Evolutionary relationships in Trypanosoma cruzi: molecular phylogenetics supports the existence of a new major lineage of strains

Trypanosoma cruzi: Genomic Diversity and Structure

Trypanosoma cruzi is the causative agent of Chagas disease, a neglected tropical disease, and one of the most important parasitic diseases worldwide. The first genome of T. cruzi was sequenced in 2005, and its complexity made assembly and annotation challenging. Nowadays, new sequencing methods have improved some strains' genome sequence and annotation, revealing this parasite's extensive genetic diversity and complexity. In this review, we examine the genetic diversity, the genomic structure, and the principal multi-gene families involved in the pathogenicity of T. cruzi. The T. cruzi genome sequence is divided into two compartments: the core (conserved) and the disruptive (variable in length and multicopy gene families among strains). The disruptive region has also been described as genome plasticity and plays a key role in the parasite survival and infection process. This region comprises several multi-gene families, including trans-sialidases, mucins, and mucin-associated surface proteins (MASPs). Trans-sialidases are the most prevalent genes in the genome with a key role in the infection process, while mucins and MASPs are also significant glycosylated proteins expressed on the parasite surface, essential for its biological functions, as host-parasite interaction, host cell invasion or protection against the host immune system, in both insect and mammalian stages. Collectively, in this review, some of the most recent advances in the structure and composition of the T. cruzi genome are reviewed.

New insights into Trypanosoma cruzi genetic diversity, and its influence on parasite biology and clinical outcomes

Chagas disease, caused by Trypanosoma cruzi, remains a serious public health problem worldwide. The parasite was subdivided into six distinct genetic groups, called "discrete typing units" (DTUs), from TcI to TcVI. Several studies have indicated that the heterogeneity of T. cruzi species directly affects the diversity of clinical manifestations of Chagas disease, control, diagnosis performance, and susceptibility to treatment. Thus, this review aims to describe how T. cruzi genetic diversity influences the biology of the parasite and/or clinical parameters in humans. Regarding the geographic dispersion of T. cruzi, evident differences were observed in the distribution of DTUs in distinct areas. For example, TcII is the main DTU detected in Brazilian patients from the central and southeastern regions, where there are also registers of TcVI as a secondary T. cruzi DTU. An important aspect observed in previous studies is that the genetic variability of T. cruzi can impact parasite infectivity, reproduction, and differentiation in the vectors. It has been proposed that T. cruzi DTU influences the host immune response and affects disease progression. Genetic aspects of the parasite play an important role in determining which host tissues will be infected, thus heavily influencing Chagas disease's pathogenesis. Several teams have investigated the correlation between T. cruzi DTU and the reactivation of Chagas disease. In agreement with these data, it is reasonable to suppose that the immunological condition of the patient, whether or not associated with the reactivation of the T. cruzi infection and the parasite strain, may have an important role in the pathogenesis of Chagas disease. In this context, understanding the genetics of T. cruzi and its biological and clinical implications will provide new knowledge that may contribute to additional strategies in the diagnosis and clinical outcome follow-up of patients with Chagas disease, in addition to the reactivation of immunocompromised patients infected with T. cruzi.

Maxicircle architecture and evolutionary insights into Trypanosoma cruzi complex

We sequenced maxicircles from T. cruzi strains representative of the species evolutionary diversity by using long-read sequencing, which allowed us to uncollapse their repetitive regions, finding that their real lengths range from 35 to 50 kb. T. cruzi maxicircles have a common architecture composed of four regions: coding region (CR), AT-rich region, short (SR) and long repeats (LR). Distribution of genes, both in order and in strand orientation are conserved, being the main differences the presence of deletions affecting genes coding for NADH dehydrogenase subunits, reinforcing biochemical findings that indicate that complex I is not functional in T. cruzi. Moreover, the presence of complete minicircles into maxicircles of some strains lead us to think about the origin of minicircles. Finally, a careful phylogenetic analysis was conducted using coding regions of maxicircles from up to 29 strains, and 1108 single copy nuclear genes from all of the DTUs, clearly establishing that taxonomically T. cruzi is a complex of species composed by group 1 that contains clades A (TcI), B (TcIII) and D (TcIV), and group 2 (1 and 2 do not coincide with groups I and II described decades ago) containing clade C (TcII), being all hybrid strains of the BC type. Three variants of maxicircles exist in T. cruzi: a, b and c, in correspondence with clades A, B, and C from mitochondrial phylogenies. While A and C carry maxicircles a and c respectively, both clades B and D carry b maxicircle variant; hybrid strains also carry the b- variant. We then propose a new nomenclature that is self-descriptive and makes use of both the phylogenetic relationships and the maxicircle variants present in T. cruzi.

