Evidence for diploidy and mating in trypanosomes

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.

Multilocus enzyme electrophoresis: a valuable technique for providing answers to problems in parasite systematics

The aim of this review is to highlight the effectiveness of the technique of multilocus enzyme electrophoresis in answering questions relating to the systematics of parasites and to highlight errors in the way the technique has been used and the results interpreted. We have approached this topic by answering specific questions that we have been asked by colleagues and students not necessarily familiar with the technique, the method of data analysis and its application. Although the technique has been applied to provide answers for taxonomic and population genetics studies, it remains under-utilised, perhaps because of recent advances in newer molecular technology. Rather than not acknowledge or dismiss the value of more traditional technology, we suggest that researchers examine problems in the systematics of parasites by the comparison of data derived from morphological, biochemical and molecular techniques.

History of sleeping sickness in East Africa

The history of human sleeping sickness in East Africa is characterized by the appearance of disease epidemics interspersed by long periods of endemicity. Despite the presence of the tsetse fly in large areas of East Africa, these epidemics tend to occur multiply in specific regions or foci rather than spreading over vast areas. Many theories have been proposed to explain this phenomenon, but recent molecular approaches and detailed analyses of epidemics have highlighted the stability of human-infective trypanosome strains within these foci. The new molecular data, taken alongside the history and biology of human sleeping sickness, are beginning to highlight the important factors involved in the generation of epidemics. Specific, human-infective trypanosome strains may be associated with each focus, which, in the presence of the right conditions, can be responsible for the generation of an epidemic. Changes in agricultural practice, favoring the presence of tsetse flies, and the important contribution of domestic animals as a reservoir for the parasite are key factors in the maintenance of such epidemics. This review examines the contribution of molecular and genetic data to our understanding of the epidemiology and history of human sleeping sickness in East Africa.

Genetic epidemiology of parasitic protozoa and other infectious agents: the need for an integrated approach

This paper emphasises the relevance of the concepts and methods of evolutionary genetics for studying the epidemiology of parasitic protozoa and other pathogenic agents. Population genetics and phylogenetic analysis both contribute to identifying the relevant evolutionary and epidemiologically discrete units of research (Discrete typing units = DTUs), that can be equated to distinct phylogenetic lines. It is necessary (i) to establish that a given species represents a reliable DTU; (ii) to see whether a given species is further structured into lower DTUs that correspond to either clonal lineages or to cryptic species, and could exhibit distinct biomedical properties (virulence, resistance to drugs, etc). DTUs at the species and subspecies level can be conveniently identified by specific genetic markers or sets of genetic markers ("tags") for epidemiological follow-up. For any kind of pathogen (protozoa, fungi, bacteria, viruses), DTUs represent the relevant units of research, not only for epidemiology, but also, for other applied researches (clinical study, pathogenicity, vaccine and drug design, immunology, etc). The development of an "integrated genetic epidemiology of infectious diseases", that would explore the respective role of, and the interactions between, the genetic diversity (and its biological consequences) of the pathogen, the host and the vector (in the case of vector-borne diseases) is called for.

Towards a unified evolutionary genetics of microorganisms

I propose here that evolutionary genetics, apart from improving our basic knowledge of the taxonomy and evolution of microbes (either eukaryotes or prokaryotes), can also greatly contribute to applied research in microbiology. Evolutionary genetics provides convenient guidelines for better interpreting genetic and molecular data dealing with microorganisms. The three main potential applications of evolutionary genetics in microbiology are (a) epidemiological follow-up (with the necessity of evaluating the stability of microbial genotypes over space and time); (b) taxonomy in the broad sense (better definition and sharper delimitation of presently described taxa, research of hidden genetic subdivisions); and (c) evaluation of the impact of the genetic diversity of microbes on their relevant properties (pathogenicity, resistance to drugs, etc). At present, two main kinds of population structure can be distinguished in natural microbial populations: (a) species that are not subdivided into discrete phylogenetic lineages (panmictic species or basically sexual species with occasional bouts of short-term clonality fall into this category); (b) species that are strongly subdivided by either cryptic speciation or clonal evolution. Improvements in available statistical methods are required to refine these distinctions and to better quantify the actual impact of gene exchange in natural microbial populations. Moreover, a codified selection of markers with appropriate molecular clocks (in other words: adapted levels of resolution) is sorely needed to answer distinct questions that address different scales of time and space: experimental, epidemic, and evolutionary. The problems raised by natural genetic diversity are very similar for all microbial species, in terms of both basic and applied science. Despite this fact, a regrettable compartmentalization among specialists has hampered progress in this field. I propose a synthetic approach, relying on the statistical improvements and technical standardizations called for above, to settle a unified evolutionary genetics of microorganisms, valid whatever the species studied, whether eukaryotic (parasitic protozoa and fungi) or prokaryotic (bacteria). Apart from benefits for basic evolutionary research, the anticipated payoff from this synthetic approach is to render routine and common-place the use of microbial evolutionary genetics in the fields of epidemiology, medicine, and agronomy.

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.

