Trypanosomes of subgenus Trypanozoon are diploid for housekeeping genes

Reproduction in Trypanosomatids: Past and Present

Simple Summary

The reproduction of trypanosomatids is a fundamental issue for host–parasite interaction, and its biological importance lies in knowing how these species acquire new defense mechanisms against the countermeasures imposed by the host, which is consistent with the theory of the endless race or the Red Queen hypothesis for the existence of meiotic sex. Moreover, the way these species re-produce may also be at the origin of novel and more virulent clades and is relevant from a thera-peutic or vaccination point of view, as sex may contribute to increased tolerance and even to the rapid acquisition of drug resistance mechanisms. Kinetoplastids are single-celled organisms, many of them being responsible for important parasitic diseases, globally termed neglected diseases, which are endemic in low-income countries. Leishmaniasis, African (sleeping sickness) and American trypanosomiasis (Chagas disease) caused by trypanosomatids are among the most ne-glected tropical scourges related to poverty and poor health systems. The reproduction of these microorganisms has long been considered to be clonal due to population genetic observations. However, there is increasing evidence of true sex and genetic exchange events under laboratory conditions. We would like to highlight the importance of this topic in the field of host/parasite in-terplay, virulence, and drug resistance.

Abstract

Diseases caused by trypanosomatids (Sleeping sickness, Chagas disease, and leishmaniasis) are a serious public health concern in low-income endemic countries. These diseases are produced by single-celled parasites with a diploid genome (although aneuploidy is frequent) organized in pairs of non-condensable chromosomes. To explain the way they reproduce through the analysis of natural populations, the theory of strict clonal propagation of these microorganisms was taken as a rule at the beginning of the studies, since it partially justified their genomic stability. However, numerous experimental works provide evidence of sexual reproduction, thus explaining certain naturally occurring events that link the number of meiosis per mitosis and the frequency of mating. Recent techniques have demonstrated genetic exchange between individuals of the same species under laboratory conditions, as well as the expression of meiosis specific genes. The current debate focuses on the frequency of genomic recombination events and its impact on the natural parasite population structure. This paper reviews the results and techniques used to demonstrate the existence of sex in trypanosomatids, the inheritance of kinetoplast DNA (maxi- and minicircles), the impact of genetic exchange in these parasites, and how it can contribute to the phenotypic diversity of natural populations.

Chromosomal copy number variation analysis by next generation sequencing confirms ploidy stability in Trypanosoma brucei subspecies

Although aneuploidy usually results in severe abnormalities in multicellular eukaryotes, recent data suggest that it could be beneficial for unicellular eukaryotes, such as yeast and trypanosomatid parasites, providing increased survival under stressful conditions. Among characterized trypanosomatids, Trypanosoma cruzi, Trypanosoma brucei and species from the genus Leishmania stand out due to their importance in public health, infecting around 20 million people worldwide. The presence of aneuploidies in T. cruzi and Leishmania was recently confirmed by analysis based on next generation sequencing (NGS) and fluorescence in situ hybridization, where they have been associated with adaptation during transmission between their insect vectors and mammalian hosts and in promoting drug resistance. Although chromosomal copy number variations (CCNVs) are present in the aforementioned species, PFGE and fluorescence cytophotometry analyses suggest that aneuploidies are absent from T. brucei. A re-evaluation of CCNV in T. b gambiense based on NGS reads confirmed the absence of aneuploidies in this subspecies. However, the presence of aneuploidies in the other two T. brucei subspecies, T. b. brucei and T. b. rhodesiense, has not been evaluated using NGS approaches. In the present work, we tested for aneuploidies in 26 T. brucei isolates, including samples from the three T. brucei subspecies, by both allele frequency and read depth coverage analyses. These analyses showed that none of the T. brucei subspecies presents aneuploidies, which could be related to differences in the mechanisms of DNA replication and recombination in these parasites when compared with Leishmania.

Evidence for viable and stable triploid Trypanosoma congolense parasites

BACKGROUND

Recent whole genome sequencing (WGS) analysis identified a viable triploid strain of Trypanosoma congolense. This triploid strain BANANCL2 was a clone of the field isolate BANAN/83/CRTRA/64 that was collected from cattle in Burkina Faso in 1983.

RESULTS

We demonstrated the viability and stability of triploidy throughout the complete life-cycle of the parasite by infecting tsetse flies with the triploid clone BANANCL2. Proboscis-positive tsetse flies efficiently transmitted the parasites to mice resulting in systemic infections. WGS of the parasites was performed at all life-cycle stages, and a method based on a block alternative allele frequency spectrum was developed to efficiently detect the ploidy profiles of samples with low read depth. This approach confirmed the triploid profile of parasites throughout their life-cycle in the tsetse fly and the mammalian host, demonstrating that triploidy is present at all stages and is stable over time.

CONCLUSION

The presence of viable field-isolated triploid parasites indicates another possible layer of genetic diversity in natural T. congolense populations. The comparison between triploid and diploid parasites provides a unique model system to study the impact of chromosome copy number variations in African trypanosomes. In addition, the consequences of triploidy can be further investigated using this stable triploid model.

