Chromosome level assembly of the hybrid Trypanosoma cruzi genome
© Weatherly et al; licensee BioMed Central Ltd. 2009
Received: 16 January 2009
Accepted: 01 June 2009
Published: 01 June 2009
In contrast to the essentially fully assembled genome sequences of the kinetoplastid pathogens Leishmania major and Trypanosoma brucei the assembly of the Trypanosoma cruzi genome has been hindered by its repetitive nature and the fact that the reference strain (CL Brener) is a hybrid of two distinct lineages. In this work, the majority of the contigs and scaffolds were assembled into pairs of homologous chromosomes based on predicted parental haplotype, inference from TriTryp synteny maps and the use of end sequences from T. cruzi BAC libraries.
Ultimately, 41 pairs of chromosomes were assembled using this approach, a number in agreement with the predicted number of T. cruzi chromosomes based upon pulse field gel analysis, with over 90% (21133 of 23216) of the genes annotated in the genome represented. The approach was substantiated through the use of Southern blot analysis to confirm the mapping of BAC clones using as probes the genes they are predicted to contain, and each chromosome construction was visually validated to ensure sufficient evidence was present to support the organization. While many members of large gene families are incorporated into the chromosome assemblies, the majority of genes excluded from the chromosomes belong to gene families, as these genes are frequently impossible to accurately position.
Now assembled, these chromosomes bring T. cruzi to the same level of organization as its kinetoplastid relatives and have been used as the basis for the T. cruzi genome in TriTrypDB, a trypanosome database of EuPathDB. In addition, they will provide the foundation for analyses such as reverse genetics, where the location of genes and their alleles and/or paralogues is necessary and comparative genome hybridization analyses (CGH), where a chromosome-level view of the genome is ideal.
The publication of the three trypanosomatid, or TriTryp, genomes in 2005 was an important advance in our understanding of the related parasites Trypanosoma cruzi, Leishmania major, and Trypanosoma brucei. However, as with other genomes [1, 2] a published genome is not necessarily a completed genome. The issue of incompleteness within the TriTryp genomes is most striking with respect to T. cruzi which, because of its highly repetitive nature and the fact that the reference strain (CL Brener) is a hybrid of
two distinct T. cruzi lineages, resulted in the publication of 32,746 contigs only partially assembled into 638 scaffolds with no complete chromosomes . Though this resulted in a genome suitable for identifying and studying individual genes, the absence of a chromosome-based assembly hinders analyses where the location of genes and their alleles and/or paralogues is important (e.g. comparative genome analysis and strategies for gene knockouts).
Fortunately, the genomes of T. brucei and L. major fared much better, providing not only nearly complete chromosomes but also synteny maps linking the majority of the core (and conserved) genes of the T. cruzi genome to T. brucei and L. major chromosomes [4–6]. In this study, we sought to organize the majority of the 32,746 T. cruzi contigs into chromosomes-size assemblies using a combination of scaffolds, synteny maps, and end sequences from T. cruzi BAC libraries. To support the soundness of the process, a set of chromosomes was validated experimentally via Southern blot analysis using individual genes as probes to confirm the predicted organization of each chromosome.
From version 5 of the T. cruzi genome  the following data were obtained: 32,746 contigs and 638 scaffolds, as well as the coordinates and annotation data of predicted genes, from the National Center for Biotechnology Information (NCBI) , results of TRIBE-MCL analysis to determine TriTryp clusters of orthologous genes (COGs) from published supplemental materials , and 1,131,562 whole genome shotgun sequence (wgs) reads from the Institute for Genome Research (TIGR). Note that the annotation data includes the predicted haplotype (i.e. "Esmeraldo-like" or "non-Esmeraldo-like") assignment for each gene. Additionally, 39,605 sequenced ends of the CHORI-105 BAC library (produced by Pieter de Jong at the Children's Hospital Oakland Research Institute and sequenced by TIGR), consisting of the TARBAC (avg. length 75 kb) and EPIFOS (avg. length 35 kb) libraries, were obtained from the GSS database at NCBI. All data were loaded into a generic feature format (GFF) database and visualized with the Generic Genome Browser (GBrowse) . Telomeric repeats were identified by searching contigs for at least 9 contiguous runs of the hexameric repeat 5'-TTAGGG-3' .
