Different allele frequencies in Trypanosoma brucei brucei and Trypanosoma brucei gambiense populations

Loop-Mediated Isothermal Amplification Test for Trypanosoma gambiense Group 1 with Stem Primers: A Molecular Xenomonitoring Test for Sleeping Sickness

The World Health Organization has targeted Human African Trypanosomiasis (HAT) for elimination by 2020 with zero incidence by 2030. To achieve and sustain this goal, accurate and easy-to-deploy diagnostic tests for Gambian trypanosomiasis which accounts for over 98% of reported cases will play a crucial role. Most needed will be tools for surveillance of pathogen in vectors (xenomonitoring) since population screening tests are readily available. The development of new tests is expensive and takes a long time while incremental improvement of existing technologies that have potential for xenomonitoring may offer a shorter pathway to tools for HAT surveillance. We have investigated the effect of including a second set of reaction accelerating primers (stem primers) to the standard T. brucei gambiense LAMP test format. The new test format was analyzed with and without outer primers. Amplification was carried out using Rotorgene 6000 and the portable ESE Quant amplification unit capable of real-time data output. The stem LAMP formats indicated shorter time to results (~8 min), were 10–100-fold more sensitive, and indicated higher diagnostic sensitivity and accuracy compared to the standard LAMP test. It was possible to confirm the predicted product using ESE melt curves demonstrating the potential of combining LAMP and real-time technologies as possible tool for HAT molecular xenomonitoring.

Human and animal Trypanosomes in Côte d'Ivoire form a single breeding population

BACKGROUND

Trypanosoma brucei is the causative agent of African Sleeping Sickness in humans and contributes to the related veterinary disease, Nagana. T. brucei is segregated into three subspecies based on host specificity, geography and pathology. T. b. brucei is limited to animals (excluding some primates) throughout sub-Saharan Africa and is non-infective to humans due to trypanolytic factors found in human serum. T. b. gambiense and T. b. rhodesiense are human infective sub-species. T. b. gambiense is the more prevalent human, causing over 97% of human cases. Study of T. b. gambiense is complicated in that there are two distinct groups delineated by genetics and phenotype. The relationships between the two groups and local T. b. brucei are unclear and may have a bearing on the evolution of the human infectivity traits.

METHODOLOGY/PRINCIPAL FINDINGS

A collection of sympatric T. brucei isolates from Côte d'Ivoire, consisting of T. b. brucei and both groups of T. b. gambiense have previously been categorized by isoenzymes, RFLPs and Blood Incubation Infectivity Tests. These samples were further characterized using the group 1 specific marker, TgSGP, and seven microsatellites. The relationships between the T. b. brucei and T. b. gambiense isolates were determined using principal components analysis, neighbor-joining phylogenetics, STRUCTURE, FST, Hardy-Weinberg equilibrium and linkage disequilibrium.

CONCLUSIONS/SIGNIFICANCE

Group 1 T. b. gambiense form a clonal genetic group, distinct from group 2 and T. b. brucei, whereas group 2 T. b. gambiense are genetically indistinguishable from local T. b. brucei. There is strong evidence for mating within and between group 2 T. b. gambiense and T. b. brucei. We found no evidence to support the hypothesis that group 2 T. b. gambiense are hybrids of group 1 and T. b. brucei, suggesting that human infectivity has evolved independently in groups 1 and 2 T. b. gambiense.

The Trypanosoma brucei gambiense secretome impairs lipopolysaccharide-induced maturation, cytokine production, and allostimulatory capacity of dendritic cells

Trypanosoma brucei gambiense, a parasitic protozoan belonging to kinetoplastids, is the main etiological agent of human African trypanosomiasis (HAT), or sleeping sickness. One major characteristic of this disease is the dysregulation of the host immune system. The present study demonstrates that the secretome (excreted-secreted proteins) of T. b. gambiense impairs the lipopolysaccharide (LPS)-induced maturation of murine dendritic cells (DCs). The upregulation of major histocompatibility complex class II, CD40, CD80, and CD86 molecules, as well as the secretion of cytokines such as tumor necrosis factor alpha, interleukin-10 (IL-10), and IL-6, which are normally released at high levels by LPS-stimulated DCs, is significantly reduced when these cells are cultured in the presence of the T. b. gambiense secretome. Moreover, the inhibition of DC maturation results in the loss of their allostimulatory capacity, leading to a dramatic decrease in Th1/Th2 cytokine production by cocultured lymphocytes. These results provide new insights into a novel efficient immunosuppressive mechanism directly involving the alteration of DC function which might be used by T. b. gambiense to interfere with the host immune responses in HAT and promote the infectious process.