ISOZYME VARIABILITY IN TRYPANOSOMA CRUZI, THE AGENT OF CHAGAS' DISEASE: GENETICAL, TAXONOMICAL, AND EPIDEMIOLOGICAL SIGNIFICANCE

A genetic interpretation of the zymograms of 524 Trypanosoma cruzi stocks from various hosts and representing a broad geographical range (United States to Southern Brazil) reveals high genetic variability (only one monomorphic locus out of 15) and suggests that this parasite has a diploid structure. The data do not give any indication of Mendelian sexuality, although many opportunities are present for genetic exchange between extremely different genotypes. The population structure of T. cruzi appears to be multiclonal and complex. The natural clones evidenced by isozyme analysis are numerous (43 different ones are recorded among 121 stocks assayed at 15 gene loci) and exhibit a large range of genotypes, in a nonhierarchical structure; it is not possible to cluster them into a few strictly delimited groups which could represent natural taxa. The available data suggest that the genetic variability of T. cruzi reflects the long separate evolution of multiple clones. It is suggested that long clonal evolution may explain the present biological and medical variability of the causative agent of Chagas' disease.

Genetic recombination between human and animal parasites creates novel strains of human pathogen

Genetic recombination between pathogens derived from humans and livestock has the potential to create novel pathogen strains, highlighted by the influenza pandemic H1N1/09, which was derived from a re-assortment of swine, avian and human influenza A viruses. Here we investigated whether genetic recombination between subspecies of the protozoan parasite, Trypanosoma brucei, from humans and animals can generate new strains of human pathogen, T. b. rhodesiense (Tbr) responsible for sleeping sickness (Human African Trypanosomiasis, HAT) in East Africa. The trait of human infectivity in Tbr is conferred by a single gene, SRA, which is potentially transferable to the animal pathogen Tbb by sexual reproduction. We tracked the inheritance of SRA in crosses of Tbr and Tbb set up by co-transmitting genetically-engineered fluorescent parental trypanosome lines through tsetse flies. SRA was readily transferred into new genetic backgrounds by sexual reproduction between Tbr and Tbb, thus creating new strains of the human pathogen, Tbr. There was no evidence of diminished growth or transmissibility of hybrid trypanosomes carrying SRA. Although expression of SRA is critical to survival of Tbr in the human host, we show that the gene exists as a single copy in a representative collection of Tbr strains. SRA was found on one homologue of chromosome IV in the majority of Tbr isolates examined, but some Ugandan Tbr had SRA on both homologues. The mobility of SRA by genetic recombination readily explains the observed genetic variability of Tbr in East Africa. We conclude that new strains of the human pathogen Tbr are being generated continuously by recombination with the much larger pool of animal-infective trypanosomes. Such novel recombinants present a risk for future outbreaks of HAT.

Genetic recombination allows transfer of harmful traits between different strains of the same pathogen and enables the emergence of genetically novel pathogen strains that the host population has not previously encountered. This can be particularly important when a pathogen acquires a virulence trait that allows it to spread beyond its normal host population. Here we show that this happens among the single-celled parasites—trypanosomes—that cause human African trypanosomiasis (HAT) or sleeping sickness carried by the tsetse fly. Genetic recombination readily occurs between the human and animal parasites when they are co-transmitted by the tsetse fly, creating new pathogen genotypes or strains. There is a single gene that confers human infectivity and each of the genotypes that inherits this gene is potentially capable of infecting humans. In this way new strains of the human pathogen can be generated by recombination between the human-infective and animal-infective trypanosomes. Such novel recombinants present a risk for future outbreaks of HAT.

Mating compatibility in the parasitic protist Trypanosoma brucei

Background

Genetic exchange has been described in several kinetoplastid parasites, but the most well-studied mating system is that of Trypanosoma brucei, the causative organism of African sleeping sickness. Sexual reproduction takes place in the salivary glands (SG) of the tsetse vector and involves meiosis and production of haploid gametes. Few genetic crosses have been carried out to date and consequently there is little information about the mating compatibility of different trypanosomes. In other single-celled eukaryotes, mating compatibility is typically determined by a system of two or more mating types (MT). Here we investigated the MT system in T. brucei.

Methods

We analysed a large series of F1, F2 and back crosses by pairwise co-transmission of red and green fluorescent cloned cell lines through experimental tsetse flies. To analyse each cross, trypanosomes were cloned from fly SG containing a mixture of both parents, and genotyped by microsatellites and molecular karyotype. To investigate mating compatibility at the level of individual cells, we directly observed the behaviour of SG-derived gametes in intra- or interclonal mixtures of red and green fluorescent trypanosomes ex vivo.

Results

Hybrid progeny were found in all F1 and F2 crosses and most of the back crosses. The success of individual crosses was highly variable as judged by the number of hybrid clones produced, suggesting a range of mating compatibilities among F1 progeny. As well as hybrids, large numbers of recombinant genotypes resulting from intraclonal mating (selfers) were found in some crosses. In ex vivo mixtures, red and green fluorescent trypanosome gametes were observed to pair up and interact via their flagella in both inter- and intraclonal combinations. While yellow hybrid trypanosomes were frequently observed in interclonal mixtures, such evidence of cytoplasmic exchange was rare in the intraclonal mixtures.