T. brucei chromosomes were used as the basis for the organization of the initial constructs of T. cruzi chromosomes. TriTryp synteny maps were downloaded from GeneDB  and were parsed using custom PERL scripts to determine an initial assignment of T. cruzi contigs and scaffolds based on the order of T. cruzi genes in the synteny map of each chromosome. Because the synteny maps are based on the order of genes on the T. brucei chromosomes, T. cruzi genes from the same contig (and thus the same region on a chromosome) may map to T. brucei genes on different chromosomes. To address this, each contig/scaffold was initially assigned to the construct that accounted for the greatest number of genes in the corresponding T. brucei chromosomes. Where available, the haplotype assignments for genes as previously annotated  were used to further designate Esmeraldo- and non-Esmeraldo-like homologous chromosomes. For situations where the haplotype was unknown or a sequenced contig was the result of the merging of sequence reads from both haplotypes, an assignment was made to the homologous chromosome accounting for the greatest number of genes on the contig or scaffold.
Mapping of BAC Clones
The sequenced ends of the BAC clones from the CHORI105 library  were mapped to the contigs based upon BLAST  analysis. The mappings were classified as "high-confidence" if they matched no more than 2 contigs with 3 or fewer base mismatches but otherwise were considered "low-confidence". In general, the high-confidence mappings were used for the basis of organization, while the low-confidence mappings provided additional support. PERL scripts were then used to identify BAC clones for which both ends mapped to the same construct, ensuring that the ends were oriented properly to represent the clone (i.e. 1 sense and 1 anti-sense) and that the distance between the mapped ends was appropriate based on the average clone size of the library (TARBAC: avg. length 75 kb, EPIFOS: avg. length 35 kb). Using an iterative process of GBrowse visualization of each construct followed by contig/scaffold rearrangement to place syntenous sequences on the same chromosome and/or to correct the orientation of the sequenced BAC ends of the CHORI-105 BAC library, the constructs were split and/or expanded to generate the chromosomes.
To simplify the chromosome assembly process, only those BAC clones that spanned scaffolds were utilized, under the assumption that the organization of the previously assembled scaffolds  is correct. However, in some instances, scaffolds were found to contain both Esmeraldo- and non-Esmeraldo-like contigs. In these cases the scaffolds were split in order to place the contigs on the appropriate homologous chromosome.
Contigs/scaffolds containing genes absent in the T. brucei genome, and thus not fully represented in the first-pass constructs based on T. cruzi/T. brucei synteny, were also placed as dictated by the mapping of BAC end sequences.
To arrive at a single chromosome model for the CL Brener genome, consensus versions of each homologous chromosome pair were constructed by merging the gene features from the aligned contigs from both the Esmeraldo- and non-Esmeraldo-like haplotypes (one representative of each allele). While choosing a representative gene for loci containing 2 fully sequenced alleles is straightforward, in many cases 1 or both alleles were not fully sequenced and thus the genes exist in pieces on separate contigs (i.e. no contiguous sequence for the complete gene). In these cases, the most likely coordinates for the full-length locus was inferred.
Using SynView  for GBrowse, synteny maps were constructed using the model T. cruzi chromosomes as reference to compare to T. brucei and L. major. These T. cruzi- based TryTryp synteny maps were used to further validate the chromosome assemblies (see below).
Dot Blot Validation
To validate the organization of the chromosomes, Southern "dot" blots were performed on overlapping BAC clones from the CHORI-105 library using genes along the chromosomes as probes. DNAs isolated from the CHORI-105 BAC library were used as targets, and gene-specific DNAs, either from a library of cloned genes in the Gateway pDONR201 plasmid  (Invitrogen) or produced by PCR amplification with gene specific primers, were used as probes in this process. Cloned genes were digested from pDONR plasmids with either BsrGI or PsiI and then labeled with Digoxigenin-11-dUTP using Roche's DIG-High Prime DNA labeling Kit and Detection Starter Kit as per manufacturer's instructions. Denatured target (BAC clone) DNA was applied to positively charged nylon membranes (Roche) and incubated with denatured, labeled probes in pre-heated hybridization buffer (DIG Easy Hyb Roche) at a hybridization temperature calculated using the Wahl formula  for 16 hrs. Membranes were then washed and spots were colormetrically detected according to the DIG High Prime DNA Labeling and Detection Starter kit I (Roche) as per manufacturer's instructions.