Detection of Group 1 Trypanosoma brucei gambiense by loop-mediated isothermal amplification

Trypanosoma brucei gambiense group 1 is the major causative agent of the Gambian human African trypanosomiasis (HAT). Accurate diagnosis of Gambian HAT is still challenged by lack of precise diagnostic methods, low and fluctuating parasitemia, and generally poor services in the areas of endemicity. In this study, we designed a rapid loop-mediated isothermal amplification (LAMP) test for T. b. gambiense based on the 3' end of the T. b. gambiense-specific glycoprotein (TgsGP) gene. The test is specific and amplifies DNA from T. b. gambiense isolates and clinical samples at 62°C within 40 min using a normal water bath. The analytical sensitivity of the TgsGP LAMP was equivalent to 10 trypanosomes/ml using purified DNA and ∼1 trypanosome/ml using supernatant prepared from boiled blood, while those of classical PCR tests ranged from 10 to 10(3) trypanosomes/ml. There was 100% agreement in the detection of the LAMP product by real-time gel electrophoresis and the DNA-intercalating dye SYBR green I. The LAMP amplicons were unequivocally confirmed through sequencing and analysis of melting curves. The assay was able to amplify parasite DNA from native cerebrospinal fluid (CSF) and double-centrifuged supernatant prepared from boiled buffy coat and bone marrow aspirate. The robustness, superior sensitivity, and ability to inspect results visually through color change indicate the potential of TgsGP LAMP as a future point-of-care test.

Conserved sequence of the TgsGP gene in Group 1 Trypanosoma brucei gambiense

The trypanosome responsible for the majority of cases of human trypanosomiasis in Africa is Group 1 Trypanosoma brucei gambiense. Currently the most reliable test for the parasite is based on a single gene, which encodes a 47kDa receptor-like T. b. gambiense-specific glycoprotein, TgsGP, expressed in the flagellar pocket of bloodstream forms. Although TgsGP has been demonstrated in T. b. gambiense throughout its geographic range, similar genes have been demonstrated in other T. brucei sspp. isolates, and there are no data on the extent of sequence variation in TgsGP. Here we have carried out a comparison of TgsGP sequences in a range of Group 1 T. b. gambiense isolates and compared the gene to homologues in other T. brucei sspp. in order to provide information to support the use of this gene as the key identification target for Group 1 T. b. gambiense. We demonstrate that the sequence of TgsGP is well conserved in Group 1 T. b. gambiense across the endemic range of gambian human trypanosomiasis and confirm that this gene is a suitable target for specific detection of this parasite. The TgsGp-like genes in some isolates of T. b. brucei, T. b. rhodesiense and Group 2 T. b. gambiense are closely similar to VSG Tb10.v4.0178, which may be the ancestral gene from which TgsGP was derived.

Molecular epidemiology of African sleeping sickness

Human sleeping sickness in Africa, caused by Trypanosoma brucei spp. raises a number of questions. Despite the widespread distribution of the tsetse vectors and animal trypanosomiasis, human disease is only found in discrete foci which periodically give rise to epidemics followed by periods of endemicity A key to unravelling this puzzle is a detailed knowledge of the aetiological agents responsible for different patterns of disease – knowledge that is difficult to achieve using traditional microscopy. The science of molecular epidemiology has developed a range of tools which have enabled us to accurately identify taxonomic groups at all levels (species, subspecies, populations, strains and isolates). Using these tools, we can now investigate the genetic interactions within and between populations of Trypanosoma brucei and gain an understanding of the distinction between human- and nonhuman-infective subspecies. In this review, we discuss the development of these tools, their advantages and disadvantages and describe how they have been used to understand parasite genetic diversity, the origin of epidemics, the role of reservoir hosts and the population structure. Using the specific case of T.b. rhodesiense in Uganda, we illustrate how molecular epidemiology has enabled us to construct a more detailed understanding of the origins, generation and dynamics of sleeping sickness epidemics.