Detection of linkage disequilibrium in Trypanosoma brucei isolated from tsetse flies and characterized by RAPD analysis and isoenzymes

This study analyses the different populations of Trypanosoma brucei spp. which may coexist within the midgut of wild tsetse flies (Stevens et al. 1994). Cloned trypanosome populations characterized by multilocus enzyme electrophoresis (MLEE) were further analysed by the random amplified polymorphic DNA (RAPD) technique, allowing detection of genetic variation at a finer level than that possible by MLEE. Genetic distance matrices derived from the results of each of the two biochemical methods were calculated and compared using a computer program based on the method of Mantel (1967). The observed correlation was used to investigate the degree of linkage disequilibrium (LD) in the data, association between unrelated polymorphic markers providing a measure of the departure from panmixia. The potential of each biochemical method to detect linkage was evaluated by an extended Mantel test. The MLEE/RAPD correlation test evidenced significant LD within the population, suggesting a predominantly clonal method of reproduction for these West African trypanosomes. Analysis of RAPD data by the extended Mantel test also showed significant LD, while the results with MLEE data were less conclusive, providing an indication of the relative potential of the two techniques to detect fine genetic variation.

Subcellular distribution and characterization of glucosephosphate isomerase in Leishmania mexicana mexicana

The glycolytic enzyme glucosephosphate isomerase (PGI) is present in two different cell compartments of Leishmania mexicana promastigotes; more than 90% of the activity was detected in the cytosol, the remainder in glycosomes. This subcellular distribution contrasts with that in Trypanosoma brucei, in which the enzyme activity has been mainly located in the glycosomes. PGI was partially purified from L. mexicana cell extracts. Throughout the purification procedure only one single PGI activity could be detected. The partially purified protein had the same subunit molecular mass (65 kDa) as the previously characterized glycosomal protein of T. brucei. Both proteins were also very similar with respect to their kinetic and antigenic properties. Using the T. brucei glycosomal PGI gene as a hybridization probe, we cloned the corresponding gene of L. mexicana. Only a single PGI locus could be detected in the L. mexicana genome. Characterization of the cloned gene showed that it codes for a polypeptide of 604 amino acids, with a molecular mass of 67,113. The sequences of the Leishmania and Trypanosoma polypeptides are 69% identical. They differ in calculated net charge (-8 versus -2, respectively) and isoelectric point (6.65 versus 7.35). Our data strongly suggest that the PGI activity in the two cell compartments of L. mexicana and T. brucei is not attributable to different isoenzymes. We discuss the possible metabolic function of the highly different enzyme distribution in the two organisms, and the molecular mechanism that could be responsible for it.

Analysis of a new genetic cross between two East African Trypanosoma brucei clones

Two clones of East African Trypanosoma brucei, with distinct homozygous isoenzyme patterns for one of three enzymes examined, were cotransmitted through the tsetse fly vector Glossina morsitans centralis. Flies with mature infections were individually fed on mice and the subsequent bloodstream from populations analysed for the presence of hybrid trypanosomes by isoenzyme analysis. Several combinations have previously been detected using this approach (Schweizer, Tait & Jenni, 1988; Sternberg et al. 1989). Four clones were isolated from one of the hybrid-containing populations. They showed a hybrid phenotype, as would be expected for the F1 progeny in a diploid Mendelian system. The analysis of the progeny clones, using two gene probes which detect restriction fragment length polymorphisms between the two parental stocks, showed that alleles had segregated at each locus and given rise to three different non-parental combinations of alleles in the hybrid progeny. Characterization of the hybrid progeny clones by PFGE (pulsed field gradient gel electrophoresis) revealed that all progeny clones were recombinant for the intermediate size chromosomes. From the analysis of the segregation of the larger chromosomes, marked by PGK (phosphoglycerate kinase) and CP (cysteine protease) gene probes, it was inferred that the progeny clones did not result from a direct fusion of diploid cells. Results with the PGK probe fit into a classical system with meiosis and subsequent fusion of the nuclei to form diploid progeny. On the other hand, blots with the CP probe as well as some of the ethidium bromide stained PFGE gels revealed the existence of non-parental size chromosomes in some of the hybrid progeny. This phenomenon was observed previously (Gibson, 1989) and further investigation is required to elucidate the mechanism.

Epidemiological relationships of Trypanosoma brucei stocks from south east Uganda: evidence for different population structures in human infective and non-human infective isolates

This study represents an analysis of trypanosome strains circulating within a confined location over a short period of time during a sleeping sickness epidemic in S.E. Uganda. A large number of Trypanosoma brucei isolates (88) were collected from a variety of hosts (man, cattle, pigs and tsetse) from villages within a 10 km radius and were analysed for variation in isoenzyme patterns, restriction fragment length polymorphism (RFLP) in repetitive DNA sequences and susceptibility to human serum. The human infective stocks form a clearly distinguishable population when compared with other stocks circulating in the domestic cattle reservoir. The data here support the occurrence of genetic exchange between the cattle stocks while an 'epidemic' population structure involving limited genetic exchange is a characteristic of the human infective stocks. Furthermore, it is shown that when both RFLP and isoenzyme analysis are carried out most stocks appear to have individual genotypes. Stocks which were formerly grouped as zymodemes are better considered as a collected of distinct individuals.