The goal of this study was to extend and improve the organization of the existing genome data for T. cruzi in order to construct a model of the core regions of the T. cruzi chromosomes. Accordingly, certain assumptions and design decisions were made to simplify the process. First, the organization of previously published scaffolds was assumed correct unless significant evidence suggested otherwise. This assumption is particularly important for the scaffolds rich in members of large gene families, as these scaffolds were included where possible but could not be validated further. Second, although some contigs are clearly the result of merging of sequences from both haplotypes, no global attempt was made to split misassembled contigs. Third, because many BAC ends mapped to multiple locations on the chromosomes or had mates that mapped to different chromosomes, completely disambiguating the organization of the contigs/scaffolds based on mapping of BAC ends was not possible, and thus placement decisions were made based on majority evidence. Finally, while some T. cruzi chromosomes are known to be triploid  the focus of this work was to differentiate the homologous chromosomes of the two parental haplotypes only, and thus all assemblies are presented as diploid. Where appropriate, we address consequences of these decisions.
TriTryp synteny maps for the T. brucei chromosomes were used as an initial platform on which to tentatively assemble 11 pairs of homologous "T. brucei-like" chromosomes from the T. cruzi genes, contigs, and scaffolds. Contigs that were not present in the T. brucei synteny maps but whose location in the T. cruzi constructs could be inferred were also added. These constructs were then systematically reorganized into T. cruzi chromosome-size pieces based on the preferential mapping of both ends of BAC clones that would link contigs or scaffolds in the proper orientation ("closed" BAC clones). Ultimately, 41 chromosomes were assembled using this approach, a number in agreement with the predicted number of T. cruzi chromosomes based upon pulsed-field gel electrophoresis (PFGE) [17–22].
Chromosome numbers were assigned based on the length of the aligned homologous chromosomes as displayed in GBrowse. Because most of the chromosome ends were not fully assembled (see below), the lengths of the chromosomes are likely underestimations. Compounding this is the fact that distinct, repetitive regions may be collapsed into a single gene or regions during assembly . As such, the numbering scheme will not match those of previous studies [17–22, 24]. In particular, chromosomes 1 and 3 as previously described [16, 25, 26] map to TcChr35 and TcChr6 respectively (see Discussion).
Breakdown of the genes/contigs/scaffolds on the assembled chromosomes
# Chromosomes (shortest: 78 kb, longest 2.3 Mb)
# Scaffolds on chromosomes
# Contigs on chromosomes
# Genes on chromosomes
# Unassignable Scaffolds
# Unassignable Annotated Contigs
# Unassignable Annotated Genes
Additional "fixes" in the reassembled T. cruzi genome
Although in most cases we accepted previously assembled scaffolds as correct, there were occasions where this approach was not compatible with synteny and BAC-end mapping results. In most of these cases, sets of homologous genes from the 2 distinct haplotypes in the CL Brener genome had been placed adjacent to each other in the formation of scaffolds and contigs. To correct for this issue, scaffolds were split (always at contig boundaries) in order to assign genes to the appropriate homologous chromosome (Additional file 2). For example, on TcChr39 scaffold CH473328 was divided into 2 pieces and these pieces assigned to the P and S chromosomes in order to place the respective alleles on the appropriate chromosome. This assignment is consistent with the annotation of these alleles as heterozygous and as part of the same COG group . Overall, 166 scaffolds were split to correct for the presumed merging of homologous sequences from the 2 haplotypes.
TcChr39 illustrates a second type of sequence merging: that of homologous Esmeraldo- and non-Esmeraldo-like raw sequence reads during the creation of the consensus sequence (Additional file 2), the evidence for which is the fact that the "best hit" mappings of BAC ends of the same clone were to contigs assigned to different chromosomes. For example, the 5' BAC end of clone "CHORI105-17N2" maps to the P chromosome on contig 504109 with 99.9% sequence identity (840/841 bases), while the 3' end maps to the S chromosome on contig 506925 with many mismatches (97.2%, 714/734 bases). Additionally, 4 genes on contig 506925 are annotated as "heterozygous" but have their predicted allelic match on the same contig. Errors of this type are noted where observed but the reassembling of these merged contigs throughout the genome was considered to be beyond the scope of this work.