Conclusions

The outcomes of individual crosses, particularly back crosses, were variable in numbers of both hybrid and selfer clones produced, and do not readily fit a simple two MT model. From comparison of the behaviour of trypanosome gametes in inter- and intraclonal mixtures, we infer that mating compatibility is controlled at the level of gamete fusion.

Meiosis and Haploid Gametes in the Pathogen Trypanosoma brucei

In eukaryote pathogens, sex is an important driving force in spreading genes for drug resistance, pathogenicity, and virulence [1]. For the parasitic trypanosomes that cause African sleeping sickness, mating occurs during transmission by the tsetse vector [2, 3] and involves meiosis [4], but haploid gametes have not yet been identified. Here, we show that meiosis is a normal part of development in the insect salivary glands for all subspecies of Trypanosoma brucei, including the human pathogens. By observing insect-derived trypanosomes during the window of peak expression of meiosis-specific genes, we identified promastigote-like (PL) cells that interacted with each other via their flagella and underwent fusion, as visualized by the mixing of cytoplasmic red and green fluorescent proteins. PL cells had a short, wide body, a very long anterior flagellum, and either one or two kinetoplasts, but only the anterior kinetoplast was associated with the flagellum. Measurement of nuclear DNA contents showed that PL cells were haploid relative to diploid metacyclics. Trypanosomes are among the earliest diverging eukaryotes, and our results support the hypothesis that meiosis and sexual reproduction are ubiquitous in eukaryotes and likely to have been early innovations [5].

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Trypanosoma brucei is a sexual organism

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Meiosis is a normal part of the trypanosome’s life cycle

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Identification of a novel haploid cell type with distinctive morphology

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First visualization of how trypanosomes mate

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.

Will the real Trypanosoma b. gambiense please stand up

Controversy has surrounded the differentiation of Trypanosoma brucei gambiense from T. b. rhodesiense (causative agents of Gambian and Rhodesian sleeping sickness, respectively) almost from the moment they were named. In the light of recent findings from biochemical and immunological characterization studies, Wendy Gibson reviews the status of T. b. gambiense to see if there is now a consensus concerning its identity.

Genetic variability within Trypanosoma brucei gambiense: evidence for the circulation of different genotypes in human African trypanosomiasis patients in Côte d'Ivoire

For 23 Ivoirian patients infected by Trypanosoma-brucei gambiense, isolation and genetic characterization using PCR and microsatellite primers were performed (in 1996-99) using 2 different isolates (A and B) from each patient. When using TBDAC 1/2, 7 genotypes were observed, and DNAs A and B for 2 patients were different. This might be the first evidence of the presence of 2 different genotypes of T. b. gambiense group 1 in the same patient.

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.

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.

Polymorphisms in Trypanosoma cruzi: evidence of genetic recombination

The ploidy of Trypanosoma cruzi is until now undetermined although analysis of isoenzymes, molecular karyotype and DNA content suggest diploidy in a very plastic genome. Also, there has been no convincing demonstration of genetic exchange and it has been proposed that reproduction is clonal. We have compared 18 T cruzi stocks and clones from the same area or host by means of isoenzyme analysis (12 loci) and restriction site polymorphisms in and around three glycolytic genes (glyceraldehyde-3-phosphate dehydrogenase, aldolase and glucosephosphate isomerase). The analysis demonstrated the presence of homozygotes and heterozygotes and is compatible with diploidy for these housekeeping genes. This strongly supports the hypothesis of genetic exchange in T cruzi and further elucidates the genetic diversity within natural T cruzi populations.

Trypanosomosis research at the Centre for Tropical Veterinary Medicine (CTVM) 1970 to 1995

This review covers aspects of research work carried out on animal trypanosomes at the Centre for Tropical Veterinary Medicine (CTVM) during the last 25 years. The review covers work on antigenic variation, tissue culture, drug resistance, immunology, biochemistry and pathology of Trypanosoma brucei, T. congolense, T. gambiense and T. evansi. It is not intended as an exhaustive review of the subject but focuses on certain aspects of these areas which are presented in relation to work carried out within the broader scientific community.

Genomic organization of an invariant surface glycoprotein gene family of Trypanosoma brucei

The genomic organization of a gene family for the invariant surface glycoprotein, ISG75 (invariant surface glycoprotein with a molecular mass of 75 kDa), from Trypanosoma brucei is described. In T. brucei strain 427 ISG75 genes are present in tandem arrays at two loci, A and B, containing 5 and 2 copies, respectively. At the 3'-end of locus A, a single gene was identified that encodes a structural isoform of ISG75. This isoform contains a unique amino-terminal domain, whereas the rest of the protein is nearly identical to the polypeptides encoded by the other genes. This isoform is transcribed into a stable mRNA, but the expression of the derived polypeptide was below the detection limit. The ISG75 gene clusters are present on chromosomal bands 9' and 10, supporting the hypothesis of Gottesdiener et al. [25] that these bands contain allelic chromosomes. The total number of ISG75 genes is strain dependent, but at least one copy of the unique isoform is present in every variant tested.