Genetic analysis of the human infective trypanosome Trypanosoma brucei gambiense: chromosomal segregation, crossing over, and the construction of a genetic map

A high-resolution genetic linkage map of the STIB 386 strain of Trypanosoma brucei gambiense is presented.

Background

Trypanosoma brucei is the causative agent of human sleeping sickness and animal trypanosomiasis in sub-Saharan Africa, and it has been subdivided into three subspecies: Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense, which cause sleeping sickness in humans, and the nonhuman infective Trypanosoma brucei brucei. T. b. gambiense is the most clinically relevant subspecies, being responsible for more than 90% of all trypanosomal disease in humans. The genome sequence is now available, and a Mendelian genetic system has been demonstrated in T. brucei, facilitating genetic analysis in this diploid protozoan parasite. As an essential step toward identifying loci that determine important traits in the human-infective subspecies, we report the construction of a high-resolution genetic map of the STIB 386 strain of T. b. gambiense.

Results

The genetic map was determined using 119 microsatellite markers assigned to the 11 megabase chromosomes. The total genetic map length of the linkage groups was 733.1 cM, covering a physical distance of 17.9 megabases with an average map unit size of 24 kilobases/cM. Forty-seven markers in this map were also used in a genetic map of the nonhuman infective T. b. brucei subspecies, permitting comparison of the two maps and showing that synteny is conserved between the two subspecies.

Conclusion

The genetic linkage map presented here is the first available for the human-infective trypanosome T. b. gambiense. In combination with the genome sequence, this opens up the possibility of using genetic analysis to identify the loci responsible for T. b. gambiense specific traits such as human infectivity as well as comparative studies of parasite field populations.

Resolution of the species problem in African trypanosomes

There is a general assumption that eukaryote species are demarcated by morphological or genetic discontinuities. This stems from the idea that species are defined by the ability of individuals to mate and produce viable progeny. At the microscopic level, where organisms often proliferate more by asexual than sexual reproduction, this tidy classification system breaks down and species definition becomes messy and problematic. The dearth of morphological characters to distinguish microbial species has led to the widespread application of molecular methods for identification. As well as providing molecular markers for species identification, gene sequencing has generated the data for accurate estimation of relatedness between different populations of microbes. This has led to recognition of conflicts between current taxonomic designations and phylogenetic placement. In the case of microbial pathogens, the extent to which taxonomy has been driven by utilitarian rather than biological considerations has been made explicit by molecular phylogenetic analysis. These issues are discussed with reference to the taxonomy of the African trypanosomes, where pathogenicity, host range and distribution have been influential in the designation of species and subspecies. Effectively, the taxonomic units recognised are those that are meaningful in terms of human or animal disease. The underlying genetic differences separating the currently recognised trypanosome taxa are not consistent, ranging from genome-wide divergence to presence/absence of a single gene. Nevertheless, if even a minor genetic difference reflects adaptation to a particular parasitic niche, for example, in Trypanosoma brucei rhodesiense, the presence of a single gene conferring the ability to infect humans, then it can prove useful as an identification tag for the taxon occupying that niche. Thus, the species problem can be resolved by bringing together considerations of utility, genetic difference and adaptation.

The origins, dynamics and generation of Trypanosoma brucei rhodesiense epidemics in East Africa

The history of sleeping sickness in East Africa has provoked controversy not only about the origins and spread of the disease, but also the identity of the causative organisms involved. Molecular methodology(1) has shed new light on the genetic makeup of the organisms involved in recent epidemics. Here, Geoff Hide, Andrew Tait, Ian Maudlin and Susan Welburn discuss these new data in relation to previous theories about the origins of epidemics in East Africa which emphasized the importance of the introduction of new strains.

