- Research article
- Open Access
Phylogenetic detection of horizontal gene transfer during the step-wise genesis of Mycobacterium tuberculosis
BMC Evolutionary Biologyvolume 9, Article number: 196 (2009)
In the past decade, the availability of complete genome sequence data has greatly facilitated comparative genomic research aimed at addressing genetic variability within species. More recently, analysis across species has become feasible, especially in genera where genome sequencing projects of multiple species have been initiated. To understand the genesis of the pathogen Mycobacterium tuberculosis within a genus where the majority of species are harmless environmental organisms, we have used genome sequence data from 16 mycobacteria to look for evidence of horizontal gene transfer (HGT) associated with the emergence of pathogenesis. First, using multi-locus sequence analysis (MLSA) of 20 housekeeping genes across these species, we derived a phylogeny that serves as the basis for HGT assignments. Next, we performed alignment searches for the 3989 proteins of M. tuberculosis H37Rv against 15 other mycobacterial genomes, generating a matrix of 59835 comparisons, to look for genetic elements that were uniquely found in M. tuberculosis and closely-related pathogenic mycobacteria. To assign when foreign genes were likely acquired, we designed a bioinformatic program called mycoHIT (mycobacterial homologue investigation tool) to analyze these data in conjunction with the MLSA-based phylogeny.
The bioinformatic screen predicted that 137 genes had been acquired by HGT at different phylogenetic strata; these included genes coding for metabolic functions and modification of mycobacterial lipids. For the majority of these genes, corroborating evidence of HGT was obtained, such as presence of phage or plasmid, and an aberrant GC%.
M. tuberculosis emerged through vertical inheritance along with the step-wise addition of genes acquired via HGT events, a process that may more generally describe the evolution of other pathogens.
Bacterial evolution can occur through either the modification of vertically inherited genes or the acquisition of new genes. In the former process, duplication of genes and neo-functionalization represent molecular events for selection to act upon. In the latter process, transformation (acquisition of naked DNA), transduction (acquisition of DNA via a phage infection) and conjugation (acquisition of DNA via a plasmid or a mobile element) represent mechanisms whereby bacteria can incorporate new genes . These modes of acquiring new genetic material are collectively referred to as horizontal gene transfer (HGT) and represent an important means by which an organism can exploit a new condition, such as a pathogenic lifestyle [2, 3].
In general, the determination that HGT has occurred is inferred by study at the species level, comparing virulent strains versus others strains within the species. In mycobacteria, gene acquisition has been implicated in the recent and rapid evolution of M. ulcerans from M. marinum via acquisition of a virulence plasmid . Additionally, M. avium subsp. paratuberculosis has acquired strain-specific genetic elements that are not present in the related environmental organism M. avium subsp. hominissuis [5, 6]. The role of HGT in the recent evolution of M. tuberculosis has also been the subject of study, looking at genes in modern M. tuberculosis lacking from isolates referred to as M. prototuberculosis [7, 8]. Unlike the recent evolution, the role of HGT in shaping evolution of microorganisms over a longer time-span, at the genus-level, has received decidedly less attention.
Recently, the availability of genome sequence data for multiple species of a genus has permitted across-species comparisons for gene content. This is particularly pertinent for the genus Mycobacterium. The majority of mycobacteria are non-pathogenic environmental organisms, but a minority of species, belonging to the slow-growing sub-lineage, cause important human diseases (M. tuberculosis, M. leprae, M. ulcerans, M. kansasii) and veterinary diseases (M. bovis, M. marinum, M. avium subsp. paratuberculosis, M. avium subsp. avium). Because the majority of pathogenic mycobacteria are situated in the same sub-lineage of the genus, we hypothesize that these different species share a common pathogenic potential, upon which selective pressures specifically resulted in the unique attributes of each organism. We further hypothesize that comparisons of all available mycobacterial genome sequences will reveal the degree to which HGT has contributed to 'old' events during the macro-evolution of pathogenic mycobacteria. To test this, we have performed a comprehensive phylogenetic screen, looking for HGT events that have contributed sequentially to the genesis of M. tuberculosis. We have also compared these results with findings derived through a compositional analysis to assess for the concordance between these methods . Together, the data present a portrait of the slow, step-wise emergence of virulence, setting the stage for further molecular studies that define the role of each gene acquisition event in the emergence of M. tuberculosis as an epidemic pathogen.