Nanopore Sequencing Significantly Improves Genome Assembly of the Protozoan Parasite Trypanosoma cruzi

Chagas disease was described by Carlos Chagas, who first identified the parasite Trypanosoma cruzi from a 2-year-old girl called Berenice. Many T. cruzi sequencing projects based on short reads have demonstrated that genome assembly and downstream comparative analyses are extremely challenging in this species, given that half of its genome is composed of repetitive sequences. Here, we report de novo assemblies, annotation, and comparative analyses of the Berenice strain using a combination of Illumina short reads and MinION long reads. Our work demonstrates that Nanopore sequencing improves T. cruzi assembly contiguity and increases the assembly size in ∼16 Mb. Specifically, we found that assembly improvement also refines the completeness of coding regions for both single-copy genes and repetitive transposable elements. Beyond its historical and epidemiological importance, Berenice constitutes a fundamental resource because it now constitutes a high-quality assembly available for TcII (clade C), a prevalent lineage causing human infections in South America. The availability of Berenice genome expands the known genetic diversity of these parasites and reinforces the idea that T. cruzi is intraspecifically divided in three main clades. Finally, this work represents the introduction of Nanopore technology to resolve complex protozoan genomes, supporting its subsequent application for improving trypanosomatid and other highly repetitive genomes.

The fecal, oral, and skin microbiota of children with Chagas disease treated with benznidazole

Background

Chagas disease is still prevalent in rural areas of South America. In endemic areas of Bolivia, school children are screened for the program of Chagas disease eradication of the Ministry of Health, and positive children are treated. Here, we compared the fecal, oral and skin microbiomes of children with or without Chagas disease, and before and after benznidazol treatment of infected children.

Methods

A total of 543 Bolivian children (5–14 years old) were tested for Chagas disease, and 20 positive children were treated with Benznidazole. Fecal samples and oral and skin swabs were obtained before and after treatment, together with samples from a group of 35 uninfected controls. The 16S rRNA genes were sequenced and analyzed using QIIME to determine Alpha diversity differences and community distances, and linear discriminant analyses to determine marker taxa by infection status or treatment.

Results

Twenty out of 543 children screened were seropositive for Chagas disease (3.7%) and were included in the study, together with 35 control children that were seronegative for the disease. Fecal samples, oral and skin swabs were taken at the beginning of the study and after the anti-protozoa therapy with Benznidazole to the chagasic children. Infected children had higher fecal Firmicutes (Streptococcus, Roseburia, Butyrivibrio, and Blautia), and lower Bacteroides and also showed some skin -but not oral- microbiota differences. Treatment eliminated the fecal microbiota differences from control children, increasing Dialister (class Clostridia) and members of the Enterobacteriaceae, and decreasing Prevotella and Coprococcus, with minor effects on the oral and skin bacterial diversity.

Conclusions

The results of this study show differences in the fecal microbiota associated with Chagas disease in children, and also evidence that treatment normalizes fecal microbiota (makes it more similar to that in controls), but is associated with oral and skin microbiota differences from control children. Since microbiota impacts in children, it is important to determine the effect of drugs on the children microbiota, since dysbiosis could lead to physiological effects which might be avoidable with microbiota restoration interventions.

A genomic scale map of genetic diversity in Trypanosoma cruzi

BACKGROUND

Trypanosoma cruzi, the causal agent of Chagas Disease, affects more than 16 million people in Latin America. The clinical outcome of the disease results from a complex interplay between environmental factors and the genetic background of both the human host and the parasite. However, knowledge of the genetic diversity of the parasite, is currently limited to a number of highly studied loci. The availability of a number of genomes from different evolutionary lineages of T. cruzi provides an unprecedented opportunity to look at the genetic diversity of the parasite at a genomic scale.

RESULTS

Using a bioinformatic strategy, we have clustered T. cruzi sequence data available in the public domain and obtained multiple sequence alignments in which one or two alleles from the reference CL-Brener were included. These data covers 4 major evolutionary lineages (DTUs): TcI, TcII, TcIII, and the hybrid TcVI. Using these set of alignments we have identified 288,957 high quality single nucleotide polymorphisms and 1,480 indels. In a reduced re-sequencing study we were able to validate ~ 97% of high-quality SNPs identified in 47 loci. Analysis of how these changes affect encoded protein products showed a 0.77 ratio of synonymous to non-synonymous changes in the T. cruzi genome. We observed 113 changes that introduce or remove a stop codon, some causing significant functional changes, and a number of tri-allelic and tetra-allelic SNPs that could be exploited in strain typing assays. Based on an analysis of the observed nucleotide diversity we show that the T. cruzi genome contains a core set of genes that are under apparent purifying selection. Interestingly, orthologs of known druggable targets show statistically significant lower nucleotide diversity values.

CONCLUSIONS

This study provides the first look at the genetic diversity of T. cruzi at a genomic scale. The analysis covers an estimated ~ 60% of the genetic diversity present in the population, providing an essential resource for future studies on the development of new drugs and diagnostics, for Chagas Disease. These data is available through the TcSNP database (http://snps.tcruzi.org).

A simple strain typing assay for Trypanosoma cruzi: discrimination of major evolutionary lineages from a single amplification product

BACKGROUND

Trypanosoma cruzi is the causative agent of Chagas' Disease. The parasite has a complex population structure, with six major evolutionary lineages, some of which have apparently resulted from ancestral hybridization events. Because there are important biological differences between these lineages, strain typing methods are essential to study the T. cruzi species. Currently, there are a number of typing methods available for T. cruzi, each with its own advantages and disadvantages. However, most of these methods are based on the amplification of a variable number of loci.