Identification of Trypanosoma evansi, Trypanosoma equiperdum and Trypanosoma brucei brucei using repetitive DNA probes

The phylogenetic relatedness of 15 stocks of Trypanosoma evansi, three stocks of Trypanosoma equiperdum and one stock of Trypanosoma brucei brucei was determined using Southern blot analysis of restriction enzyme digested DNA, probed with two repetitive DNA sequences from T. b. brucei. A dendrogram derived by cluster analysis of restriction fragment length polymorphism (RFLP) revealed three groups of related stocks. Group 1 included 14 stocks of T. evansi and one stock of T. equiperdum. Group 2 included two stocks of T. equiperdum and one stock of T. evansi. Group 3 included the one stock of T. brucei brucei. Group 2 is more closely related to Group 3 than Group 1, by analysis of the banding patterns. Further analysis of the T. evansi in Group 1 revealed that the patterns of isolates from different provinces in China were identical, but differed from T. evansi isolated from Africa, South America and the Philippines. These results provide insight into the origins of T. evansi and suggest that RFLP may be a useful means of distinguishing closely related trypanosomes.

Detection of trypanosome infections in the saliva of tsetse flies and buffy-coat samples from antigenaemic but aparasitaemic cattle

Relatively simple protocols employing non-radioactive DNA probes have been used for the detection of African trypanosomes in the blood of mammalian hosts and the saliva of live tsetse flies. In combination with the polymerase chain reaction (PCR), the protocols revealed trypanosomes in buffy-coat samples from antigenaemic but aparasitaemic cattle and in the saliva of live, infected tsetse flies. Furthermore, the protocols were used to demonstrate concurrent natural infections of single tsetse flies with different species of African trypanosomes.

Cell-to-cell interactions suggesting a sexual process in Herpetomonas megaseliae (Kinetoplastida: Trypanosomatidae)

Giemsa-stained smears of Herpetomonas megaseliae cultures in LIT medium displayed on several occasions not only the typically dividing promastigotes but also pairs of apposed parasites attached by their posterior ends, where a cellular enlargement and a ring-like border generally occurred; other pairs formed by an elongated promastigote and a bell-shaped cell were also found. In both cases, either each of the joined cells could have its own nucleus or one of them was anucleate and the other presented two nuclei that were sometimes so close that they appeared to be a single nucleus. These findings, along with others seeming to be intermediate steps between them, strongly suggested that a sexual process, involving parasite union followed by nuclear migration from one cell to the other and then by fusion, was taking place in such cultures.

Isoenzyme comparison of Trypanozoon isolates from two sleeping sickness areas of south-eastern Uganda

The study characterized 151 Trypanozoon isolates from south-east Uganda by isoenzyme electrophoresis. Stocks were from a range of hosts, including man, cattle, pigs, dogs and Glossina fuscipes fuscipes: 104 isolates were from the Busoga area, 47 were from the Tororo district. Stocks were characterized on thin layer starch gel using eight enzyme systems: ALAT, ASAT, ICD, MDH, ME, NHD, NHI, PGM. Enzyme profiles were generally typical of East Africa; new patterns for ICD and ME were detected. Trypanosomes were classified on the basis of their profile by similarity coefficient analysis and the unweighted pair-group method using arithmetic averages (UPGMA). The majority of trypanosomes were classified in one or other of two genetically distinct groups which corresponded to the strain groups busoga and zambezi, both of which are associated with Rhodesian sleeping sickness in East Africa. Contingency table analyses indicated associations between certain isoenzymes of ICD and PGM, according to host and geographical origin. Significant relationships between trypanosome strain group and geographic origin were also demonstrated for some host groups.

Molecular variation in Giardia

Molecular characterisation of species within the genus Giardia has revealed that much of the phenotypic heterogeneity, particularly within the species G. duodenalis, has a genetic basis. The source of this genetic variation appears to arise from predominantly asexual, clonal reproduction, although occasional bouts of sexual reproduction cannot be ruled out. Genetic variation is extensive with some clones widely distributed and others seemingly unique and localised to a particular endemic focus. Little attention has been given to the molecular epidemiology of Giardia infections. Future studies should be directed at studying the ecology and dynamics of transmission of Giardia clones, particularly in localised areas, and to evaluating the factors that serve to maintain genetic diversity between clones, especially the role of inter-clonal competition. Future research using molecular techniques should aim to identify and follow Giardia clones in nature and correlate genetic typing with important clinical and epidemiological characteristics such as virulence, drug sensitivity and zoonotic potential.

Molecular variation in trypanosomes

Several species of the genus Trypanosoma cause parasitic diseases of considerable medical and veterinary importance throughout Africa, Asia and the Americas. These parasites exhibit considerable intra-species genetic diversity and variation, which has complicated their taxonomic classification. This diversity and variation can be defined at the level of both the genome and of individual genes. The nuclear genome shows considerable inter- and intra-species plasticity in terms of chromosome number and size (molecular karyotype). The mitochondrial (kDNA) genome also varies considerably between species, especially in terms of minicircle size and organization. There is also considerable intra-specific sequence diversity in minicircles and within the Variable Region of the maxicircle. Restriction enzyme analysis of this diversity has lead to the concept of 'schizodemes'. At the gene level, isoenzyme analysis has proven very useful for strain and isolate identification, with the classification into numerous 'zymodemes'. Considerable antigenic diversity has also been identified in T. cruzi and T. brucei, with the development of 'serodemes' in the latter. In addition to this inter-strain diversity, African trypanosomes (T. brucei, T. congolense, and T. vivax) exhibit the phenomenon of antigenic variation, where individual parasites are able to express any one of hundreds of different copies of the Variant Surface Glycoprotein gene at any particular time. The molecular mechanisms underlying antigenic variation are now understood in considerable detail. The implication of this molecular diversity and variation are discussed in terms of trypanosome taxonomy and disease control.