Genome sequence data
M. kansasii ATCC 12478T, M. avium subsp. avium ATCC 25291T and M. intracellulare ATCC 13950T were purchased from the American Type Culture Collection. After growth in Middlebrook 7H9 media (Difco Laboratories, Detroit, MI) containing 0.05% Tween 80 (Sigma-Aldrich, St louis, MO) and 10% of Albumin-Dextrose-Catalase (Becton Dickinson and co., Sparks, MD), genomic DNA was extracted using a protocol optimized for mycobacterial DNA, as previously described . Genomic DNA was used to prepare a library for shotgun sequencing at the University of Minnesota Biomedical Genomics Center (M. avium subsp. avium) or the McGill University and Genome Quebec Innovation Center (M. kansasii and M. intracellulare). At both centers, sequences were determined by the Genome Sequencer™ FLX (Roche Applied Science). The genome of M. kansasii is estimated to be 6.4 Mb in length, with 33× coverage, and has been deposited in GenBank as projectID number 30907. The genome of M. avium subsp avium is estimated to be 4.8 Mb in length, with 17× coverage, and is deposited in GenBank as projectID number 30909. The genome of M. intracellulare is estimated to be 5.4 Mb in length, with 26× coverage, and is deposited in GenBank as projectID number 27955.
For other genomes, sequences were obtained at NCBI http://www.ncbi.nlm.nih.gov/genomes/lproks.cgi using the parenthesized accession numbers: M. tuberculosis H37Rv (NC_000962) , M. marinum M (NC_010612) , M. ulcerans Agy99 (NC_008611) , M. leprae TN (NC_002677) , M. avium subsp. hominisuis 104 (NC_008595), M. avium subsp. paratuberculosis K10 (NC_002944) , M. smegmatis str. MC2 155 (NC_008596), M. vanbaalenii PYR-1 (NC_008726), M. gilvum PYR-GCK (NC_009338), M. sp. KMS (NC_008705), M. sp. MCS (NC_008146), M. sp. JLS (NC_009077), M. abscessus 19977T (NC_010397), Rhodococcus jostii RHA1 (NC_008268) , Nocardia farcinica IFM 10152 (NC_00631)  and Nocardiodes sp. JS615 (NC_008699).
Multilocus Sequence Analysis-based phylogeny
Multilocus sequence analysis of 20 house-keeping genes from each mycobacterial genome was performed in order to uncover their phylogenetic relationships. Chosen genes were deemed essential for in vitro growth  and conserved across all mycobacterial species and the two out-group species. These genes were also present in the markedly decayed genome of M. leprae, providing further evidence for their conservation. The genes are distributed throughout the M. tuberculosis H37Rv genome: Rv0005-gyrB, Rv0041-leuS, Rv0350-dnaK, Rv0440-groEL2, Rv0667-rpoB, Rv0684-fusA1, Rv0704-rplB, Rv1098-fum, Rv1310-atpD, Rv1437-pgk, Rv1650-pheT, Rv1832-gcvB, Rv2220-glnA1, Rv2555c-alaS, Rv-2748c-ftsK, Rv2839c-infB, Rv2916c-ffh, Rv3240-secA1, Rv3459c-rpsK, and Rv3646-topA. For an out-group, we used orthologues of these genes in two related members of the order Actinomycetales: Rhodococcus jostii and Nocardia farcinica IFM 10152.
For each gene, representative sequences were obtained and aligned in MEGA 4.1 . Resulting FASTA files were entered and concatenated in START2  to form a final 38787 bp in-frame semantide. Next, using the Neighbor-Joining method, we generated an un-rooted phylogenetic tree at the amino acid level in MEGA 4.1, with 500 bootstrap replicates. Phylogenies were also generated for each individual protein to determine the concordance of findings on a gene-by-gene basis (data not shown). Notably, because the situation of M. leprae varied considerably on a gene-by-gene basis, we have removed this organism from the MLSA-based tree.