METHODOLOGY/PRINCIPAL FINDINGS

We present a simple typing assay for T. cruzi, based on the amplification of a single polymorphic locus: the TcSC5D gene. When analyzing sequences from this gene (a putative lathosterol/episterol oxidase) we observed a number of interesting polymorphic sites, including 1 tetra-allelic, and a number of informative tri- and bi-allelic SNPs. Furthermore, some of these SNPs were located within the recognition sequences of two commercially available restriction enzymes. A double digestion with these enzymes generates a unique restriction pattern that allows a simple classification of strains in six major groups, corresponding to DTUs TcI-TcIV, the recently proposed Tcbat lineage, and TcV/TcVI (as a group). Direct sequencing of the amplicon allows the classification of strains into seven groups, including the six currently recognized evolutionary lineages, by analyzing only a few discriminant polymorphic sites.

CONCLUSIONS/SIGNIFICANCE

Based on these findings we propose a simple typing assay for T. cruzi that requires a single PCR amplification followed either by restriction fragment length polymorphism analysis, or direct sequencing. In the panel of strains tested, the sequencing-based method displays equivalent inter-lineage resolution to recent multi- locus sequence typing assays. Due to their simplicity and low cost, the proposed assays represent a good alternative to rapidly screen strain collections, providing the cornerstone for the development of robust typing strategies.

Recent, independent and anthropogenic origins of Trypanosoma cruzi hybrids

The single celled eukaryote Trypanosoma cruzi, a parasite transmitted by numerous species of triatomine bug in the Americas, causes Chagas disease in humans. T. cruzi generally reproduces asexually and appears to have a clonal population structure. However, two of the six major circulating genetic lineages, TcV and TcVI, are TcII-TcIII inter-lineage hybrids that are frequently isolated from humans in regions where chronic Chagas disease is particularly severe. Nevertheless, a prevalent view is that hybridisation events in T. cruzi were evolutionarily ancient and that active recombination is of little epidemiological importance. We analysed genotypes of hybrid and non-hybrid T. cruzi strains for markers representing three distinct evolutionary rates: nuclear GPI sequences (n = 88), mitochondrial COII-ND1 sequences (n = 107) and 28 polymorphic microsatellite loci (n = 35). Using Maximum Likelihood and Bayesian phylogenetic approaches we dated key evolutionary events in the T. cruzi clade including the emergence of hybrid lineages TcV and TcVI, which we estimated to have occurred within the last 60,000 years. We also found evidence for recent genetic exchange between TcIII and TcIV and between TcI and TcIV. These findings show that evolution of novel recombinants remains a potential epidemiological risk. The clearly distinguishable microsatellite genotypes of TcV and TcVI were highly heterozygous and displayed minimal intra-lineage diversity indicative of even earlier origins than sequence-based estimates. Natural hybrid genotypes resembled typical meiotic F1 progeny, however, evidence for mitochondrial introgression, absence of haploid forms and previous experimental crosses indicate that sexual reproduction in T. cruzi may involve alternatives to canonical meiosis. Overall, the data support two independent hybridisation events between TcII and TcIII and a recent, rapid spread of the hybrid progeny in domestic transmission cycles concomitant with, or as a result of, disruption of natural transmission cycles by human activities.

Analyses of 32 loci clarify phylogenetic relationships among Trypanosoma cruzi lineages and support a single hybridization prior to human contact

Background

The genetic diversity of Trypanosoma cruzi, the etiological agent of Chagas disease, has been traditionally divided in two major groups, T. cruzi I and II, corresponding to discrete typing units TcI and TcII-VI under a recently proposed nomenclature. The two major groups of T. cruzi seem to differ in important biological characteristics, and are thus thought to represent a natural division relevant for epidemiological studies and development of prophylaxis. To understand the potential connection between the different manifestations of Chagas disease and variability of T. cruzi strains, it is essential to have a correct reconstruction of the evolutionary history of T. cruzi.

Methodology/Principal Findings

Nucleotide sequences from 32 unlinked loci (>26 Kilobases of aligned sequence) were used to reconstruct the evolutionary history of strains representing the known genetic variability of T. cruzi. Thorough phylogenetic analyses show that the original classification of T. cruzi in two major lineages does not reflect its evolutionary history and that there is only strong evidence for one major and recent hybridization event in the history of this species. Furthermore, estimates of divergence times using Bayesian methods show that current extant lineages of T. cruzi diverged very recently, within the last 3 million years, and that the major hybridization event leading to hybrid lineages TcV and TcVI occurred less than 1 million years ago, well before the contact of T. cruzi with humans in South America.

Conclusions/Significance

The described phylogenetic relationships among the six major genetic subdivisions of T. cruzi should serve as guidelines for targeted epidemiological and prophylaxis studies. We suggest that it is important to reconsider conclusions from previous studies that have attempted to uncover important biological differences between the two originally defined major lineages of T. cruzi especially if those conclusions were obtained from single or few strains.

