Genetic heterogeneity in the Trypanosoma brucei gambiense genome analysed by random amplification of polymorphic DNA

Genetic structure of Trypanosoma congolense "forest type" circulating in domestic animals and tsetse flies in the South-West region of Cameroon

Despite the economic impact of trypanosome infections, few investigations have been undertaken on the population genetics and transmission dynamics of animal trypanosomes. In this study, microsatellite markers were used to investigate the population genetics of Trypanosoma congolense “forest type”, with the ultimate goal of understanding its transmission dynamics between tsetse flies and domestic animals. Blood samples were collected from pigs, sheep, goats and dogs in five villages in Fontem, South-West region of Cameroon. In these villages, tsetse were captured, dissected and their mid-guts collected. DNA was extracted from blood and tsetse mid-guts and specific primers were used to identify T. congolense “forest type”. All positive samples were genetically characterized with seven microsatellite markers. Genetic analyses were performed on samples showing single infections of T. congolense “forest type”. Of the 299 blood samples, 137 (46%) were infected by T. congolense “forest type”. About 3% (54/1596) of tsetse fly mid-guts were infected by T. congolense “forest type”. Of 182 samples with T. congolense “forest type”, 52 were excluded from the genetic analysis. The genetic analysis on the 130 remaining samples revealed polymorphism within and between subpopulations of the target trypanosome. The dendrogram of genetic similarities was subdivided into two clusters and three sub-clusters, indicating one major and several minor genotypes of T. congolense “forest type” in tsetse and domestic animals. The low F_STvalues suggest low genetic differentiation and no sub-structuration within subpopulations. The same T. congolense genotypes appear to circulate in tsetse and domestic animals.

Trypanosoma brucei s.l.: Microsatellite markers revealed high level of multiple genotypes in the mid-guts of wild tsetse flies of the Fontem sleeping sickness focus of Cameroon

To identify Trypanosoma brucei genotypes which are potentially transmitted in a sleeping sickness focus, microsatellite markers were used to characterize T. brucei found in the mid-guts of wild tsetse flies of the Fontem sleeping sickness focus in Cameroon. For this study, two entomological surveys were performed during which 2685 tsetse flies were collected and 1596 (59.2%) were dissected. Microscopic examination revealed 1.19% (19/1596) mid-gut infections with trypanosomes; the PCR method identified 4.7% (75/1596) infections with T. brucei in the mid-guts. Of these 75 trypanosomes identified in the mid-guts, Trypanosoma brucei gambiense represented 0.81% (13/1596) of them, confirming the circulation of human infective parasite in the Fontem focus. Genetic characterization of the 75 T. brucei samples using five microsatellite markers revealed not only multiple T. brucei genotypes (47%), but also single genotypes (53%) in the mid-guts of the wild tsetse flies. These results show that there is a wide range of trypanosome genotypes circulating in the mid-guts of wild tsetse flies from the Fontem sleeping sickness focus. They open new avenues to undertake investigations on the maturation of multiple infections observed in the tsetse fly mid-guts. Such investigations may allow to understand how the multiple infections evolve from the tsetse flies mid-guts to the salivary glands and also to understand the consequence of these evolutions on the dynamic (which genotype is transmitted to mammals) of trypanosomes transmission.

Trypanosoma brucei gambiense: study of population genetic structure of Central African stocks using amplified fragment length polymorphism (AFLP)

To understand the maintenance and resurgence of historical Human African Trypanosomiasis (HAT) foci, AFLP was used to genotype 100 Central African Trypanosoma brucei s.l. stocks. This technique confirmed the high genetic stability of T. b. gambiense group 1 stocks and the micro genetic variability within Central African T. b. gambiense stocks. It revealed several T. b. gambiense genotypes and allowed the identification of minor and major genotypes in HAT foci. The coexistence of these genotypes in the same focus suggests that clustering of stocks according to HAT focus does not provide the true genetic picture of trypanosome circulating within the disease focus because the minor genotypes are generally underestimated. The presence of minor and major genotypes in HAT foci may explain the persistence and the resurgence of Central African sleeping sickness foci.

Trypanosoma brucei s.l.: Characterisation of stocks from Central Africa by PCR analysis of mobile genetic elements

To better understand the epidemiology of sleeping sickness in the Central African sub-region, notably the heterogeneity of Human African Trypanosomiasis (HAT) foci, the mobile genetic element PCR (MGE-PCR) technique was used to genotype Trypanosoma brucei s.l. (T. brucei s.l.) isolates from this sub-region. Using a single primer REV B, which detects positional variation of the mobile genetic element RIME, via amplification of flanking regions, MGE-PCR revealed a micro genetic variability between Trypanosoma brucei gambiense (T. b. gambiense) isolates from Central Africa. The technique also revealed the presence of several T. b. gambiense genotypes and allowed the identification of minor and major ubiquitous genotypes in HAT foci. The presence of several T. b. gambiense genotypes in HAT foci may explain the persistence and the resurgence phenomena of the disease and also the epidemic and the endemic status of some Central African sleeping sickness foci. The MGE-PCR technique represents a simple, rapid, and specific method to differentiate Central African T. brucei s.l. isolates.