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.

Genetic processes within an epidemic of sleeping sickness in Uganda

Reproductive processes within the current Ugandan epidemic of sleeping sickness are investigated. Genotype frequencies derived from isoenzyme patterns in 44 stocks of Trypanosoma brucei s.l. collected in 1988 from Tororo, south-east Uganda are analysed by single and multiple loci methods. In the single locus method, the hypothesis of random mating is tested by agreement with Hardy-Weinberg equilibrium. The multiple loci method uses a contingency table approach to detect non-random associations between pairs of loci; this equates to the detection of disequilibrium. The results do not support the concept of a randomly mating population of T. brucei within the current epidemic. Results from the epidemic data set are discussed in relation to the broader problem of genetic exchange in Trypanozoon.

Comparative studies of Trypanosoma (Duttonella) vivax isolates from Colombia

The characterization of four Trypanosoma vivax isolates from Colombia in South America showed that although minor phenotypic differences existed between them, these parasites are antigenically related and belong to a single serodeme. Characterization by isoenzyme assay, karyotyping and DNA probe analysis, showed the Colombian isolates to be more similar to the West African than to Kenyan T. vivax. There was, however, little serological cross-reactivity between South American and African groups of T. vivax. Although the T. vivax isolates from Colombia were pathogenic for dairy calves which showed the typical sign of progressive emaciation, these parasites failed to infect mice or tsetse and could not be cultivated as bloodstream forms in vitro. This study represents initial attempts to establish the phenotypic and serological diversity amongst T. vivax isolates from South America.

Allelic polymorphism of the Trypanosoma brucei polyubiquitin gene

We have characterized a second T. brucei polyubiquitin gene (UbB) that is highly similar in the coding and flanking regions to a previously described T. brucei polyubiquitin gene (UbA). However, UbB differs from UbA in 2 respects: (1) the predicted carboxy-terminal amino acid of UbB is methionine, as opposed to leucine in UbA, and (2) UbB contains approximately 13 ubiquitin repeats, as opposed to approximately 30 repeats in UbA. In Southern blots of intact T. brucei DNA separated by pulsed field gel electrophoresis, the polyubiquitin sequences have been shown to reside on band 19, which may contain 3 chromosomes. Three experiments that target a neomycin-resistance gene to the polyubiquitin locus demonstrate a one-to-one ratio of polyubiquitin 3-flanking sequences, which suggests that UbA and UbB are alleles rather than duplications. Four additional strains of T. brucei and one strain of T. equiperdum show variation in their polyubiquitin gene size, suggesting that this is a common polymorphism.

Molecular Biology of African Trypanosomes: Development of New Strategies to Combat an Old Disease

African trypanosomes are protozoan parasites that cause a number of diseases of man and domesticated animals in large regions of sub-Saharan Africa. The diseases have proven to be particularly difficult to prevent or to effectively treat due to features of both the trypanosome and the insect vector, the tsetse fly. The habitat of the tsetse and its resistance to insecticides have rendered vector control efforts ineffective. Attempts to develop a vaccine against the African trypanosomes has been dwarfed by the parasite's ability to change the composition of its exposed surface antigens. This process of antigenic variation allows the parasite to avoid the host's immune response and presents the host with a seemingly endless antigenic repertoire. Since conventional approaches to the control of African trypanosomiasis have largely met with failure, there has been a renewed interest in identifying novel aspects of the biology, biochemistry, and molecular biology of trypanosomes that might be exploited to develop new targets for vaccines or chemotherapy. Importantly, this research has opened a virtual Pandora's box of exciting biochemical and molecular surprises, which makes the African trypanosomes not only important medical pathogens but also an exciting experimental system for the basic scientist. In this review, the authors will describe some of the most recent and intriguing developments in the field of molecular parasitology.

Genetic structure of natural populations of the free-living amoeba, Naegleria lovaniensis. Evidence for sexual reproduction

The genetic structure of two populations of Naegleria lovaniensis, comprising 71 isolates collected from the same local geographical area was investigated by isoenzyme analysis. Allelic variation at seven polymorphic enzymatic loci allowed identification of 45 distinctive genotype associations. Analysis of single locus variation reveals that most of them are close to Hardy-Weinberg equilibrium, which indicates segregation and free recombination between alleles. The recovery of a relatively high number of distinct genotypic associations (most of them being unique), and the absence of linkage disequilibrium between genotypes at the different loci also support the existence of recombination. Although we have no idea about the process involved, the results clearly indicate that genetic exchanges occur, at least occasionally, in natural populations of N. lovaniensis.