Trypanosoma cruzi is the protozoan parasite that causes Chagas disease, a major health problem in Latin America. The genetic diversity of this parasite has been traditionally divided in two major groups: T. cruzi I and II, which can be further divided in six major genetic subdivisions (subgroups TcI-TcVI). T. cruzi I and II seem to differ in important biological characteristics, and are thought to represent a natural division relevant for epidemiological studies and development of prophylaxis. Having a correct reconstruction of the evolutionary history of T. cruzi is essential for understanding the potential connection between the genetic and phenotypic variability of T. cruzi with the different manifestations of Chagas disease. Here we present results from a comprehensive phylogenetic analysis of T. cruzi using more than 26 Kb of aligned sequence data. We show strong evidence that T. cruzi II (TcII-VI) is not a natural evolutionary group but a paraphyletic lineage and that all major lineages of T. cruzi evolved recently (<3 million years ago [mya]). Furthermore, the sequence data is consistent with one major hybridization event having occurred in this species recently (< 1 mya) but well before T. cruzi entered in contact with humans in South America.

Flow cytometric analysis and microsatellite genotyping reveal extensive DNA content variation in Trypanosoma cruzi populations and expose contrasts between natural and experimental hybrids

Trypanosoma cruzi exhibits remarkable genetic heterogeneity. This is evident at the nucleotide level but also structurally, in the form of karyotypic variation and DNA content differences between strains. Although natural populations of T. cruzi are predominantly clonal, hybrid lineages (TcIId and TcIIe) have been identified and hybridisation has been demonstrated in vitro, raising the possibility that genetic exchange may continue to shape the evolution of this pathogen. The mechanism of genetic exchange identified in the laboratory is unusual, apparently involving fusion of diploid parents followed by genome erosion. We investigated DNA content diversity in natural populations of T. cruzi in the context of its genetic subdivisions by using flow cytometric analysis and multilocus microsatellite genotyping to determine the relative DNA content and estimate the ploidy of 54 cloned isolates. The maximum difference observed was 47.5% between strain Tu18 cl2 (TcIIb) and strain C8 cl1 (TcI), which we estimated to be equivalent to ∼73 Mb of DNA. Large DNA content differences were identified within and between discrete typing units (DTUs). In particular, the mean DNA content of TcI strains was significantly less than that for TcII strains (P < 0.001). Comparisons of hybrid DTUs TcIId/IIe with corresponding parental DTUs TcIIb/IIc indicated that natural hybrids are predominantly diploid. We also measured the relative DNA content of six in vitro-generated TcI hybrid clones and their parents. In contrast to TcIId/IIe hybrid strains these experimental hybrids comprised populations of sub-tetraploid organisms with mean DNA contents 1.65–1.72 times higher than the parental organisms. The DNA contents of both parents and hybrids were shown to be relatively stable after passage through a mammalian host, heat shock or nutritional stress. The results are discussed in the context of hybridisation mechanisms in both natural and in vitro settings.

Trypanosoma cruzi: adaptation to its vectors and its hosts

American trypanosomiasis is a parasitic zoonosis that occurs throughout Latin America. The etiological agent, Trypanosoma cruzi, is able to infect almost all tissues of its mammalian hosts and spreads in the environment in multifarious transmission cycles that may or not be connected. This biological plasticity, which is probably the result of the considerable heterogeneity of the taxon, exemplifies a successful adaptation of a parasite resulting in distinct outcomes of infection and a complex epidemiological pattern. In the 1990s, most endemic countries strengthened national control programs to interrupt the transmission of this parasite to humans. However, many obstacles remain to the effective control of the disease. Current knowledge of the different components involved in elaborate system that is American trypanosomiasis (the protozoan parasite T. cruzi, vectors Triatominae and the many reservoirs of infection), as well as the interactions existing within the system, is still incomplete. The Triatominae probably evolve from predatory reduvids in response to the availability of vertebrate food source. However, the basic mechanisms of adaptation of some of them to artificial ecotopes remain poorly understood. Nevertheless, these adaptations seem to be associated with a behavioral plasticity, a reduction in the genetic repertoire and increasing developmental instability.

TcSNP: a database of genetic variation in Trypanosoma cruzi

The TcSNP database (http://snps.tcruzi.org) integrates information on genetic variation (polymorphisms and mutations) for different stocks, strains and isolates of Trypanosoma cruzi, the causative agent of Chagas disease. The database incorporates sequences (genes from the T. cruzi reference genome, mRNAs, ESTs and genomic sequences); multiple sequence alignments obtained from these sequences; and single-nucleotide polymorphisms and small indels identified by scanning these multiple sequence alignments. Information in TcSNP can be readily interrogated to arrive at gene sets, or SNP sets of interest based on a number of attributes. Sequence similarity searches using BLAST are also supported. This first release of TcSNP contains nearly 170 000 high-confidence candidate SNPs, derived from the analysis of annotated coding sequences. As new sequence data become available, TcSNP will incorporate these data, mapping new candidate SNPs onto the reference genome sequences.