An isoenzyme survey ofTrypanosoma bruceis.l. from the Central African subregion: population structure, taxonomic and epidemiological considerations

In order to improve our knowledge about the taxonomic status and the population structure of the causative agent of Human African Trypanosomiasis in the Central African subregion, 169 newly isolated stocks, of which 16 came from pigs, and 5 reference stocks, were characterized by multilocus enzyme electrophoresis, for 17 genetic loci. We identified 22 different isoenzyme profiles or zymodemes, many of which showed limited differences between them. These zymodemes were equated to multilocus genotypes. UPGMA dendrograms revealed one main group:Trypanosoma brucei gambiensegroup I and 3T. brucei‘non-gambiense’ stocks.T. b. gambiensegroup I zymodemes were very homogenous, grouping all the human stocks and 31% of the pig stocks. Two main zymodemes (Z1 and Z3) grouping 74% of the stocks were found in different remote countries. The genetic distances were relatively high inT. brucei‘non-gambiense’ zymodemes, regrouping 69% of pig stocks. The analysis of linkage disequilibrium was in favour of a predominantly clonal population structure. This was supported by the ubiquitous occurrence of the main zymodemes, suggesting genetic stability in time and space of this parasite's natural clones. However, in some cases an epidemic population structure could not be ruled out. Our study also suggested that the domestic pig was a probable reservoir host forT. b. gambiensegroup I in Cameroon.

Species concepts for trypanosomes: from morphological to molecular definitions?

The way species and subspecies names are applied in African trypanosomes of subgenera Trypanozoon and Nannomonas is reviewed in the light of data from molecular taxonomy. In subgenus Trypanozoon the taxonomic importance of pathogenicity, host range and distribution appear to have been inflated relative to actual levels of genetic divergence. The opposite is true for subgenus Nannomonas, where current taxonomic usage badly underrepresents genetic diversity. Data from molecular characterisation studies are revealing a growing number of genotypes, which may represent distinct taxa. Unfortunately few of these genotypes are yet supported by sufficient biological data to be recognized taxonomically. But we may be missing fundamental epidemiological information, because of our inability to distinguish these trypanosomes in host blood morphologically or in tsetse by their developmental cycle. Molecular taxonomy has led the way in identifying these new genotypes and now offers the key to elucidating the biology of these organisms.

Will the real Trypanosoma brucei rhodesiense please step forward?

The sleeping sickness trypanosomes Trypanosoma brucei rhodesiense and T. brucei gambiense are morphologically indistinguishable from each other and from T. brucei brucei, which does not infect humans. The relationships between these three subspecies have been controversial. Several years ago, the characterization of T. brucei gambiense was reviewed in an attempt to clarify and draw together the results, and to put them in the context of the biology of the organism. The discovery of a gene associated with human-serum resistance in T. brucei rhodesiense and the consequent reappraisal of the identity of this trypanosome prompt this companion article.

Natural immunity to human African trypanosomiasis: trypanosome lytic factors and the blood incubation infectivity test

This review focuses on the epidemiology of human African trypanosomiasis: why it occurs in humans, the current methods of surveillance, and the drugs available to treat it. Emphasis is placed on the identification of human-infective trypanosomes by the blood incubation infectivity test. This test distinguishes between trypanosomes that are non-infective for humans and those that are potentially infective. Currently the test requires incubation of parasites with human serum before injection into mice; any surviving parasites are considered human-infective. The factors in serum that kill all non-human-infective parasites are known as trypanosome lytic factors. The paper details the biochemistry of these factors and recommends standardization of the test based on current knowledge. This test can be used to screen animals with trypanosomiasis, in order to evaluate their role during endemic and epidemic human African trypanosomiasis.