Phylogenetic analysis of freshwater fish trypanosomes from Europe using ssu rRNA gene sequences and random amplification of polymorphic DNA

The taxonomy and phylogenetic relationships of fish trypanosomes are uncertain. A collection of 22 cloned trypanosome isolates from 14 species of European freshwater fish and 1 species of African freshwater fish were examined by molecular phylogenetic analysis. The small subunit ribosomal RNA (ssu rRNA) genes of 8 clones were sequenced and compared with ssu rRNA gene sequences from a wider selection of vertebrate trypanosome isolates by phylogenetic analysis. All trypanosomes from freshwater fish fell in a single clade, subdivided into 3 groups. This clade sits within a larger, robust clade containing trypanosomes from marine fish and various amphibious vertebrates. All 22 trypanosome clones were analysed by random amplification of polymorphic DNA. The resulting dendrogram shows 3 groups, which are congruent with the groups identified in the ssu rRNA gene phylogeny. Two of the groups contain the majority of trypanosome isolates and within-group variation is slight. These groups do not separate purported trypanosome species distinguished by morphology or host origin, and thus these criteria do not appear to be reliable guides to genetic relationships among fish trypanosomes. However, we suggest that the 2 groups themselves may represent different species of fish trypanosomes. The polymorphic DNA markers we have identified will facilitate future comparisons of the biology of these 2 groups of fish trypanosomes.

[Contributions and limits of the diagnosis of human African trypanosomiasis]

Human African trypanosomiasis, or sleeping sickness, is still a worrying problem in Africa. Sleeping sickness is a disease for which a systematic monitoring is necessary, particularly for the trypanosomiasis caused by Trypanosoma brucei gambiense, which is characterized by a long asymptomatic stage. In the absence of specific clinical signs, mass screening of populations remains the only way to control the disease and to avoid its spreading. The lack of sensitivity and specificity of the diagnosis tests classically used led to the development of molecular tools. PCR amplification of parasite specific sequences has considerably improved the diagnostic of the parasitic infection, the stage diagnosis as well as the post-therapeutic follow-up. But there are limits with a use in routine and research is still necessary to make PCR a real tool for control of sleeping sickness.

Molecular profiles of Trypanosoma brucei, T. evansi and T. equiperdum stocks revealed by the random amplified polymorphic DNA method

A total of 20 random primers (10-mers) were used to amplify RAPD markers from the genomic DNA of four Trypanosoma brucei stocks from East and West Africa, four T. evansi stocks from Africa, Asia and South America and one T. equiperdum stock from Asia. Between 65 and 88 reproducible fragments ranging from 0.25 to 2.15 kb were generated from these stocks depending on the stock/primer combination. The similarity coefficient (SC) among the stocks of T. brucei from Kenya, Nigeria, Tanzania and Zambia ranged from 62.9% to 74.0% (average: 67.6%). The SC among the stocks of T. evansi from Kenya, China and Brazil was 76.4%-95.5% (average: 86.4%), while the SC between T. evansi stock from China and Brazil was 95.5%. For T. evansi and T. equiperdum, the SC among the stocks ranged from 81.2% to 94.4% (average: 87.6%). As for the SC among the stocks of T. brucei and T. evansi, it was found to be from 54.7% to 80.3% (average: 68.0%) and the SC among stocks of T. brucei and T. equiperdum was from 59.4% to 76.9% (average: 68.1%). Our results indicate that the stocks of T. evansi from China and from Brazil are more closely related to the stock of T. equiperdum from China than to the stocks of T. evansi isolated from Kenya and to the stocks of T. brucei. In addition, our results further support the hypothesis that T. evansi stocks from China and Brazil could have arisen from a single lineage. The possible evolution of T. evansi and T. equiperdum is also discussed.

Genetic relatedness among Trypanosoma evansi stocks by random amplification of polymorphic DNA and evaluation of a synapomorphic DNA fragment for species-specific diagnosis

In this study we employed randomly amplified polymorphic DNA patterns to assess the genetic relatedness among 14 Brazilian Trypanosoma evansi stocks from domestic and wild hosts, which are known to differ in biological characteristics. These akinetoplastic stocks were compared with one another, to three Old World (Ethiopia, China and Philippines) dyskinetoplastic stocks of T. evansi, and also with Trypanosoma equiperdum, Trypanosoma brucei brucei, Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense. Randomly amplified polymorphic DNA analysis showed limited heterogeneity in T. evansi stocks from different hosts and geographical regions of the world, or in other species of the subgenus Trypanozoon. However, minor variations generated random amplification of polymorphic DNA analysis disclosed a pattern consisting of a unique synapomorphic DNA fragment (termed Te664) for the T. evansi cluster that was not detected in any other trypanosome species investigated. Pulsed field gel electrophoresis analysis demonstrated that the Te664 fragment is a repetitive sequence, dispersed in intermediate and minichromosomes of T. evansi. Based on this sequence, we developed a conventional PCR assay for the detection of T. evansi using crude preparations of blood collected either on glass slides or on filter paper as template DNA. Our results showed that this assay may be useful as a diagnostic tool for field-epidemiological studies of T. evansi.

