G. m. morsitans in Southern Africa.

Diversity and phylogenetic relationships of Glossina populations in Nigeria and the Cameroonian border region

Background

Tsetse flies are vectors of trypanosomes, parasites that cause devastating disease in humans and livestock. In the course of vector control programmes it is necessary to know about the Glossina species present in the study area, the population dynamics and the genetic exchange between tsetse fly populations.

Results

To achieve an overview of the tsetse fly diversity in Nigeria and at the Nigeria-Cameroon border, tsetse flies were trapped and collected between February and March 2014 and December 2016. Species diversity was determined morphologically and by analysis of Cytochrome C Oxidase SU1 (COI) gene sequences. Internal transcribed spacer-1 (ITS-1) sequences were compared to analyse variations within populations. The most dominant species were G. m. submorsitans, G. tachinoides and G. p. palpalis. In Yankari Game Reserve and Kainji Lake National Park, G. submorsitans and G. tachinoides were most frequent, whereas in Old Oyo National Park and Ijah Gwari G. p. palpalis was the dominant species. Interestingly, four unidentified species were recorded during the survey, for which no information on COI or ITS-1 sequences exists. G. p. palpalis populations showed a segregation in two clusters along the Cameroon-Nigerian border.

Conclusions

The improved understanding of the tsetse populations in Nigeria will support decisions on the scale in which vector control is likely to be more effective. In order to understand in more detail how isolated these populations are, it is recommended that further studies on gene flow be carried out using other markers, including microsatellites.

Tsetse fly evolution, genetics and the trypanosomiases - A review

This reviews work published since 2007. Relative efforts devoted to the agents of African trypanosomiasis and their tsetse fly vectors are given by the numbers of PubMed accessions. In the last 10 years PubMed citations number 3457 for Trypanosoma brucei and 769 for Glossina. The development of simple sequence repeats and single nucleotide polymorphisms afford much higher resolution of Glossina and Trypanosoma population structures than heretofore. Even greater resolution is offered by partial and whole genome sequencing. Reproduction in T. brucei sensu lato is principally clonal although genetic recombination in tsetse salivary glands has been demonstrated in T. b. brucei and T. b. rhodesiense but not in T. b. gambiense. In the past decade most genetic attention was given to the chief human African trypanosomiasis vectors in subgenus Nemorhina e.g., Glossina f. fuscipes, G. p. palpalis, and G. p. gambiense. The chief interest in Nemorhina population genetics seemed to be finding vector populations sufficiently isolated to enable efficient and long-lasting suppression. To this end estimates were made of gene flow, derived from F_ST and its analogues, and Ne, the size of a hypothetical population equivalent to that under study. Genetic drift was greater, gene flow and Ne typically lesser in savannah inhabiting tsetse (subgenus Glossina) than in riverine forms (Nemorhina). Population stabilities were examined by sequential sampling and genotypic analysis of nuclear and mitochondrial genomes in both groups and found to be stable. Gene frequencies estimated in sequential samplings differed by drift and allowed estimates of effective population numbers that were greater for Nemorhina spp than Glossina spp. Prospects are examined of genetic methods of vector control. The tsetse long generation time (c. 50 d) is a major contraindication to any suggested genetic method of tsetse population manipulation. Ecological and modelling research convincingly show that conventional methods of targeted insecticide applications and traps/targets can achieve cost-effective reduction in tsetse densities.

Genetic diversity of Glossina fuscipes fuscipes along the shores of Lake Victoria in Tanzania and Kenya: implications for management

BACKGROUND

Tsetse flies (Diptera: Glossinidae) are sole vectors for trypanosomiasis, which affect human health and livestock productivity in Africa. Little is known about the genetic diversity of Glossina fuscipes fuscipes, which is an important species in Tanzania and Kenya. The main objective of the study was to provide baseline data to determine the genetic variability and divergence of G. f. fuscipes in the Lake Victoria basin of Tanzania and Kenya in order to guide future vector control efforts in the region.

FINDINGS

Two hundred and seventy five G. f. fuscipes from 8 sites along the shores of Lake Victoria were screened for genetic polymorphisms at 19 microsatellite loci. Samples were collected from two sites in Kenya and six sites in Tanzania. Four of the Tanzanian sites were located in the Rorya district, on the eastern shores of Lake Victoria, while the other two sites were from Ukerewe and Bukoba districts from the southern and western Lake Victoria shores, respectively. Four genetically distinct allopatric clusters were revealed by microsatellite analysis, which sorted the sampling sites according to geography, with sites separated by as little as ~65 km belonging to distinct genetic clusters, while samples located within ~35 km from each other group in the same cluster.