Measure of molecular diversity within the Trypanosoma brucei subspecies Trypanosoma brucei brucei and Trypanosoma brucei gambiense as revealed by genotypic characterization

We have evaluated whether sequence polymorphisms in the rRNA intergenic spacer region can be used to study the relatedness of two subspecies of Trypanosoma brucei. Thirteen T. brucei isolates made up of 6 T. b. brucei and 7 T. b. gambiense were analyzed using restriction fragment length polymorphism (RFLP). By PCR-based restriction mapping of the ITS1-5.8S-ITS2 ribosomal repeat unit, we found a fingerprint pattern that separately identifies each of the two subspecies analyzed, with unique restriction fragments observed in all but 1 of the T. b. gambiense "human" isolates. Interestingly, the restriction profile for a virulent group 2 T. b. gambiense human isolate revealed an unusual RFLP pattern different from the profile of other human isolates. Sequencing data from four representatives of each of the two subspecies indicated that the intergenic spacer region had a conserved ITS-1 and a variable 5.8S with unique transversions, insertions, or deletions. The ITS-2 regions contained a single repeated element at similar positions in all isolates examined, but not in 2 of the human isolates. A unique 4-bp [C(3)A] sequence was found within the 5.8S region of human T. b. gambiense isolates. Phylogenetic analysis of the data suggests that their common ancestor was a nonhuman animal pathogen and that human pathogenicity might have evolved secondarily. Our data show that cryptic species within the T. brucei group can be distinguished by differences in the PCR-RFLP profile of the rDNA repeat.

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.

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.

Trypanosoma brucei s.l: evolution, linkage and the clonality debate

The Index of Association (IA) has been proposed by Maynard Smith et al. (1993) as a general method for characterizing the population structures of microorganisms as either: clonal, epidemic, cryptic species or panmictic. With reference to the current debate surrounding the mode of reproduction in parasitic protozoa, this study explores (i) the suitability and limitations of the IA for characterizing populations of Trypanosoma brucei s.l., and (ii) the idea that the significance of genetic differences between populations may be better understood if the evolution, spread and temporal stability of certain parasite genotypes are also considered. Four populations of T. brucei from Côte d'Ivoire, Uganda and Zambia are analysed using the IA and a complementary test for linkage disequilibrium, test f of Tibayrenc, Kjellberg & Ayala (1990). The two populations from Uganda are characterized as epidemic, while the others appear more or less clonal; the merits of the two methods are compared. The implications of the various population classifications are discussed with reference to genotype longevity in each region; the evolution and biomedical consequences of the genetic non-homogeneity of T. brucei are reviewed.

Families of adenylate cyclase genes in Trypanosoma brucei

Four genes for adenylate cyclase have been characterized in Trypanosoma brucei. One of them, esag 4 (for expression site associated gene 4) is present in different VSG (variant surface glycoprotein) gene expression sites and, thus, is only expressed in the bloodstream form of the parasite. The others, termed gresag 4.1, 4.2 and 4.3 (for genes related to esag 4) are expressed in both bloodstream and procyclic forms. In addition, we cloned a esag 4-related gene from T. congolense. Here we characterize the genomic organization of gresag 4.1 and 4.3. While gresag 4.3 is unique, gresag 4.1 exists as a multigenic family of at least nine members located on a 3-Mb chromosome. Six of them are clustered in a region of 300 kb, three copies being tandemly linked. The determination of the nucleotide sequence of a conserved 1.6 kb PstI fragment demonstrated the presence of two separate subgroups in this family. This gene arrangement is present in different isolates of T.b. brucei/rhodesiense/gambiense. Several gresag 4.1 copies are transcribed in both bloodstream and procyclic forms.

Population genetics of Trypanosoma brucei in central Africa: taxonomic and epidemiological significance

In order to estimate the value of population genetics for both the taxonomy of trypanosomes belonging to the species Trypanosoma brucei and a better understanding of Human African Trypanosomiasis (HAT), we undertook a cellulose acetate electrophoresis isoenzyme study involving 55 stocks isolated from man and animals in Congo, Zaire and Cameroun. Out of the 24 loci surveyed, 15 exhibited variability, which made it possible to delimit 23 zymodemes, divided into 2 groups. The first group equated to the classical subspecies Trypanosoma brucei gambiense, while the second corresponded to the classical subspecies Trypanosoma brucei brucei. These results broadly agree with the current taxonomy, and are corroborated by RFLP analysis of kDNA. Statistical analysis indicates a basically clonal reproduction system of the trypanosomes in the area studied; the zymodemes are equivalent to natural clones (or a family of closely related clones), stable in space and time. Epidemiological hypotheses are proposed according to the geographic distribution of the clones in this area.