Sequence diversity and evolution of multigene families in Trypanosoma cruzi

Several copies of genes belonging to three multigene families present in the genome of Trypanosoma cruzi were sequenced and comparatively analyzed across six different strains of the parasite belonging to the T. cruzi I lineage (Colombiana, Silvio X10 and Dm28c), the T. cruzi II lineage (Esmeraldo and JG) and a hybrid strain (CL Brener). For all three gene families analyzed, our results support the division in T. cruzi I and II lineages. Furthermore, in agreement with its hybrid nature, sequences derived from the CL Brener clone clustered together with T. cruzi II sequences as well as with a third group of sequences. Paralogous sequences encoding Amastin, an amastigote surface glycoprotein and TcAG48, an antigenic RNA binding protein, which are clustered in the parasite genome, present higher intragenomic variability in T. cruzi II and CL Brener strains, when compared to T. cruzi I strains. Paralogous sequences derived from the TcADC gene family, which encode various isoforms of adenylyl cyclases and are dispersed throughout the T. cruzi genome, exhibit similar degree of variability in all strains, except in the CL Brener strain, in which the sequences were more divergent. Several factors including mutation rates and gene conversion mechanisms, acting differently within the T. cruzi population, may contribute to create such distinct levels of sequence diversity in multigene families that are clustered in the T. cruzi genome.

Molecular phylogeny of Trypanosoma cruzi from Central America (Guatemala) and a comparison with South American strains

Molecular phylogenetic analysis was carried out for 21 strains of Trypanosoma cruzi, nine of which were obtained from Guatemala and 12 from South America. Phylogenetic trees were constructed using the nucleotide sequences of two nuclear gene regions, dihydrofolate reductase-thymidylate synthase (DHFR-TS) and trypanothione reductase (TR), and contiguous portions of two mitochondrial genes, cytochrome oxidase subunit II (COII) and reduced nicotinamide adenine dinucleotide dehydrogenase subunit 1 (ND1). Possible genetic exchange between the rather divergent lineages of T. cruzi II from South America was suggested in the trees of the two nuclear genes. T. cruzi I strains obtained from Guatemala and Colombia were identical in all the genes examined, but other T. cruzi I isolates from South America were rather polymorphic in the DHFR-TS and mitochondrial genes. No genetic exchange was identified between T. cruzi I populations from Central and South America in the present study.

Identification of Trypanosoma cruzi sublineages by the simple method of single-stranded conformation DNA polymorphism (SSCP)

Fifty-eight stocks of Trypanosoma cruzi from Latin America were genetically characterized using the methods of the polymerase chain reaction (PCR) and the single-stranded conformation DNA polymorphism (SSCP) with four genes, mini-exon, 24Salpha rRNA, 18Sr RNA, cruzipain, and a RAPD fragment DNA region, P7-P8. All the isolates examined were assigned to T. cruzi I or subgroups of T. cruzi II by these methods. From these results, the SSCP analysis, which was simple to perform and highly sensitive to sequence variation, seemed to be a good modality for characterizing T. cruzi, particularly for subgroups of T. cruzi II. However, in several isolates of T. cruzi II, the subgroups determined with the SSCP of 24Salpha rRNA were not consistent with those determined with other genes, the SSCP of 18S rRNA and cruzipain, and the PCR of P7-P8, possibly because of the occurrence of rare genetic exchanges or mutations or both in natural populations of this parasite. The SSCP patterns of 24Salpha rRNA and 18S rRNA were highly variable in the T. cruzi I isolates; therefore, analyses using both genes are considered to be one possible method for the characterization of isolates within T. cruzi I.

Comparative karyotyping as a tool for genome structure analysis of Trypanosoma cruzi

As a part of the Trypanosoma cruzi genome project, 239 genetic markers were hybridised to PFGE separated DNA from T. cruzi, in order to determine the number and size of chromosomes and to aid the assembly of the genome sequence. We used three strains, T. cruzi IIe CL Brener (the genome project reference strain) and two T. cruzi I strains, Sylvio X10/7 and CAI/72, to perform a comparative study of their karyotypes and to determine marker linkage. A densitometry analysis of the separations estimated the total chromosome numbers to be 55 in CL Brener and 57 in the two other strains. In all, 45 markers hybridised to single chromosomal bands and 103 markers to two bands in CL Brener, while the number of markers in Sylvio X10/7 and CAI/72 were 102/68 and 61/105, respectively. Size differences between homologous chromosomes were often large, up to 1900 kb (173%). The average difference was 36% for CL Brener and 23.5% for the T. cruzi I strains. Larger differences in CL Brener are consistent with a recent hybrid origin. Forty markers distributed into 15 linkage groups were found to identify specific chromosomes or chromosomes pairs. While the same markers are generally linked in all three strains, the sizes of the chromosomes vary extensively, indicating large chromosomal rearrangements. These data provide valuable information for the finishing of the CL Brener genome sequence.