Observations from the Assembled Chromosomes
In this work we have attempted to complete the job of assembly of the T. cruzi CL Brener genome using all available sequence information. The result is a genome of 41 chromosome pairs ranging in size from 78 kb to 2.4 Mb. This assembly is somewhat unique to currently sequenced genomes in that both homologous chromosomes of this widely heterozygous hybrid strain required construction before a consensus model for each chromosome could be derived. Previous pulsed field gradient electrophoresis (PFGE) studies [17–22] have estimated that the chromosomes of T. cruzi range in size from 300 kb to over 3 Mb. While the range of the assembled chromosomes is less than those from the PFGE studies, the difference is likely due to the gene family rich contigs that were not able to be placed in this assembly. However, a previous orthogonal-field-alternation gel electrophoresis (OFAGE) study  has described chromosomes as small as 100 kb.
Previous chromosome-level studies [16, 25, 26] in T. cruzi have focused on two assemblies named chromosomes "1" (corresponding to TcChr35 herein) and "3" (TcChr6) based on the ordering of gel bands in PFGE analyses. According to these studies, chromosome "1" exists as 2 homologous chromosomes of size 450 kb and 1.3 Mb, while chromosome "3" is present as 2 homologs of 600 kb and 1.0 Mb. However, while the PFGE analyses predict different sized homologous chromosomes for both "1" and "3", in our assemblies the homologous chromosomes of each are roughly the same size (> 1 Mb for TcChr35 and ~400 kb for TcChr6). In the case of chromosome "3" (TcChr6), where the size of the assembled homologous chromosomes is smaller than their estimated lengths from the PFGE study, it is likely that the size discrepancy is due to unassigned gene family rich contigs in the sub-telomeric and/or telomeric regions. This justification is consistent with the previous finding that the deletion of the 400 kb sequence responsible for the size differences of the homologous chromosomes of "3" resulted in no phenotypic consequences . However, there is a clear contradiction between the organization of TcChr35 and the model for chromosome "1" proposed in  as both assembled homologous chromosomes in TcChr35 are larger than the reported 450 kb homolog. This study notes that the difference in size between the two homologs of chromosome "1" cannot be due to additional sequence between the Tcsod locus and the downstream telomere. However in the assembled TcChr35, the Tcsod locus is essentially in the middle of the homologous chromosomes. Synteny between the chromosomes of L. major (Lm Chr32) and T. brucei (Tb Chr11) supports the organization of TcChr35 as assembled herein, as does the BAC clone mappings of the Esmeraldo-like homologous chromosome. However, despite the allelic synteny across the entire chromosome, the lack of spanning BAC clones on the non-Esmeraldo-like chromosome from the Tcsod locus to the rest of the chromosome does not rule out the possibility that there exists a small homolog of that chromosome ending near the Tcsod locus (as described for chromosome "1" in ). If this were the case, then the remainder of the non-Esmeraldo-like chromosome is either a separate chromosome homolog or is a portion of another chromosome altogether.
The fixes and observations described above emphasize perhaps the most confounding issue with using the initial T. cruzi genome assembly . Copies of heterozygous alleles are often annotated as independent genes when in fact they are alleles on the homologous chromosomes in this hybrid strain (a problem with the assembly). At the same time, many families of genes include truly distinct genes in discrete loci (an aspect of the organism). These characteristics make it challenging to determine when genes are heterozygous alleles mapping to the same locus or are paralogous genes at different loci. This latter decision is of course further complicated by the fact that the complete sequence for many genes is not present in the assembled contigs. Viewing regions of the assembled chromosomes where both haplotypes are represented facilitates this determination; special consideration must be made for syntenous genes where one or both of the alleles exist at the end of contigs because many of these are truncated and should be merged with another "gene" on the adjacent contig.