Genetic variation in Trypanosoma brucei and the epidemiology of sleeping sickness in the Lambwe Valley, Kenya

A contingency table approach was used to explore the influence of location, host species and time on the genetic composition of a Trypanosoma brucei population in Lambwe Valley, Kenya. Significant differences in zymodeme frequencies were noticed over comparatively short geographical distances suggesting that transmission of T. brucei is somewhat localized. A significant association was observed between zymodeme and the mammalian host from which T. brucei was derived. The association was consistent in different localities in Lambwe valley and remained stable for at least 32 months. These observations indicate that zymodemes are adapted to different host species and that genetic exchange has not disrupted host associations over this time-scale. A major change in the composition of the T. brucei population during a sleeping sickness outbreak in 1980 was confirmed. But while new zymodemes emerged, a decline in overall diversity was noted during times of high sleeping sickness incidence. The results can be explained by selection of T. brucei zymodemes for particular transmission cycles. Although it is not necessary to invoke genetic exchange, sex may help T. brucei to adapt to changes in selection pressures. Such a hypothesis helps to explain why T. brucei appears largely clonal in the short term, even though population studies indicate that sex is responsible for much genetic diversity in the long term. It also explains why neighbouring populations of T. brucei are composed of a different range of zymodemes formed from the same alleles. Such a view implies that genetic exchange has an important role in the microevolution of T. brucei populations.

Trypanosoma cf. carassii: the combination of malic enzyme patterns supports the theory of diploidy in trypanosomes

Electrophoretic analysis of Trypanosoma cf. carassii strains from cyprinid fish revealed three basic types of enzyme patterns of malic enzyme (ME) in forms from culture. Two enzyme patterns were one-banded and differed only slightly in electrophoretic mobility. The third pattern consisted of three bands, the two marginals corresponding to respective bands of one-banded patterns and the third located in the middle. ME is thought to be dimeric in trypanosomes and therefore the triple-banded pattern may be regarded as the hybrid from combination of the former two. This fact supports the concept of diploidy in fish trypanosomes.

Multilocus isozyme identification of Trypanosoma brucei stocks isolated in central Africa: evidence for an animal reservoir of sleeping sickness in Congo

Six Congolese and 3 Zairian Trypanosoma brucei stocks were studied by isozyme cellulose acetate electrophoresis. Twenty isozyme systems were used, of which only 5 showed variability. These 5 polymorphic systems made it possible to identify 5 different zymodemes. Zymodemes isolated from man were recorded both from pig and sheep too, which confirms the results of previous authors. This favors the existence of an animal reservoir of human African trypanosomiasis in the Congo, which could play a role in the transmission of the disease, at least by the maintenance of residual foci.

Trypanosoma brucei rhodesiense: characterisation of stocks from Zambia, Kenya, and Uganda using repetitive DNA probes

We have previously described a system for characterising the relationships between trypanosome stocks of the T.brucei group based on Southern blotting with repetitive DNA probes followed by cluster analysis of resultant banding patterns (G. Hide et al. Molec. Bioch. Parasitol. 39, 213-226, 1990). In this study, we extend this analysis to examine the relationships between trypanosome stocks isolated from major sleeping sickness foci in Zambia, Kenya, and Uganda. We show that the trypanosome strains responsible for disease in Zambia are quite distinct from those sampled from the Kenya/Uganda foci. Furthermore, the human serum resistant stocks isolated from the Kenya/Uganda foci which were isolated from man (or from animals) were found to form a tight group in the cluster analysis, while stocks isolated from nonhuman sources in the same area or stocks from elsewhere were found in separate groups. Thus, the human infective trypanosome strains found in these foci may have common origins and have, perhaps, arisen by clonal selection from a common source.

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.

Evidence that the mechanism of gene exchange in Trypanosoma brucei involves meiosis and syngamy

All pairwise combinations of three cloned stocks of Trypanosoma brucei (STIB 247L, STIB 386AA and TREU 927/4) were co-transmitted through tsetse flies (Glossina morsitans) and screened for the production of hybrid trypanosomes. Clones of metacyclic and bloodstream trypanosomes from flies harbouring mature infections containing hybrid trypanosomes were established and screened for several isoenzyme and restriction fragment length polymorphisms. For each of the three combinations of parents, some progeny clones were observed to be of a phenotype and genotype indicating that genetic exchange had occurred during development of the trypanosomes in flies. These hybrid clones shared three salient features: (1) where the parents were homozygous variants the progeny were heterozygous, (2) where one of the parents was heterozygous, allelic segregation was observed and (3) the progeny clones were shown to be recombinant when two or more markers for which one of the parents was heterozygous were examined. These results are consistent with the progeny being an F1 in a diploid mendelian genetic system involving meiosis and syngamy. Our observations show that all possible combinations of the three stocks may undergo genetic exchange. A marker analysis of a series of clones each derived from single metacyclic trypanosomes showed that individual flies transmit a mixture of trypanosome genotypes corresponding to F1 progeny and to parental types, indicating that genetic exchange was a non-obligatory event in the life-cycle of the trypanosome. In addition, a preliminary analysis of the phenotype of procyclic stage trypanosomes derived from flies infected with two stocks, indicates that genetic exchange is unlikely to occur at this stage.