Two Hybridization Events Define the Population Structure of Trypanosoma cruzi

Genetic variation in Trypanosoma cruzi is likely a key determinant in transmission and pathogenesis of Chagas disease. We have examined nine loci as markers for the extant T. cruzi strains. Four distinct alleles were found for each locus, corresponding to the sequence classes present in the homozygous discrete typing units (DTUs) I, IIa, IIb, and IIc. The alleles in DTUs IIa and IIc showed a spectrum of polymorphism ranging from DTU I-like to DTU IIb-like, in addition to DTU-specific sequence variation. DTUs IId and IIe were indistinguishable, showing DTU homozygosity at one locus and heterozygosity with DTU IIb and IIc allelic sequences at eight loci. Recombination between the DTU IIb and IIc alleles is evidenced from mosaic polymorphisms. These data imply that two discrete hybridization events resulted in the formation of the current DTUs. We propose a model in which a fusion between ancestral DTU I and IIb strains gave rise to a heterozygous hybrid that homogenized its genome to become the homozygous progenitor of DTUs IIa and IIc. The second hybridization between DTU IIb and IIc strains that generated DTUs IId and IIe resulted in extensive heterozygosity with subsequent recombination of parental genotypes.

Genotypic variation among lineages of Trypanosoma cruzi and its geographic aspects

Isozyme analysis with 18 enzyme loci was conducted on 146 isolates of Trypanosoma cruzi from Mexico, Guatemala, Colombia, Ecuador, Peru, Brazil, Bolivia, Paraguay and Chile. Forty-four different MLGs (groups of isolates with identical multilocus genotypes) were identified and a phylogeny was constructed. The phylogenetic tree consisted of two main groups (T. cruzi I, T. cruzi II), and the latter was further divided into two subgroups (T. cruzi IIa, T. cruzi IIb-e). Evidence of hybridization between different MLGs of T. cruzi II was found, which means that genetic exchanges seem to have occurred in South American T. cruzi. On the other hand, the persistence of characteristic T. cruzi I and T. cruzi II isozyme patterns in single small villages in Bolivia and Guatemala suggested that genetic exchange is very rare between major lineages. A significant difference in genetic diversity was shown between T. cruzi I and T. cruzi II from several indices of population genetics. Two possibilities could explain this genetic variation in the population: differences in evolutionary history and/or different tendencies to exchange genetic material. Broad-scale geographic distributions of T. cruzi I and T. cruzi IIb-e were different; T. cruzi I occurred in Central America and south to Bolivia and Brazil, while T. cruzi IIb-e occurred in the central and southern areas of South America, overlapping with T. cruzi I in Brazil and Bolivia.

Genetic subdivisions within Trypanosoma cruzi (Discrete Typing Units) and their relevance for molecular epidemiology and experimental evolution

Background

This paper summarizes the main results obtained on Trypanosoma cruzi genetic diversity and population structure since this parasite became the theme of many genetic and molecular studies in the early seventies.

Results

T. cruzi exibits a paradigmatic pattern of long-term, clonal evolution, which has structured its natural populations into several discrete genetic subdivisions or "Discrete Typing Units" (DTU). Rare hybridization events are nevertheless detectable in natural populations and have been recently obtained in the laboratory.

Conclusions

The DTUs and natural clones of T. cruzi constitute relevant units for molecular epidemiology and experimental evolution. Experimental mating opens the way to an in-depth knowledge of this parasite's formal genetics.

Evaluation of Trypanosoma cruzi hybrid stocks based on chromosomal size variation

Although all classical lines of evidence point to the fact that Trypanosoma cruzi has a predominantly clonal evolution, accumulating data show that some T. cruzi stocks are the result of hybridisation events. We evaluated whether chromosomal polymorphism would give evolutionary information on hybrid isolates. Twenty-three coding sequences were mapped on the chromosomes of nine parasite stocks, four of which are putative hybrids (CL Brener and rDNA group 1/2). Phenetic analyses of karyotype data were based on the absolute chromosomal size difference index (aCSDI), a method that assumes that the genomic distance between two organisms is the sum of the size differences between their homologous chromosomes. aCSDI-based dendrograms obtained from a variable number of probes (3-18 probes) defined in all the cases three clusters: two corresponding, respectively, to T. cruzi I and T. cruzi II groups; and a third one, to rDNA group 1/2. CL Brener was alternatively positioned in T. cruzi II or rDNA group 1/2 clusters. Three clusters were also observed in the dendrogram constructed with restriction fragment length polymorphism (RFLP) data from 18 probes. The topology of the chromosome and RFLP dendrograms is similar, with a significant correlation coefficient (r=0.86062; P<0.0001), supporting a strong structuring of the clusters. This study also revealed that hybrid stocks have a larger proportion of two different-sized homologous chromosomes, as compared with non-hybrid strains. Overall, our results show that chromosomes are valuable characters for identification of evolutionary groups, in particular, T. cruzi hybrid organisms.