A successful backcross in Trypanosoma brucei

Genetic exchange can take place between different strains of Trypanosoma brucei ssp. when they are cotransmitted via the tsetse fly vector, but the mechanism and limits of compatibility between strains are ill-defined as yet. Following the recovery of several hybrid genotypes with single drug resistance from a cross of drug resistant parental strains, we attempted a series of backcrosses and F1 crosses, selecting hybrids by double drug resistance. Of 4 backcrosses, one produced hybrid progeny, the analysis of which is presented here, but none of the 4 F1 crosses produced hybrid progeny. However, among experimental flies from the 8 crosses, although there were large numbers of salivary gland infections, very few consisted of a mixture of parental clones, a prerequisite for mating. In the successful backcross both parents were diploid, but none of the crosses involving triploid clones produced hybrid progeny. The hybrid-secreting fly from the backcross contained a mixture of hybrid and parental clones. The hybrid clones had approximately 3n DNA contents relative to the 2n parental clones and fell into 2 groups with respect to restriction site polymorphisms in kinetoplast DNA maxi-circles. Fingerprinting by random PCR amplification using 8 different arbitrary primers showed minor variation between the hybrid clones.

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.

Karyotype analysis of the monogenetic trypanosomatid Leptomonas collosoma

In order to develop a genetic system for the monogenetic trypanosomatids, we have analyzed the molecular karyotype of Leptomonas collosoma based on chromosome separation by clamped homogeneous electric field (CHEF) gel electrophoresis. The chromosome location of 5 RNA coding genes (SL, U6, 5S, 7SL and rRNA) and 2 protein coding genes (for HSP83 and alpha-tubulin) was determined. All of the L. collosoma genes examined were found on at least 2 chromosomes, which differ in size in the range of 100-500 kb, suggesting that the organism is diploid. The weighted sum of L. collosoma chromosomes separated by CHEF analysis was approximately 62 +/- 3 Mb, whereas the genome size determined by FACS was estimated at approx. 80 Mb. This suggests that some of the homologous chromosomes differ in their size. The analysis presented here may facilitate studies on the function of individual genes, and on the genetic stability of this organism.

Cloning and expression of the dihydrofolate reductase-thymidylate synthase gene from Trypanosoma cruzi

We have cloned, sequenced and expressed the Trypanosoma cruzi gene encoding the bifunctional protein dihydrofolate reductase-thymidylate synthase (DHFR-TS). The strategy followed for the isolation of positive clones from a genomic library was based on the construction of a probe by the amplification of highly conserved sequences of the TS domain by the polymerase chain reaction. Translation of the open reading frame of 1563 bp yields a polypeptide of 521 amino acids with a molecular mass of 58829 Da. For heterologous expression of T. cruzi DHFR-TS in Escherichia coli, the entire coding sequence was amplified by polymerase chain reaction and cloned into the plasmid vector pKK223.3. The presence of catalytically active DHFR-TS was demonstrated by complementation of the Thy- E. coli strain chi 2913 and the DHFR- Thy- E. coli strain PA414. The gene is expressed as an active protein which constitutes approximately 2% of the total cell soluble protein. Recombinant bifunctional enzyme and the DHFR domain have been purified by methotrexate-Sepharose chromatography to yield 1-2 mg of active DHFR-TS per litre of culture. Southern and electrophoretic analyses using the coding sequence as probe indicated that the T. cruzi enzyme is encoded by a single copy gene which maps to two bands of approximately 990 kb and 1047 kb. It appears that T. cruzi is diploid for the DHFR-TS gene which is located on two different-sized homologous chromosomes.

Genetic exchange in Trypanosoma brucei: evidence for meiosis from analysis of a cross between drug-resistant transformants

Genetic exchange in Trypanosoma brucei spp. can occur when two strains are cotransmitted through the tsetse fly vector, but it is non-obligatory and a comparatively rare event. To increase recovery of hybrids, we crossed drug resistant parental strains and selected hybrids by double drug resistance [15]. Analysis of 29 hybrid clones from five separate genetic exchange events shows independent segregation of marker genes and a high frequency of triploidy, both of which phenomena have been observed previously for other trypanosome crosses. However, in addition we provide evidence of genetic recombination involving the tubulin locus. These three observations strongly support the hypothesis that genetic exchange starts with a meiotic division in T. brucei.

Alternative splicing within and between alleles of the ATPase gene 1 locus of Trypanosoma brucei

The P-type ATPase gene TBA1 of Trypanosoma brucei belongs to a polycistronic transcription unit. We analyzed the structure and expression of a 4-kb region located immediately downstream from TBA1. This region is unique and contains two large open reading frames transcribed into stable mRNAs. These putative genes, termed ADG1 and ADG2, can respectively encode a 24-kDa and a 81-kDa protein. The intergenic spacings between the polyadenylation sites and the next 3' splice acceptor sites are very short: 148 bp between TBA1 and ADG1, and 127 bp between ADG1 and ADG2. Transcripts from each of the two ADG1 alleles can be detected, indicating that both homologs are transcribed. These transcripts are differentially spliced due to a single base difference which destroys in one homolog the AG acceptor site present in the other. In the 'mutant' allele an alternative downstream splice acceptor site is used. Despite its sequence conservation in both alleles, this splice site is only used in the allele lacking the upstream AG acceptor site. The major population of ADG1 transcripts exhibit a long 5'-untranslated extension and no 3'-terminal tail, but a minor population shows a smaller 5'-untranslated region due alternative splicing closer to the initiation codon of the gene. The steady-state amounts of transcripts from individual genes in this region are differentially stage-regulated.