Genetic exchange in African trypanosomes

African trypanosomes are important pathogens of humans and domestic animals, but little was known, until recently, of the genetic system of these parasites. Recent results demonstrate the existence of nonobligatory genetic exchange between different stocks of T. brucei. A number of models have been put forward for the mechanism of genetic exchange, including a fusion model with subsequent random loss of chromosomes and a more conventional mendelian system.

Genetic exchange and evolutionary relationships in protozoan and helminth parasites

The study of genetic exchange systems and the use of genetic analysis has been relatively limited in parasites leading to considerable gaps in our basic knowledge. This lack of knowledge makes it difficult to draw firm conclusions as to how these systems evolved. An additional problem is also raised by the difficulties in defining evolutionary distances particularly with the unicellular protozoa, using classical ultrastructural and cytological criteria. While these difficulties have by no means been overcome, the use of rapid sequencing techniques applied to the ribosomal genes has allowed measurement of evolutionary distances, and considerable advances in our understanding of the genetic exchange systems in a few parasitic protozoa have recently been made. The conclusions from these recent sets of analyses are reviewed and then examined together in order to discuss the evolution of genetic exchange systems in parasitic protozoa. The evolutionary distances defined by ribosome sequence analysis show that parasites are an extremely divergent group, with distances which, in some cases, are orders of magnitude greater than the distances between mammals and fish; furthermore these studies suggest that the parasitic protozoa or their free-living ancestors are extremely ancient. These findings support the view that parasitism has occurred independently many times and that the parasitic life-style has been adopted by evolutionarily distinct groups. The recent observation of a non-obligatory genetic system in the diploid but evolutionary ancient kinetoplastid Trypanosoma brucei suggests that diploidy and meiosis are extremely old. The observation, in parasitic protozoa and helminths, that selfing or non-obligatory mating is a common feature suggests that these processes may be strategies to overcome the cost of meiosis. In this context, the question of what selective forces maintain genetic exchange is discussed.

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.

Genetic exchange in Trypanosoma brucei

The discovery of genetic exchange in African trypanosomes belonging to the Trypanosoma brucei group is an important development in our understanding of these organisms. Genetic exchange is a feature of major importance in relation to population structure and speciation. Furthermore, a convenient laboratory-based mating system would be of considerable value as a tool in trypanosomiasis research. It is now known that although cyclical development of trypanosomes within the tsetse fly does not require mating to occur, genetic exchange may take place under Conditions in which genetically distinct trypanosomes develop within the same fly. During the past few years there has been a considerable body of research on laboratory crosses, and a number of controversial and apparently contradictory models of the mechanism of genetic exchange and the ploidy of different life cycle stages have been proposed. In this article, Andy Tait and Mike Turner review the present state of knowledge regarding gene exchange in T. brucei, and attempt to reconcile the various observations and models available.

Analysis of a genetic cross between Trypanosoma brucei rhodesiense and T. b. brucei

Two trypanosome clones, representing East and West African homozygotes at 2 isoenzyme loci (T. b. rhodesiense MHOM/ZM/74/58 [CLONE B] and T. b. brucei MSUS/CI/78/TSW 196 [CLONE A]), were cotransmitted through tsetse flies and the resulting trypanosome populations checked for the presence of non-parental karyotypes by pulsed-field gel electrophoresis. Ten clones isolated from these populations proved to have 5 different recombinant genotypes by analysis of nuclear and kinetoplast DNA (kDNA) polymorphisms. It is inferred that genetic exchange occurred between the 2 trypanosome clones in the fly, as previously reported for 2 other T. brucei spp. clones by Jenni and colleagues. For the most part, the hybrid clones shared many characteristics with both parents and their genotypes were consistent with segregation and reassortment of parental alleles. The least amount of genetic material exchanged was kDNA alone. Regarding the mechanism of genetic exchange, several hybrid clones had identical and unique nuclear DNA polymorphisms, but different kDNA type. Assuming that the same reassortment of nuclear markers is unlikely to occur by chance, these clones most probably arose from a predecessor carrying both types of kDNA.

Genetic evidence that metacyclic forms of Trypanosoma brucei are diploid

The hypothesis that metacyclic trypanosomes are haploid has been tested genetically. Five cloned stocks of Trypanosoma brucei (each having four known isoenzyme markers and six known restriction fragment length polymorphisms) have been independently transmitted through tsetse flies. Fifteen individual metacyclic organisms were taken from flies with mature cyclical infections and used to establish fresh clones. All the sub-clones from all the flies proved to be identical to the starting (parental) stocks, with respect to all the markers examined, including those markers which were heterozygous in the parental stocks. We conclude that metacyclic trypanosomes are diploid, and are not the product of an obligatory meiosis.