Single-nucleotide polymorphisms of the Trypanosoma cruzi MSH2 gene support the existence of three phylogenetic lineages presenting differences in mismatch-repair efficiency

We have identified single-nucleotide polymorphisms (SNPs) in the mismatch-repair gene TcMSH2 from Trypanosoma cruzi. Phylogenetic inferences based on the SNPs, confirmed by RFLP analysis of 32 strains, showed three distinct haplogroups, denominated A, B, and C. Haplogroups A and C presented strong identity with the previously described T. cruzi lineages I and II, respectively. A third haplogroup (B) was composed of strains presenting hybrid characteristics. All strains from a haplogroup encoded the same specific protein isoform, called, respectively, TcMHS2a, TcMHS2b, and TcMHS2c. The classification into haplogroups A, B, and C correlated with variation in the efficiency of mismatch repair in these cells. When microsatellite loci of strains representative of each haplogroup were analyzed after being cultured in the presence of hydrogen peroxide, new microsatellite alleles were definitely seen in haplogroups B and C, while no evidence of microsatellite instability was found in haplogroup A. Also, cells from haplogroups B and C were considerably more resistant to cisplatin treatment, a characteristic known to be conferred by deficiency of mismatch repair in eukaryotic cells. Altogether, our data suggest that strains belonging to haplogroups B and C may have decreased mismatch-repair ability when compared with strains assigned to the haplogroup A lineage.

Evidence for multiple hybrid groups in Trypanosoma cruzi

A role for parasite genetic variability in the spectrum of Chagas disease is emerging but not yet evident, in part due to an incomplete understanding of the population structure of Trypanosoma cruzi. To investigate further the observed genotypic variation at the sequence and chromosomal levels in strains of standard and field-isolated T. cruzi we have undertaken a comparative analysis of 10 regions of the genome from two isolates representing T. cruzi I (Dm28c and Silvio X10) and two from T. cruzi II (CL Brener and Esmeraldo). Amplified regions contained intergenic (non-coding) sequences from tandemly repeated genes. Multiple nucleotide polymorphisms correlated with the T. cruzi I/T. cruzi II classification. Two intergenic regions had useful polymorphisms for the design of classification probes to test on genomic DNA from other known isolates. Two adjacent nucleotide polymorphisms in HSP 60 correlated with the T. cruzi I and T. cruzi II distinction. 1F8 nucleotide polymorphisms revealed multiple subdivisions of T. cruzi II: subgroups IIa and IIc displayed the T. cruzi I pattern; subgroups IId and IIe possessed both the I and II patterns. Furthermore, isolates from subgroups IId and IIe contained the 1F8 polymorphic markers on different chromosome bands supporting a genetic exchange event that resulted in chromosomes V and IX of T. cruzi strain CL Brener. Based on these analyses, T. cruzi I and subgroup IIb appear to be pure lines, while subgroups IIa/IIc and IId/IIe are hybrid lines. These data demonstrate for the first time that IIa/IIc are hybrid, consistent with the hypothesis that genetic recombination has occurred more than once within the T. cruzi lines.

Mechanism of genetic exchange in American trypanosomes

The kinetoplastid Protozoa are responsible for devastating diseases. In the Americas, Trypanosoma cruzi is the agent of Chagas' disease--a widespread disease transmissible from animals to humans (zoonosis)--which is transmitted by exposure to infected faeces of blood-sucking triatomine bugs. The presence of genetic exchange in T. cruzi and in Leishmania is much debated. Here, by producing hybrid clones, we show that T. cruzi has an extant capacity for genetic exchange. The mechanism is unusual and distinct from that proposed for the African trypanosome, Trypanosoma brucei. Two biological clones of T. cruzi were transfected to carry different drug-resistance markers, and were passaged together through the entire life cycle. Six double-drug-resistant progeny clones, recovered from the mammalian stage of the life cycle, show fusion of parental genotypes, loss of alleles, homologous recombination, and uniparental inheritance of kinetoplast maxicircle DNA. There are strong genetic parallels between these experimental hybrids and the genotypes among natural isolates of T. cruzi. In this instance, aneuploidy through nuclear hybridization results in recombination across far greater genetic distances than mendelian genetic exchange. This mechanism also parallels genome duplication.

Evidence for genetic exchange and hybridization in Trypanosoma cruzi based on nucleotide sequences and molecular karyotype

Trypanosoma cruzi is thought to undergo predominant clonal evolution, as determined by population genetics studies. However, this model does not exclude occasional recombination, which existence is strongly suggested by several recent studies. We sequenced a portion of the maxicircle cytochrome b (CYb) gene and of the nuclear rRNA promoter region from representative strains of six T. cruzi genetic lineages isolated from anthroponotic environments and man (lineages IIb, IId and IIe), sylvatic environments (lineages IIa and IIc) or both (lineage I). Phylogenetic analyses based on the two genes were incongruent. Remarkably, in lineage IIe, CYb and rRNA sequences were very closely related to those of lineages IIc and IIb, respectively. One stock of lineage IId showed rRNA sequence heterogeneity, with both IIb-like and IIc-like copies. Analysis of the size variation of six distinct pairs of putative homologous chromosomes revealed a bimodal distribution of chromosomal sizes across T. cruzi. Notably, stocks of lineages IId and IIe had several chromosomal pairs distributed in distinct modes, with the corresponding modes individually found in lineages IIb and IIc. Together, these data indicate the origin of lineages IId and IIe by hybridization between representatives of lineages IIb and IIc. CYb and rRNA sequences clustered into three and four major lineages, respectively. Data were in agreement with the distinction of six genetic lineages, but not with their proposed grouping into two primary lineages, as lineage II was not monophyletic. Based on a CYb substitution rate of 1% per million years (Myr), the major lineages are estimated to have diverged around 10 million years ago.