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.

Insertion of the promoter for a variant surface glycoprotein gene expression site in an RNA polymerase II transcription unit of procyclic Trypanosoma brucei

The variant-specific surface glycoprotein (VSG) genes of Trypanosoma brucei are invariably expressed near the ends of chromosomes (telomeres). We have targeted a VSG gene expression site (ES) promoter driving a selectable marker gene (neomycin phosphotransferase) into a chromosome-internal transcription unit, the tubulin gene array of procyclic trypanosomes. To avoid read through transcription of the marker gene from the tubulin promoter, we targeted the ES promoter in inverse orientation relative to tubulin gene transcription. The only correctly targeted transformant obtained contained the marker gene close to the border of the tubulin gene array, and expression of this gene was relatively low. Possible reasons for the low targeting efficiency and expression level are discussed.

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.

The glycosomes of the Kinetoplastida

Glycosomes are the microbodies of the organisms belonging to the order of the Kinetoplastida, comprising trypanosomes and leishmanias, both pathogens to man. The organelles sequester a number of glycolytic enzymes that are normally located in the cytosol in other eukaryotic organisms, and share some enzymes with peroxisomes and glyoxysomes of other protists, plants and animals. Proteins enter the glycosome by a mechanism of post-translational translocation which involves in some, but not all, cases a C-terminal oligopeptide sequence.

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.

Genomic organization and context of a trypanosome variant surface glycoprotein gene family

We have defined the genomic organization and genomic context of a Trypanosoma brucei brucei gene family encoding variant surface glycoproteins (VSGs). This gene family is neither tandemly repeated nor closely linked in the genome, and is not located on small or intermediate size chromosomes. Two dispersed repeated sequence elements, RIME-ingi and the upstream repeat sequence, are linked to members of this gene family; however, the upstream repeat sequences are closely linked only to the basic copy. In other isolates of T.b. brucei this gene family appears conserved with some variation; a restriction fragment length polymorphism found among these isolates suggests the hypothesis that VSG genes may occasionally be diploid. A model accounting for both the generation of dispersed families of VSG genes, and for the interstrain variability of VSG genes, is proposed.

A ribosomal RNA gene promoter at the telomere of a mini-chromosome in Trypanosoma brucei

The parasitic protozoan Trypanosoma brucei has some hundred mini-chromosomes of 50-150 kb, which mainly consist of telomeric repeats, sub-telomeric repeats and internal 177-bp repeats. Their primary function seems to be to expand the repertoire of non-transcribed sub-telomeric variant surface glycoprotein (VSG) genes. Here we report that two of the smaller mini-chromosomes (55 and 60 kb) contain sequences homologous to the ribosomal RNA gene promoter region. We have targeted by homologous recombination the neomycin phosphotransferase (neo(r)) gene behind the promoter on the 55 kb chromosome and show that this promoter mediates the efficient synthesis of properly trans-spliced and polyadenylated neo mRNA. The resulting high resistance to G418 (a neo analogue) is stable in the absence of drug showing that mitotic segregation of this mini-chromosome is precise. Downstream of the transcription start the wild-type version of the ribosomal promoter is flanked by telomeric repeats. The absence of the sub-telomeric repeats found in other T.brucei chromosome ends suggests that the rDNA-telomeric junction has been formed by de novo addition of telomeric repeats to a broken chromosome end (healing). Our results provide a plausible explanation for the alpha-amanitin-resistant transcription of telomeric repeats in T.brucei reported by Rudenko and Van der Ploeg and they show that trypanosomes can efficiently use RNA polymerase I for the expression of sub-telomeric genes, supporting the notion that the alpha-amanitin-resistant transcription of sub-telomeric VSG genes may also be catalyzed by this enzyme.

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.

Molecular karyotype analysis in Leishmania

The advent of pulsed field electrophoresis has allowed a direct approach to the karyotype of Leishmania. The molecular karyotype thus obtained is a stable characteristic of a given strain, although minor modifications may occur during in vitro maintenance. Between 20 and 28 chromosomal bands can be resolved depending on the strain, ranging in size from approximately 250 to 2600 kb. The technique has revealed a striking degree of polymorphism in the size and number of the chromosomal bands between different strains, and this seems independent of the category (species, zymodeme, population) to which the strains belong. It appears that only certain strains originating from the same geographic area may share extensive similarities. This polymorphism can largely be accounted for by chromosome size variations, which can involve up to 25% of the chromosome length. As a result, homologous chromosomes can exist in versions of markedly different size within the same strain. When this occurs with several different chromosomes, the interpretation of PFE patterns appears difficult without prior identification of the size-variable chromosomes and of the chromosome homologies. DNA deletions and amplifications have been shown to account for some of these size modifications, but other mechanisms are probably involved; nevertheless, interchromosomal exchange does not seem to play a major role in these polymorphisms. These chromosomal rearrangements, yet in an early stage of characterization, exhibit two relevant features: they seem (1) to affect essentially the subtelomeric regions and (2) to occur in a recurrent nonrandom manner. Chromosomal rearrangements sharing the same characteristics have been identified in yeast and other protozoa such as Trypanosoma and Plasmodium. The significance of this hypervariability for the biology of the parasite remains unknown, but it can be expected that such mechanisms have been maintained for some purpose; genes specifically located near chromosome ends might benefit from rapid sequence change, alternating activation, or polymorphism of expression. The chromosomal plasticity could represent a general mode of mutation in these parasites, in parallel with genetic exchange which may be uncommon in nature. The molecular characterization of these rearrangements, the identification of each chromosome with the help of physical restriction maps and linkage maps, and the collation of such data on a number of strains and species should allow a significant progress in the understanding of the genetics of Leishmania, in particular as regards ploidy, generation of phenotypic diversity, and genome evolution. Finally, like other models, this is susceptible to improve our knowledge of DNA-DNA interactions and of the chromosome functional structure and dynamics.