Duplication and transcription of procyclin genes in Trypanosoma brucei

The genes encoding procyclin, the major glycoprotein expressed on the surface of procyclic forms of Trypanosoma brucei, comprise a multigene family. It has previously been demonstrated that procyclin genes in cloned trypanosome strains from Kenya and Uganda show restriction fragment polymorphisms. A detailed study of the Kenyan strain 227 has revealed that procyclin genes are arranged in tandem at 3 distinct loci (Pro A, B and C) and that the polymorphism is due to the duplication of 1.3 kb in the Pro A locus, which has generated an additional procyclin gene. Northern blot analysis has shown that at least 2 loci are transcribed and that a minimum of 3 procyclin genes are expressed within a cloned line. The transcription of procyclin genes is resistant to 1 mg ml-1 alpha-amanitin, whereas that of the 5' flanking gene in the Pro A locus is sensitive. This observation suggests that the two genes form part of separate transcription units with a promoter between them.

Glucosephosphate isomerase from Trypanosoma brucei. Cloning and characterization of the gene and analysis of the enzyme

In Trypanosoma brucei the enzyme glucose-6-phosphate isomerase, like most other enzymes of the glycolytic pathway, resides in a microbody-like organelle, the glycosome. Here we report a detailed study of this enzyme, involving a determination of its kinetic properties and the cloning and sequence analysis of its gene. The gene codes for a polypeptide of 606 amino acids, with a calculated Mr of 67280. The protein predicted from the gene sequence has 54-58% positional identity with its yeast and mammalian counterparts. Compared to those other glucose-6-phosphate isomerases the trypanosomal enzyme contains an additional 38-49 amino acids in its N-terminal domain, as well as a number of small insertions and deletions. The additional amino acids are responsible for the 5-kDa-larger subunit mass of the T. brucei enzyme, as measured by gel electrophoresis. The glucose-6-phosphate isomerase of the trypanosome has no excess of positive residues and, consequently, no high isoelectric point, in contrast to the other glycolytic enzymes that are present in the glycosome. However, similar to other glycosomal proteins analyzed so far, specific clusters of positive residues can be recognized in the primary structure. Comparison of the kinetic properties of the T. brucei glucose-6-phosphate isomerase with those of the yeast and rabbit muscle enzymes did not reveal major differences. The three enzymes have very similar pH profiles. The affinity for the substrate fructose 6-phosphate (Km = 0.122 mM) and the inhibition constant for the competitive inhibitor gluconate 6-phosphate (Ki = 0.14 mM) are in the same range as those of the similar enzymes. The Km shows the same strong dependence on salt as the rabbit muscle enzyme, although somewhat less than the yeast glucose-6-phosphate isomerase. The trypanocidal drug suramin inhibits the T. brucei and yeast enzymes to the same extent (Ki = 0.29 and 0.36 mM, respectively), but it had no effect on the rabbit muscle enzyme. Agaricic acid, a potent inhibitor of various glycosomal enzymes of T. brucei, has also a strong, irreversible effect on glucose-6-phosphate isomerase, while leaving the yeast and mammalian enzymes relatively unaffected.

Evidence for diploidy in metacyclic forms of African trypanosomes

The DNA contents of bloodstream form trypanosomes (life cycle stages circulating in the blood of the vertebrate host) of four African Trypanosoma species and of metacyclic forms (the life cycle stage that is injected into the vertebrate by the tsetse fly during its bite) of the same four species were measured by cytofluorometry of individual cells or nuclei. The results showed unambiguously that the metacyclic forms cannot be considered to be products of meiosis containing only half of the DNA of bloodstream forms, in contrast to what was previously reported for Trypanosoma brucei [Zampetti-Bosseler, F., Schweizer, J., Pays, E., Jenni, L. & Steinert, M. (1986) Proc. Natl. Acad. Sci. USA 83, 6063-6064] during an attempt to localize the gametes in the life cycle after experimental evidence of sexual gene exchange in this parasite was reported.

Gene exchange in African trypanosomes: frequency and allelic segregation

The existence of a system of genetic exchange in Trypanosoma brucei is now established, but the frequency with which mating occurs and the mechanisms by which genes are exchanged are still unknown. This paper presents the results of a study of one pair of trypanosome stocks, which show that mating is a non-obligatory but frequent event in a life-cycle stage within the insect vector. Analysis of ten progeny clones using a total of eleven markers (iso-enzymes and DNA probes detecting restriction fragment length polymorphisms) has indicated that segregation of alleles occurs at five of these loci. The segregation patterns of a polymorphic EcoRI site in the maxi-circle of the kinetoplast DNA (kDNA) show that the progeny inherit one or other of the parental kDNA types. These results demonstrate that segregation of alleles occurs and that new combinations of alleles at different loci are generated in the progeny clones. The implications of these findings for defining the mechanism of gene exchange are discussed in relation to a simple mendelian genetic system involving meiosis and syngamy.

Trypanosoma brucei contains two RNA polymerase II largest subunit genes with an altered C-terminal domain

We have identified and cloned four trypanosomal RNA polymerase largest subunit genes. Here, we present the molecular analysis of two genes, Trp4.8 and Trp5.9. The sequence of these genes shows that they are almost identical to each other and indicates that they encode the largest subunit of RNA polymerase II. Both genes contain a C-terminal extension that is clearly distinct from that of other eukaryotic RNA polymerase II genes, because it lacks the common tandemly repeated heptapeptide sequence and is rich in acidic amino acids. It shares many potential phosphorylation sites, however, with the C-terminal extension of other eukaryotic RNA polymerase II large subunits. The presence of two RNA polymerase II loci suggests that a fourth RNA polymerase could be formed. Interestingly, the fourth gene is only found in species exhibiting antigenic variation.