Numerical taxonomy of Trypanozoon based on polymorphisms in a reduced range of enzymes

Numerical analyses of Trypanozoon taxonomy are presented, based on the isoenzyme data of Stevens et al. (1992). The previous study used a reduced range of enzymes compared with earlier work; the analyses indicate the value of this rationalized system. Both recently isolated trypanosome stocks and previously studied populations were included, allowing detailed comparison with earlier studies. Relationships between zymodemes were calculated with an improved similarity coefficient program, using Jaccard's coefficient (1908), and by Nei's method (1972). Dendrograms were constructed from the matrices produced with the group-average method. The groupings produced by both numerical methods were in close agreement, and the clusters of related principal zymodemes largely matched the species, subspecies and strain groups proposed by previous workers. Trypanozoon biochemical taxonomy is reviewed and the groupings reinforced by this study are: the mainly East African strain groups, busoga, zambezi, kakumbi, kiboko and sindo; T.b. gambiense and the bouaflé strain group from West Africa, and T. evansi; an intermediate bouaflé/busoga group was also recognized.

A simplified method for identifying subspecies and strain groups in Trypanozoon by isoenzymes

To characterize trypanosomes from the subgenus Trypanozoon, 272 stocks in 111 zymodemes were analysed by the polymorphisms seen in a rationalized range of nine enzymes, resolved by electrophoresis, mostly on cellulose acetate. Several highly polymorphic or invariant enzymes used previously were omitted, while two new enzymes, NHD and SOD were included; the isoenzymes seen for SOD were interpreted as two separate enzymes, SODA and SODB. Isoenzyme band patterns were analysed by two complementary numerical methods to elucide taxonomic relationships within the subgenus; groups of zymodemes corresponding to subspecies and strain groups were defined, which agreed closely with previous studies. Except for one zymodeme, Trypanosoma evansi could not be clearly distinguished from the bouaflé strain group. This strain group had enzymic features that overlapped to some extent those of the busoga group. Trypanosoma brucei gambiense and the zambezi, kakumbi, kiboko and sindo groups were clearly defined. Eight zymodemes could not be classified. A rapid identification system using a limited number of enzymes is presented.

High homology between variant surface glycoprotein gene expression sites of Trypanosoma brucei and Trypanosoma gambiense

The AnTat 11.17 variant surface glycoprotein (VSG) is synthesized in both metacyclic and bloodstream forms of Trypanosoma gambiense. We have characterized the AnTat 11.17 gene, and analyzed its expression site (ES) in the bloodstream form by Southern and Northern blotting with probes from the Trypanosoma brucei AnTat 1.3A VSG ES, and by run-on transcription. The AnTat 11.17 ES is located at the end of a 700-kb chromosome. It appears to contain all the genes (ESAGs, for Expression Site-Associated Genes) present in the AnTat 1.3A VSG ES, with the possible exception of ESAG 1. Limited nucleotide sequence analysis of ESAG cDNAs from the AnTat 11.17 ES shows considerable conservation with ESAGs of T. brucei. The transcription promoter of the AnTat 11.17 VSG ES, localized by virtue of the specific accumulation of promoter-proximal transcripts which occurs following UV irradiation, was found to be at the same relative position to the first ESAG (ESAG 7) as in AnTat 1.3A.

A family of repeated DNA sequences in Toxoplasma gondii: cloning, sequence analysis, and use in strain characterization

A Toxoplasma gondii genomic library was constructed in lambda EMBL3. Repeated fragments were detected by hybridization with radiolabeled total DNA from the parasite and one recombinant was chosen due to its strong hybridization signal. By using electrophoretic and hybridization analysis, four cross-hybridizating restriction fragments were selected and sequenced. The determined nucleotide sequence of these fragments (TGR1A, TGR1E, TGR2, and TGR4) has shown a complex system of conserved and degenerated repeats in which TGR1E corresponds to the most conserved element. This last sequence was used to investigate restriction fragment length polymorphisms among several T. gondii strains by Southern blotting.