Recurrent polymorphisms in small chromosomes of Leishmania tarentolae after nutrient stress or subcloning

Molecular karyotypes of the UC, LEM87 and LEM115 Leishmania tarentolae strains were obtained. All strains had 24-28 chromosomal bands which varied in size between 300 kb and 2.9 Mb. Several recurrent chromosomal polymorphisms occurred in LEM115 after nutrient shock or subcloning. One type of polymorphism involves the truncation of a 365-kb chromosome which contains the miniexon genes. This specific chromosome breakage appears to be induced by the nutrient shock or subcloning process and also occurs spontaneously during routine passage. Another polymorphism is the appearance of a 90-kb minichromosome (115-SNA1) after severe nutrient shock. This appears to be selection of a pre-existing cell type from a mosaic population. The 115-SNA1 minichromosome has sequence homology with a minichromosome in LEM87 cells but shows no homology with any chromosomes in 115wt or other strains. The copy number of 115-SNA1 varies with culture conditions, suggesting a relaxed centromeric control. The nature and origin of this minichromosome is not known.

The cytosolic and glycosomal isoenzymes of glyceraldehyde-3-phosphate dehydrogenase in Trypanosoma brucei have a distant evolutionary relationship

Trypanosoma brucei contains two isoenzymes for glyceraldehyde-3-phosphate dehydrogenase: one enzyme resides in a microbody-like organelle, the glycosome; the other is found in the cytosol. Previously we have reported the characterization of the gene for the glycosomal enzyme [Michels, P. A. M., Poliszczak, A., Osinga, K. A., Misset, O., Van Beeumen, J., Wierenga, R. K., Borst, P. & Opperdoes, F. R. (1986) EMBO J. 5, 1049-1056]. Here we describe the cloning and analysis of the gene that codes for the cytosolic isoenzyme. The gene encodes a polypeptide of 330 amino acids, with a calculated molecular mass of 35440 Da. The two isoenzymes are only 55% identical. The cytosolic glyceraldehyde-3-phosphate dehydrogenase differs from the glycosomal enzyme in the following respects: (a) its subunit molecular mass is 3.4 kDa smaller due to the absence of insertions and a small C-terminal extension which are unique to the glycosomal protein; (b) the cytosolic enzyme has a lower pI (7.9, as compared to 9.3 for the glycosomal isoenzyme), which is due to a reduction in the excess of positively charged amino acids (the calculated net charges of the polypeptides are +2 and +11, respectively). We have compared the amino acid sequences of the two T. brucei glyceraldehyde-3-phosphate dehydrogenases, with 24 available sequences of the corresponding enzyme of other organisms from various phylogenetic groups. On the basis of this comparison an evolutionary tree was constructed. This analysis strongly supports the theory that T. brucei diverged early in evolution from the main eukaryotic branch of the phylogenetic tree. Further, two separate branches for the lineages leading to Trypanosoma are inferred from the amino acid sequences, suggesting that the genes for the two glyceraldehyde-3-phosphate dehydrogenases of the trypanosome are distantly related and must have been acquired independently by the trypanosomal ancestor. The branching determined with the glycosomal enzyme precedes that found with the cytosolic enzyme. The available data do not allow us to decide which of the two genes originally belonged to the trypanosome lineage and which entered the cell later by horizontal gene transfer.

Stable integrative transformation of Trypanosoma brucei that occurs exclusively by homologous recombination

A calmodulin-neomycin-resistance fusion gene was introduced into Trypanosoma brucei by electroporation, and stably transformed cell lines were obtained. In all of the transformants, the fusion gene had integrated into the host genome at the cognate locus, evidently by homologous recombination within flanking calmodulin DNA. This unusual observation distinguishes trypanosomes as the only eukaryote other than yeast known to undergo gene targeting in essentially 100% of the stable transformants. It should now be possible to systematically manipulate the trypanosome genome, directing predetermined mutations to virtually any chromosomal locus.

Chromosome organization of the protozoan Trypanosoma brucei

The genome of the protozoan Trypanosoma brucei is known to be diploid. Karyotype analysis has, however, failed to identify homologous chromosomes. Having refined the technique for separating trypanosome chromosomes (L. H. T. Van der Ploeg, C. L. Smith, R. I. Polvere, and K. Gottesdiener, Nucleic Acids Res. 17:3217-3227, 1989), we can now provide evidence for the presence of homologous chromosomes. By determining the chromosomal location of different genetic markers, most of the chromosomes (14, excluding the minichromosomes), could be organized into seven chromosome pairs. In most instances, the putative homologs of a pair differed in size by about 20%. Restriction enzyme analysis of chromosome-sized DNA showed that these chromosome pairs contained large stretches of homologous DNA sequences. From these data, we infer that the chromosome pairs represent homologs. The identification of homologous chromosomes gives valuable insight into the organization of the trypanosome genome, will facilitate the genetic analysis of T. brucei, and suggests the presence of haploid gametes.