The clonal theory of parasitic protozoa: 12 years on

The question of population structure in parasitic protozoa has recently gained a renewed topicality with significant contributions on medically important pathogens, such as Plasmodium falciparum, Toxoplasma gondii and Cryptosporidium parvum. The proposals that initiated this debate are reviewed here and the subsequent developments of the clonal theory, in light of recent contributions, are examined.

Sequence variation in the dihydrofolate reductase-thymidylate synthase (DHFR-TS) and trypanothione reductase (TR) genes of Trypanosoma cruzi

Dihydrofolate reductase-thymidylate synthase (DHFR-TS) and trypanothione reductase (TR) are important enzymes for the metabolism of protozoan parasites from the family Trypanosomatidae (e.g. Trypanosoma spp., Leishmania spp.) that are targets of current drug-design studies. Very limited information exists on the levels of genetic polymorphism of these enzymes in natural populations of any trypanosomatid parasite. We present results of a survey of nucleotide variation in the genes coding for those enzymes in a large sample of strains from Trypanosoma cruzi, the agent of Chagas' disease. We discuss the results from an evolutionary perspective. A sample of 31 strains show 39 silent and five amino acid polymorphisms in DHFR-TS, and 35 silent and 11 amino acid polymorphisms in TR. No amino acid replacements occur in regions that are important for the enzymatic activity of these proteins, but some polymorphisms occur in sites previously assumed to be invariant. The sequences from both genes cluster in four major groups, a result that is not fully consistent with the current classification of T. cruzi in two major groups of strains. Most polymorphisms correspond to fixed differences among the four sequence groups. Two tests of neutrality show that there is no evidence of adaptive divergence or of selective events having shaped the distribution of polymorphisms and fixed differences in these genes in T. cruzi. However, one nearly significant reduction of variation in the TR sequences from one sequence group suggests a recent selective event at, or close to, that locus.

Characterisation of large and small subunit rRNA and mini-exon genes further supports the distinction of six Trypanosoma cruzi lineages

It has been proposed that isolates of Trypanosoma cruzi, the agent of American trypanosomiasis, can be ordered into two primary phylogenetic lineages, first based on multilocus enzyme electrophoresis and random amplified polymorphic DNA, and subsequently based on the 24Salpha rRNA and mini-exon genes. Recent multilocus enzyme electrophoresis and random amplified polymorphic DNA data have additionally shown that the major multilocus enzyme electrophoresis/random amplified polymorphic DNA lineage II is further subdivided into five smaller lineages, designated IIa-IIe. In this study, the precise correspondence between the multilocus enzyme electrophoresis/random amplified polymorphic DNA and rRNA/mini-exon lineages was investigated. Using the 24Salpha rRNA and mini-exon markers in combination, five sets of strains were distinguished, corresponding to the multilocus enzyme electrophoresis/random amplified polymorphic DNA lineages I, IIa, IIc, IId and to lineages IIb/IIe together, respectively. The previous categorisation into only two primary lineages based on 24Salpha rRNA and mini-exon characterisation is explained, in part, by the lack of representativeness of the breadth of T. cruzi diversity in earlier study samples. Additionally, a PCR assay based on a length-variable region of the 18S rRNA gene distinguished lineage IIe from lineage IIb. Thus, the six multilocus enzyme electrophoresis/random amplified polymorphic DNA lineages could be readily identified by combining data from the 24Salpha rRNA, mini-exon and 18S rRNA characterisation assays, further supporting the relevance of these genetic units for T. cruzi strain classification and subspecific nomenclature. The recently proposed groups T. cruzi I and T. cruzi II correspond to multilocus enzyme electrophoresis/random amplified polymorphic DNA lineages I and IIb, respectively. Our findings show that T. cruzi lineage characterisation based on a single marker (either mini-exon or 24Salpha rRNA) has insufficient resolution, and leads to important reinterpretations of recent epidemiological and evolutionary studies based on the oversimplified rRNA/mini-exon dichotomic classification of T. cruzi isolates.

Nucleotide sequences provide evidence of genetic exchange among distantly related lineages of Trypanosoma cruzi

Simple phylogenetic tests were applied to a large data set of nucleotide sequences from two nuclear genes and a region of the mitochondrial genome of Trypanosoma cruzi, the agent of Chagas' disease. Incongruent gene genealogies manifest genetic exchange among distantly related lineages of T. cruzi. Two widely distributed isoenzyme types of T. cruzi are hybrids, their genetic composition being the likely result of genetic exchange between two distantly related lineages. The data show that the reference strain for the T. cruzi genome project (CL Brener) is a hybrid. Well-supported gene genealogies show that mitochondrial and nuclear gene sequences from T. cruzi cluster, respectively, in three or four distinct clades that do not fully correspond to the two previously defined major lineages of T. cruzi. There is clear genetic differentiation among the major groups of sequences, but genetic diversity within each major group is low. We estimate that the major extant lineages of T. cruzi have diverged during the Miocene or early Pliocene (3-16 million years ago).