Genetic exchange in Trypanosoma brucei brucei: variable chromosomal location of housekeeping genes in different trypanosome stocks

A Trypanosoma brucei brucei clone from West Africa was crossed with another T. b. brucei clone from the East African kiboko group. This group is defined by characteristic isoenzyme patterns and kinetoplast DNA maxicircle polymorphisms, and is associated with a wild animal-tsetse transmission cycle. Three types of clone were isolated from the cross, 2 of which were hybrid. The hybrids were heterozygotic at 7 loci where the parents were homozygotic and the hybrids also had molecular karyotypes different from those of both parents. Both molecular karyotypes had an extra non-parental band, which was shown to have a different origin in the 2 sets of clones by Southern analysis with various housekeeping gene probes. This analysis also revealed that although the GPI and PGK genes reside on the same chromosome in parent J10, they are on different chromosomes in parent 196. Hybridisation of PFG blots carrying a variety of other trypanosome stocks confirmed that the GPI gene is not always in the same linkage group as the PGK gene cluster. Given that genetic exchange in trypanosomes involves meiosis, such differences in gene linkage will give rise to progeny with incorrect gene dosage, i.e., certain crosses will be partially infertile. This incipient speciation may explain why natural populations of T. brucei spp. are observed not to be in a randomly mating equilibrium.

The molecular epidemiology of parasites

The explosion of new techniques, made available by the rapid advance in molecular biology, has provided a battery of novel approaches and technology which can be applied to more practical issues such as the epidemiology of parasites. In this review, we discuss the ways in which this new field of molecular epidemiology has contributed to and corroborated our existing knowledge of parasite epidemiology. Similar epidemiological questions can be asked about many different types of parasites and, using detailed examples such as the African trypanosomes and the Leishmania parasites, we discuss the techniques and the methodologies that have been or could be employed to solve many of these epidemiological problems.

Elusive trypanosomes

Professor Kershaw's encouragement of the development of anion-exchange separation of African trypanosomes from blood led to two decades of activity when, for the first time, considerable progress was made in the intrinsic characterization of these parasites. Such characterization depended on establishing high infections in laboratory rodents. However, the collection of samples from the field was restricted by the failure of certain trypanosomes either to infect, or to multiply adequately in, rodents. More recently, in vitro culture has come to play an increasingly important role in producing material. By obtaining procyclic forms directly from wild tsetse flies, or by transforming low numbers of bloodstream forms in field samples to the procyclic phase in experimental tsetse, trypanosomes of poor or nil infectivity to rodents were readily cultured in the large amounts required for biochemical characterization. A number of specimens of a new kind of Nannomonas, of Trypanosoma simiae, of T. grayi, and of an antigenically distinct T. brucei gambiense were found. Evidence is presented that many other kinds of trypanosome may be eluding isolation by their inability to infect rodents.

Evidence for kinetoplast and nuclear DNA homogeneity in Trypanosoma evansi isolates

The kinetoplast DNA minicircles from 13 stocks of trypanosomes designated as Trypanosoma evansi were digested with various restriction enzymes. We also examined the distribution of restriction site polymorphisms in the nuclear DNA of 9 of these stocks, using 7 different variable surface glycoprotein (VSG) and non-VSG probes. Restricted kinetoplast DNA (kDNA) fragments of some of these strains were cloned into M13 or PUC 18 vectors and sequenced. The restriction and sequence mapping showed that most of T. evansi isolates belonged to the A1 and A2 types of Borst and to two new closely related types A3 and A4. A notable exception was RoTat 4/1 derived from a Sudanese stock which was found to display a characteristic brucei-like minicircle heterogeneity. The T. evansi minicircles analysed are not only homogeneous in sequence but also the region similar to the conserved region in Trypanosoma brucei and Trypanosoma equiperdum is flanked on its 5' end by a palindromic repeat of part of the conserved region. The highly conserved sequence GGGCGGT which appears to correspond to the initiation of synthesis of one of the Okazaki fragments contains an additional G and is located as in T. brucei and T. equiperdum about 73 bp 5' from the ORI. The nuclear DNA analysis confirms the kDNA study in that all the T. evansi stocks are members of a very homogeneous group in terms of sequence divergence. Moreover, our analysis also confirms that T. evansi is more closely related to the West African T. b. brucei and T. b. gambiense than to other African trypanosomes.