The T. cruzi genome contains many non-gene-family, homozygous genes (i.e. with only a single annotated allele) that disrupt the allelic synteny of homologous chromosomes (Figure 2a, Additional file 2). These sequences are likely the result of the merging of sequence from both the Esmeraldo- and non-Esemeraldo-like haplotypes, an indication that the homologous chromosomes are, as expected, quite similar. However, there are many sequenced BAC ends whose exact sequence does not exist in the annotated genome, such as in cases where one BAC end maps to a contig on a particular homologous chromosome with near perfect sequence identity, while the best match of the other end is to a contig on the other homologous chromosome with many mismatches (Additional file 2). Given that the genome was sequenced to 14× coverage at an error rate of < = 1.5%, the absence of these sequences is surprising. Further examination of the raw sequence reads may reveal that the particular sequences exist but were not utilized in the assembly process. Regardless, the current analysis has mapped to these chromosomes the candidate BAC clones that could be fully sequenced in order to correct these errors and close the remaining gaps in the chromosomes.
The assembled chromosomes provide a physical platform on which to study gene function and variation in T. cruzi. For example, the chromosome structure provided here will be particularly useful for planning and confirming gene knockouts and thus determining the function of hypothetical genes or confirming the function of annotated genes. As RNAi does not appear to function in T. cruzi, gene knockout remains a primary method linking phenotypes or functions to particular gene products. In addition, the chromosomes will facilitate strain comparisons, either by techniques such as CGH or subsequent sequencing of additional strains.
Telomeric and sub-telomeric regions of the chromosomes may never be fully sequenced; these regions are likely too redundant to assemble properly and yet too variable as a whole between strains of T. cruzi to be ultimately informative, except as examples of the degree of variability that is possible in T. cruzi. As assembled, over 23% of the annotated genes in the genome are members of large gene families, but it has been suggested that there may be upwards of 20,000 additional genes in these families that are not present in the genome due to the collapsing of reads during assembly . The large number of gene families and the substantial number of members of these families will be interesting to further explore, as the biological function of such large and diverse families of genes is not totally clear. It is hypothesized that the location near chromosome ends facilitates rearrangement in these genes and thus provides a source for new variants . Since members of these families are major targets of anti-T. cruzi immune responses, it is likely that this variation has a role in immune evasion. It would be of interest to determine if gene family clusters that are integrated amongst the core genes in the T. cruzi genome are less prone to rearrangement over time or variation between strains relative to those on chromosome ends, as would be predicted.
As a caveat, one of the risks in assembling the chromosomes as described here is that a mosaic may result given the repetitive and hybrid nature of the T. cruzi genome. Though the majority of BAC clones used for organization were mapped unambiguously to the appropriate chromosomes, it must be noted that the organization was based on the most likely location of each scaffold/contig; there were many clones whose BAC ends either mapped to different chromosomes, a contradiction to the placement of the associated sequences, or mapped to scaffolds/contigs that were not placed on any chromosome. Thus these assemblies represent a model for the chromosomes of T. cruzi, and, though they are still incomplete, they are a vast improvement on what was previously available.
The T.cruzi genome that was published in 2005 provided a foundation for discovery-based research of this human pathogen. However, because of the repetitive nature of the genome and the fact that the reference strain is a widely heterozygous hybrid of two distinct lineages, sequencing and assembly was hindered and resulted in the publication of many small pieces instead of complete chromosomes, as was the case for T. brucei and L. major. In this work, the small pieces (contigs and scaffolds) were assembled into chromosomes based on the predicted parental haplotype, inferences from the TriTryp synteny maps, and end sequences from T. cruzi BAC libraries. The approach was substantiated through the use of Southern blot analysis to confirm the mapping of the BAC clones using as probes the genes they are predicted to contain, and each chromosome was visually validated to ensure sufficient evidence (BAC clones linking scaffolds) was present to support the organization. These chromosomes extend the foundation of the published genome to facilitate gene-level analyses, such as gene knockouts, by assisting the determination of the number of copies of a particular gene and global analyses such as CGH studies for comparisons of chromosomal differences between strains of T. cruzi. The assembled chromosomes, as well as the mapped BAC clones that were used to construct them, are available on TriTrypDB, a trypanosome database of the EuPathDB organization .
We would like to thank Pieter de Jong at the Children's Hospital Oakland Research Institute for the T. cruzi BAC libraries, members of the TIGR-SBRI-Karolinska Trypanosome Sequencing Consortium for access to shotgun sequencing reads and Haiming Wang of the Kissinger Research Group, University of Georgia for valuable assistance with GBrowse/SynView configuration.
This work was supported by US NIH Program Project Grant P01 AI0449790 to RLT.
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