Implications of genetic exchange in the study of protozoan infections

Genetic exchange is now known to occur during the life-cycle of many parasitic protozoa, including malaria parasites, coccidia and trypanosomes. The process is studied by making deliberate crosses between cloned organisms differing in clearly defined markers. In malaria parasites, crosses have been made between parasites differing in characters such as isoenzymes, antigens and other proteins, drug sensitivity, and chromosome and other DNA polymorphisms. Crosses are made by transmitting a mixture of gametes of each clone through mosquitoes to allow cross-fertilization to take place, and examining the resulting progeny by cloning for organisms exhibiting non-parental combinations of characters. The inheritance of many characters, such as antigen and protein variants, is in accordance with Mendelian expectations for a haploid organism. Recombination occurs at a higher than expected frequency. Studies on chromosomes have show that crossing-over events commonly occur following meiosis of hybrid zygotes. Repetitive DNA and subtelomeric regions of chromosomes appear to be particularly susceptible to such recombination events. In trypanosomes, crosses between clones of Trypanosoma brucei have shown that hybrids are formed during tsetse fly transmission. The organism appears to be mainly diploid, but some characters including certain chromosomes seem to be inherited in a non-Mendelian manner.

Detection of hybrid phenotypes in African trypanosomes by high resolution two-dimensional gel electrophoresis

High resolution two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and autoradiography were used to analyze the protein phenotypes of Trypanosoma brucei (T.b. brucei and T.b. gambiense) clones suspected of being hybrids. Procyclic culture forms of parental and suspected hybrid trypanosomes were biosynthetically labeled with [35S]methionine and labeled proteins were resolved by multiple 2D-PAGE (the ISO-DALT system) to allow accurate inter-gel comparisons. Autoradiography of the gels showed that the parental clones had qualitative differences in at least seven sets of spots. Five of these sets represented charge differences and one represented proteins of altered relative molecular mass (Mr) and charge. Autoradiographs of the gels of the putative hybrid trypanosomes showed both forms of the proteins found separately in the parental clones indicating that new, nonparental phenotypes had been generated by transmission of mixed trypanosome clones through tsetse flies. The 2D-PAGE patterns from parasites cultivated for extended periods were identical, showing that the individual cloned parasites were phenotypically stable. The results indicate that analytical 2-D gels can be used to study the phenotypes of "parental" or "hybrid" African trypanosomes without having any previous knowledge of the molecular characteristics of the parasites. In addition, the technique allows an extension of phenotypic analysis to hundreds of different proteins in populations of cloned parasites.

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

Restriction fragment length polymorphism (RFLP) has been analysed in Trypanosoma brucei DNA following hybridization with different DNA probes. This polymorphism seems to be due to allelic variation, and not to variation between sequence duplicates, since the genomic environment of the probed polymorphic fragments is conserved over considerable distances. In an analysis of 35 non-gambiense stocks, we found different combinations of homozygotes and heterozygotes for the four RFLP probes used, in keeping with previous observations that genetic reassortment occurs in T. b. brucei. Moreover, the non-gambiense populations from West and East Africa can be differentiated according to their characteristic allele frequencies. In sharp contrast, we found that the 49 T. b. gambiense stocks, analysed with the same probes, share the same single allelic combination and are all homozygous for each one of the four markers. This characteristic gambiense allele combination is very common among Western non-gambiense isolates, but rare or absent among Eastern ones. Two stocks isolated from man in West Africa turned out to be non-gambiense by all molecular criteria examined, including total nuclear DNA content. Taken together, these observations suggest that human serum-resistant variants may appear among the West African T. b. brucei population, and that T. b. gambiense evolved from one of these resistant variants as a man-adapted subspecies that became genetically isolated from the rest of the West African trypanosome population.

Structure and sequence of the gene for the largest subunit of trypanosomal RNA polymerase III

As the first step in the analysis of the transcription process in the African trypanosome, Trypanosoma brucei, we have started to characterise the trypanosomal RNA polymerases. We have previously described the gene encoding the largest subunit of RNA polymerase II and found that two almost identical RNA polymerase II genes are encoded within the genome of T. brucei. Here we present the identification, cloning and sequence analysis of the gene encoding the largest subunit of RNA polymerase III. This gene contains a single open reading frame encoding a polypeptide with a Mr of 170 kD. In total, eight encoding a polypeptide with a Mr of 170 kD. In total, eight highly conserved regions with significant homology to those previously reported in other eukaryotic RNA polymerase largest subunits were identified. Some of these domains contain functional sites, which are conserved among all eukaryotic largest subunit genes analysed thus far. Since these domains make up a large part of each polypeptide, independent of the RNA polymerase class, these data strongly support the hypothesis that these domains provide a major part of the transcription machinery of the RNA polymerase complex. The additional domains which are uniquely present in the largest subunit of RNA polymerase I and II, respectively, two large hydrophylic insertions and a C-terminal extension, might be a determining factor in specific transcription of the gene classes.