Molecular characterization of field isolates of human pathogenic trypanosomes

The accurate identification of each of the three subspecies of Trypanosoma brucei remains a challenging problem in the epidemiology of sleeping sickness. Advances in molecular characterization have revealed a much greater degree of heterogeneity within the species than previously supposed. Only group 1 T. b. gambiense stands out as a separate entity, defined by several molecular markers. T. b. rhodesiense is generally too similar to sympatric T. b. brucei strains to be distinguished from them by any particular molecular markers. Nevertheless, characterization of trypanosome isolates from humans and other animals has allowed the identification of potential reservoir hosts of T. b. rhodesiense. The recent discovery of a gene for human serum resistance may provide a useful marker for T. b. rhodesiense in the future. There have been few attempts to find associations between genetic markers and other biological characters, except human infectivity. However, virulence or fly transmissibility have been correlated with molecular markers in some instances.

Theileria annulata: genetic characterization of Spanish isolates by isoenzyme electrophoresis and random amplified polymorphic DNA

Isoenzyme electrophoresis and RAPD techniques were used to study the genetic polymorphism of different Spanish isolates of Theileria annulata in the schizont and piroplasm stages. Enzyme activity attributable to the parasite was detected in only 5 of the 13 loci studied with isoenzyme electrophoresis. Of these, differences between the cell lines studied were found only in the loci GPI, ICD, and FH. Only 6 of the 11 primers used in the RAPD generated reproducible genomic DNA fingerprints. None of the amplification products generated using primers ILO 509, ILO 525, ILO 872, and ILO 875 hybridized with DNA of the bovine cell line not infected by T. annulata, BL20, indicating that this technique can be used with either piroplasm DNA or DNA from parasite schizonts after first passing it through DEAE cellulose columns. The results obtained with both characterization techniques demonstrate a moderate degree of polymorphism among the Spanish isolates of this protozoan.

Genetic exchange in the trypanosomatidae

The only trypanosomatid so far proved to undergo genetic exchange is Trypanosoma brucei, for which hybrid production after co-transmission of different parental strains through the tsetse fly vector has been demonstrated experimentally. Analogous mating experiments have been attempted with other Trypanosoma and Leishmania species, so far without success. However, natural Leishmania hybrids, with a combination of the molecular characters of two sympatric species, have been described amongst both New and Old World isolates. Typical homozygotic and heterozygotic banding patterns for isoenzyme and deoxyribonucleic acid markers have also been demonstrated amongst naturally-occurring T. cruzi isolates. The mechanism of genetic exchange in T. brucei remains unclear, although it appears to be a true sexual process involving meiosis. However, no haploid stage has been observed, and intermediates in the process are still a matter for conjecture. The frequency of sex in trypanosomes in nature is also a matter for speculation and controversy, with conflicting results arising from population genetics analysis. Experimental findings for T. brucei are discussed in the first section of this review, together with laboratory evidence of genetic exchange in other species. The second section covers population genetics analysis of the large body of data from field isolates of Leishmania and Trypanosoma species. The final discussion attempts to put the evidence from experimental and population genetics into its biological context.

Wide variation in DNA content among isolates of Trypanosoma brucei ssp

The DNA contents of 18 Trypanosoma brucei ssp. stocks were compared using flow cytometry, karyotype analysis and quantitation of repetitive DNA by Southern blotting and hybridisation. The DNA contents of Type 1 T. b. gambiense stocks were lower than those of non-gambiense stocks, but both groups showed a wide range of variation in DNA content. Amongst T. b. gambiense stocks. Mabia at the lower end of the range had 14% less DNA than Dal 972 at the top of the range. Similarly, amongst non-gambiense stocks. 117R at the lower end of the range had 14% less DNA than LM55 at the top of the range. The T. b. gambiense stock Mabia had 29% less DNA than non-gambiense stock LM55. The DNA content of Type II T. b. gambiense stocks had minichromosomes albeit fewer than non-gambiense stocks. This result was verified by hybridisation with probes for satellite DNA and a telomere-specific repeat. Hybridisation with the probe for the beta-tubulin genes also revealed an apparent reduction of gene copy number T. b. gambiense relative to non-gambiense stocks. In conclusion, there is a wide range of variation in genome size in T. brucei ssp., with T. b. gambiense stocks at the lower end of the range. The reduction in genome size correlates with loss of repeated genes and non-coding sequences in T. b. gambiense stocks, and is not continued to chromosomes of a particular size.