A clonal theory of parasitic protozoa: the population structures of Entamoeba, Giardia, Leishmania, Naegleria, Plasmodium, Trichomonas, and Trypanosoma and their medical and taxonomical consequences

We propose a general theory of clonal reproduction for parasitic protozoa, which has important medical and biological consequences. Many parasitic protozoa have been assumed to reproduce sexually, because of diploidy and occasional sexuality in the laboratory. However, a population genetic analysis of extensive data on biochemical polymorphisms indicates that the two fundamental consequences of sexual reproduction (i.e., segregation and recombination) are apparently rare or absent in natural populations of the parasitic protozoa. Moreover, the clones recorded appear to be stable over large geographical areas and long periods of time. A clonal population structure demands that the medical attributes of clones be separately characterized; ubiquitous clones call for priority characterization. Uniparental reproduction renders unsatisfactory Linnean taxonomy; this needs to be supplemented by the "natural clone" as an additional taxonomic unit, which is best defined by means of genetic markers.

Population genetics of Trypanosoma brucei and the epidemiology of human sleeping sickness in the Lambwe Valley, Kenya

Numerical taxonomy was used to review isoenzyme variation in isolates of Trypanosoma brucei obtained from cattle, tsetse, humans and wildlife from the Lambwe Valley, Kenya. From isoenzyme information alone, it was possible to classify isolates as to source through the use of linear discriminant functions analysis, with an error rate of only 2% in humans, and 14% over all groups. Differentiation was mostly dependent on patterns in the enzymes ASAT, PEP1, and ICD. Parasites from non-human sources, especially tsetse, were characterized by high isoenzyme diversity, and many unique zymodemes. Observed frequencies of genotypes for ICD, ALAT, and ASAT did not agree with expected frequencies based on random mating of a diploid organism. Deviations were particularly large for tsetse isolates, and were mostly due to a deficiency of one homozygote. Cluster analysis revealed complex relationships among isolates, with patterns evolving through time. Major human zymodemes from the 1970s clustered together with most wildlife isolates from East Africa. This chronic human-wildlife transmission cycle was characterized by ASAT pattern I. Other, minor human zymodemes were associated with a human-cattle transmission cycle characterized by ASAT pattern VII. These original chronic transmission cycles appeared to change in 1980 with the appearance of two new zymodemes in humans. These zymodemes involved changes in ALAT and/or PGM to patterns typical of tsetse and cattle isolates. A resultant epidemic was halted with repeated aerial spraying of endosulfan in 1981. Since then, a variety of new zymodemes of unknown human infectivity have appeared. The origins of these changes are discussed in terms of genetic exchange in tsetse, adaptation to human and cattle transmission cycles, and selection resulting from chronic use of insecticides.

The identification of Trypanosoma brucei subspecies using repetitive DNA sequences

We describe the use of repetitive DNA probes to characterise the relationships between different stocks of African trypanosomes representing the subspecies of Trypanosoma brucei. Probes derived from the ribosomal RNA genes (coding region and nontranscribed spacer) and another repetitive DNA sequence were used to characterise trypanosome stocks by Southern blotting. Numerical taxonomy methods applied to the resulting restriction enzyme patterns were used to derive a dendrogram depicting the relationships between the stocks examined. We show that three groups of West African human infective stocks can be distinguished: firstly, a group containing exclusively T. b. gambiense; secondly, a group which is indistinguishable from animal isolates in West Africa; and thirdly, a single stock which is indistinguishable from East African T. b. rhodesiense. In addition, we observe that T. b. rhodesiense stocks from East Africa are indistinguishable from animal isolates from the same area. Finally, we show that a group of T. b. rhodesiense stocks, isolated from a 1978 sleeping sickness outbreak in Zambia, are probably derived from a single parasite strain, and that this strain is distinct from T. b. rhodesiense parasites from Kenya and Uganda.