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.

An expression-site-associated gene family of trypanosomes is expressed in vivo and shows homology to a variant surface glycoprotein gene

Utilizing first-strand cDNA from different stages, a gene family was identified that is expressed in bloodstream form trypanosomes but not in cultured procyclic forms. This family of 50-100 genes, termed bloodstream-specific 1 (BS1), shares a chromosomal distribution pattern similar to the variant surface glycoprotein (VSG) genes and the expression-site-associated genes (ESAGs). The BS1 genes are expressed in several variants of Trypanosoma brucei brucei and in Trypanosoma brucei gambiense. Sequence analysis of five members of this gene family reveals the recently described ESAG 6 and ESAG 7 genes as well as the ESAG X gene to be members of this family. We have been unable to localize the BS1 gene product in the cell but show that chronically infected rabbit serum recognizes recombinant BS1 protein arguing for expression in vivo. Finally we note that the derived protein sequence for the BS1 genes suggests an evolutionary relationship with at least one variant surface glycoprotein gene, and hence these studies may provide clues to understanding the molecular origins of antigenic variation in trypanosomes.

Chromosome organization of the protozoan Trypanosoma brucei

The genome of the protozoan Trypanosoma brucei is known to be diploid. Karyotype analysis has, however, failed to identify homologous chromosomes. Having refined the technique for separating trypanosome chromosomes (L. H. T. Van der Ploeg, C. L. Smith, R. I. Polvere, and K. Gottesdiener, Nucleic Acids Res. 17:3217-3227, 1989), we can now provide evidence for the presence of homologous chromosomes. By determining the chromosomal location of different genetic markers, most of the chromosomes (14, excluding the minichromosomes), could be organized into seven chromosome pairs. In most instances, the putative homologs of a pair differed in size by about 20%. Restriction enzyme analysis of chromosome-sized DNA showed that these chromosome pairs contained large stretches of homologous DNA sequences. From these data, we infer that the chromosome pairs represent homologs. The identification of homologous chromosomes gives valuable insight into the organization of the trypanosome genome, will facilitate the genetic analysis of T. brucei, and suggests the presence of haploid gametes.

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.

Chromosomal localization of seven cloned antigen genes provides evidence of diploidy and further demonstration of karyotype variability in Trypanosoma cruzi

The karyotype of Trypanosoma cruzi was studied by pulsed field gel electrophoresis (PFGE) in conditions that allowed 20-25 chromosome bands to be detected. However, several of these bands were present in non-equimolar amounts, suggesting that the total chromosome number is considerably higher. The patterns obtained with the different cloned and uncloned strains were unique, suggesting that the karyotype of T. cruzi is highly variable. The chromosomal localizations of seven cloned genes were determined by Southern blotting of PFGE-separated chromosomes. Three of the clones gave rise to similar patterns and mapped on a chromosome or a family of chromosomes larger than 1.6 Mb. Two clones mapped on either single or pairs of chromosomes, which in some cases differed considerably in size between the different strains tested, suggesting that extensive chromosome rearrangements occur in T. cruzi. Another clone hybridized to several chromosomes in most strains and probably represents a family of genes. Lastly, one clone hybridized to nearly all chromosomes. Many of the clones hybridized to pairs of restriction fragments in the different strains, suggesting that they are allelic. For one of the clones it was possible to provide further evidence for the allelic nature of the fragments by establishing detailed restriction maps around them and by showing that the two fragments in a pair hybridized to chromosomes which differed slightly in size. Taken together, the results infer that the genome of T. cruzi epimastigotes is diploid.

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.

Specific probes for Trypanosoma (Trypanozoon) evansi based on kinetoplast DNA minicircles

Trypanosoma evansi is difficult to distinguish from other members of subgenus Trypanozoon, save for its inability to develop cyclically in the tsetse fly and its characteristic kinetoplast DNA (kDNA). We have used cloned kDNA minicircle fragments as specific probes to distinguish T. evansi from other trypanosomes of subgenus Trypanozoon. Two probes were required, each specific for one of the subgroups of T. evansi previously described. Probe A reacted only with the major isoenzyme group of T. evansi stocks, which have minicircle type A and occur in South America, Kenya, Sudan, Nigeria and Kuwait. The probe did not hybridise with various Trypanosoma brucei spp. stocks, Trypanosoma vivax, Trypanosoma congolense or Trypanosoma simiae, nor with trypanosomes of the minor isoenzyme group of T. evansi stocks found in Kenya with type B minicircles. Probe B was specific for the latter. The probes were sensitive down to a level of 100 trypanosomes in a dot blot. These probes thus provide a simple means of distinguishing T. evansi from T. brucei spp. using comparatively few trypanosomes and without resort to tsetse transmission experiments.