CONCLUSION

Our results suggest that there is ongoing genetic admixture within sampling sites located ~35 km from each other, while sites located ~65 km apart are genetically isolated from each other. Similar patterns emerged from a parallel study on G. f. fuscipes analyzed from the Lake Victoria Uganda shores. From a control perspective these results suggest that for sites within the same genetic cluster, control efforts should be carried out in a coordinated fashion in order to avoid re-invasions. Future work should focus on better quantifying the extent and spatial patterns of the observed genetic discontinuities of the G. f. fuscipes populations along the Tanzanian shores. This will aid in their control by providing guidelines on the geographical extent of the area to be treated at the same time.

Genetic diversity and population structure of the tsetse fly Glossina fuscipes fuscipes (Diptera: Glossinidae) in Northern Uganda: Implications for vector control

Uganda is the only country where the chronic and acute forms of human African Trypanosomiasis (HAT) or sleeping sickness both occur and are separated by < 100 km in areas north of Lake Kyoga. In Uganda, Glossina fuscipes fuscipes is the main vector of the Trypanosoma parasites responsible for these diseases as well for the animal African Trypanosomiasis (AAT), or Nagana. We used highly polymorphic microsatellite loci and a mitochondrial DNA (mtDNA) marker to provide fine scale spatial resolution of genetic structure of G. f. fuscipes from 42 sampling sites from the northern region of Uganda where a merger of the two disease belts is feared. Based on microsatellite analyses, we found that G. f. fuscipes in northern Uganda are structured into three distinct genetic clusters with varying degrees of interconnectivity among them. Based on genetic assignment and spatial location, we grouped the sampling sites into four genetic units corresponding to northwestern Uganda in the Albert Nile drainage, northeastern Uganda in the Lake Kyoga drainage, western Uganda in the Victoria Nile drainage, and a transition zone between the two northern genetic clusters characterized by high level of genetic admixture. An analysis using HYBRIDLAB supported a hybrid swarm model as most consistent with tsetse genotypes in these admixed samples. Results of mtDNA analyses revealed the presence of 30 haplotypes representing three main haplogroups, whose location broadly overlaps with the microsatellite defined clusters. Migration analyses based on microsatellites point to moderate migration among the northern units located in the Albert Nile, Achwa River, Okole River, and Lake Kyoga drainages, but not between the northern units and the Victoria Nile drainage in the west. Effective population size estimates were variable with low to moderate sizes in most populations and with evidence of recent population bottlenecks, especially in the northeast unit of the Lake Kyoga drainage. Our microsatellite and mtDNA based analyses indicate that G. f. fuscipes movement along the Achwa and Okole rivers may facilitate northwest expansion of the Rhodesiense disease belt in Uganda. We identified tsetse migration corridors and recommend a rolling carpet approach from south of Lake Kyoga northward to minimize disease dispersal and prevent vector re-colonization. Additionally, our findings highlight the need for continuing tsetse monitoring efforts during and after control.

Northern Uganda is an epidemiologically important region affected by human African trypanosomiasis (HAT) because it harbors both forms of the HAT disease (T. b. gambiense and T. b. rhodesiense). The geographic location of this region creates the risk that these distinct foci could merge, which would complicate diagnosis and treatment, and may result in recombination between the two parasite strains with as yet unknown consequences. Both strains require a tsetse fly vector for transmission, and in Uganda, G. f. fuscipes is the major vector of HAT. Controlling the vector remains one of the most effective strategies for controlling trypanosome parasites. However, vector control efforts may not be sustainable in terms of long term reduction in G. f. fuscipes populations due to population rebounds. Population genetics data can allow us to determine the likely source of population rebounds and to establish a more robust control strategy. In this study, we build on a previous broad spatial survey of G. f. fuscipes genetic structure in Uganda by adding more than 30 novel sampling sites that are strategically spaced across a region of northern Uganda that, for historical and political reasons, was severely understudied and faces particularly high disease risk. We identify natural population breaks, migration corridors and a hybrid zone with evidence of free interbreeding of G. f. fuscipes across the geographic region that spans the two HAT disease foci. We also find evidence of low effective population sizes and population bottlenecks in some areas that have been subjects of past control but remain regions of high tsetse density, which stresses the risk of population rebounds if monitoring is not explicitly incorporated into the control strategy. We use these results to make suggestions that will enhance the design and implementation of control activities in northern Uganda.