Trypanosoma brucei: two-dimensional gel analysis of the major glycosomal proteins during the life cycle

Kinetoplastid organisms possess a unique organelle, the glycosome, which compartmentalizes the Embden-Meyerhof segment of glycolysis and several other metabolic pathways. In Trypanosoma brucei many of the enzyme activities localized to the glycosome are stage regulated. Two-dimensional gel analysis was used to examine the characteristics, expression, and biosynthesis of the major glycosomal proteins. Two-dimensional gel maps of glycosomes from slender bloodforms and late intermediate-stumpy bloodforms (the precursors of procyclic forms) were indistinguishable, while those of procyclic form glycosomes showed extensive differences. Glycosomal phosphoenolpyruvate carboxykinase and malate dehydrogenase were identified to have subunit molecular weights of 60 and 34 kDa, respectively. We detected two hitherto undescribed glycosomal proteins, one of which is found only in bloodforms. All of the major proteins, except glucose phosphate isomerase, were highly basic. Stage regulation of glycosomal enzyme activities correlated with stage regulation of specific protein biosynthesis.

Interclonal variations in molecular karyotype in Leishmania infantum imply a 'mosaic' strain structure

The molecular karyotypes of 36 clones derived from 8 strains of Leishmania infantum were examined by pulsed-field gel electrophoresis. Although there appeared to be a high degree of genetic relatedness between the clones and the parent strain, a limited degree of polymorphism was noted in 50% of the clones, expressed mainly as the presence of an additional chromosome or as a chromosome size modification. Repeated subcloning in one strain showed that chromosomal rearrangements could occur during the cloning process. Chromosome homologies were examined by Southern analysis with chromosome-specific DNA probes. The results suggest a disomy for some chromosomes, but cannot exclude aneuploidy. The mechanisms possibly leading to such heterogeneity are discussed: they could involve frequent DNA amplification/deletion, and imply a 'mosaic' structure of the cultured strains or clones, with different individuals possessing differently sized versions of the same chromosomes.

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.

Genomic organization, chromosomal localization, and developmentally regulated expression of the glycosyl-phosphatidylinositol-specific phospholipase C of Trypanosoma brucei

The surface of the bloodstream form of the African trypanosome, Trypansoma brucei, is covered with about 10(7) molecules of the variant surface glycoprotein (VSG), a protein tethered to the plasma membrane by a glycosyl-phosphatidylinositol (GPI) membrane anchor. This anchor is cleavable by an endogenous GPI-specific phospholipase C (GPI-PLC). GPI-PLC activity is down regulated when trypanosomes differentiate from the bloodstream form to the procyclic form found in the tsetse fly vector. We have mapped the GPI-PLC locus in the trypanosome genome and have examined the mechanism for this developmental regulation in T. brucei. Southern blot analysis indicates a single-copy gene for GPI-PLC, with two allelic variants distinguishable by two NcoI restriction fragment length polymorphisms. The gene was localized solely to a chromosome in the two-megabase compression region by contour-clamped homogeneous electric field gel electrophoresis. No rearrangement of the GPI-PLC gene occurs during differentiation to procyclic forms, which could potentially silence GPI-PLC gene expression. Enzymological studies give no indication of a diffusible inhibitor of GPI-PLC activity in procyclic forms, and Western immunoblot analysis reveals no detectable GPI-PLC polypeptide in these forms. Therefore, it is highly unlikely that the absence of GPI-PLC activity in procyclic forms is due to posttranslational control. Northern (RNA) blot analysis reveals barely detectable levels of GPI-PLC mRNA in procyclic forms; therefore, regulation of GPI-PLC activity in these forms correlates with the steady-state mRNA level.

Genomic organization, chromosomal localization, and developmentally regulated expression of the glycosyl-phosphatidylinositol-specific phospholipase C of Trypanosoma brucei

The surface of the bloodstream form of the African trypanosome, Trypansoma brucei, is covered with about 10(7) molecules of the variant surface glycoprotein (VSG), a protein tethered to the plasma membrane by a glycosyl-phosphatidylinositol (GPI) membrane anchor. This anchor is cleavable by an endogenous GPI-specific phospholipase C (GPI-PLC). GPI-PLC activity is down regulated when trypanosomes differentiate from the bloodstream form to the procyclic form found in the tsetse fly vector. We have mapped the GPI-PLC locus in the trypanosome genome and have examined the mechanism for this developmental regulation in T. brucei. Southern blot analysis indicates a single-copy gene for GPI-PLC, with two allelic variants distinguishable by two NcoI restriction fragment length polymorphisms. The gene was localized solely to a chromosome in the two-megabase compression region by contour-clamped homogeneous electric field gel electrophoresis. No rearrangement of the GPI-PLC gene occurs during differentiation to procyclic forms, which could potentially silence GPI-PLC gene expression. Enzymological studies give no indication of a diffusible inhibitor of GPI-PLC activity in procyclic forms, and Western immunoblot analysis reveals no detectable GPI-PLC polypeptide in these forms. Therefore, it is highly unlikely that the absence of GPI-PLC activity in procyclic forms is due to posttranslational control. Northern (RNA) blot analysis reveals barely detectable levels of GPI-PLC mRNA in procyclic forms; therefore, regulation of GPI-PLC activity in these forms correlates with the steady-state mRNA level.

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.