Detection of M. tuberculosisorthologues in other mycobacterial genomes
To detect M. tuberculosis genes that do or do not have orthologues in the other genomes, we performed an alignment search with the StandAlone TBLASTN program , using the 3989 predicted proteins from M. tuberculosis as the query sequences to search for matches in the genomic DNA of other organisms. With this initial search, we obtained a matrix of 63824 scores (3989 protein sequences blasted against 16 genomes) providing two types of output: categorical (hit versus no hit) and quantitative (degree of similarity). To categorically assign that there was no hit, we employed the default E-value (or Expectation value) of e-10 which is provided at NCBI and has been used in a similar study . Thus, if the statistical significance ascribed to a comparison is greater than this E value, we assigned a percentage of similarity of 0% to that comparison.
To analyze quantitative results, we wished to assign absence of an M. tuberculosis orthologue or presence of an M. tuberculosis orthologue in the database genome. When no hit was reported (0% similarity), we determined that this M. tuberculosis protein has no orthologue in the corresponding database genome. To determine presence of an orthologue, we performed a sensitivity analysis using different thresholds of the similarity score. In the simplest model, any hit was considered evidence for an orthologue (referred hereafter as the 'any hit' threshold). To reduce the likelihood of false-positive assignments due to low-level similarity, we performed an empiric analysis on sequence-comparison data to define incrementally more stringent thresholds. As exemplified in the Additional file 1 (Figure S1), the frequency distribution of BLAST similarity scores obtained between M. tuberculosis and M. kansasii was non-Gaussian, with a right skew. Therefore, we did not use thresholds guided by the mean and the standard deviation. Rather, we determined the degree of similarity at the 95th percentile of hits between the organisms under comparison, then at the 90th percentile and finally at the 85th percentile. As the similarity score that met these percentiles varied across different genome comparisons, we determined these thresholds for each bi-organism comparison before applying these cut-offs to assign orthology.
Phylogeny-based detection of horizontal gene transfer in M. tuberculosis
To enumerate genes potentially acquired via horizontal gene transfer, we looked for proteins found only in certain subsets of the MLSA-based phylogeny. We assumed that a protein encoded by a foreign gene will be present in at least some of the descendants of the bacteria that acquired it, but will be absent from other closely related bacteria. As an example, for a M. tuberculosis protein with orthologues in M. kansasii, M. marinum and M. ulcerans, but no orthologue in other mycobacterial sequences, we inferred that this gene was potentially acquired by the common ancestor of M. tuberculosis, M. kansasii, M. marinum and M. ulcerans. The alternative hypothesis would be that this gene was present in the ancestral Actinobacteria but lost independently three times (from M. avium-intracellulare, from rapid-growing mycobacteria, and from Rhodococcus jostii). This alternative hypothesis was further considered in a secondary analysis described below under 'sub-classification analysis.'
Therefore, using the MLSA-based phylogeny, we assigned gene acquisitions into four lists. List A included genes acquired by the ancestor of all slow-growing mycobacteria, potentially including elements mediating common pathogenicity mechanisms across these different organisms. List B included genes acquired by the common ancestor of M. tuberculosis, M. kansasii, M. marinum and M. ulcerans. List C included genes acquired by the common ancestor of M. tuberculosis and M. kansasii. List D included genes unique to M. tuberculosis, acquired after the common ancestor with M. kansasii (see Figure 1).
To facilitate the process of assembling the matrices of similarities into potential HGT assignments, we developed a software utility called mycoHIT (mycobacterial Homology Investigation Tool). The Software and a help file are available at http://www.molepi.mcgill.ca. Briefly, mycoHIT is a JAVA application presented through a graphical user interface. This program serves to perform BLAST searches, parses the TBLASTN result files with PERL scripts, in order to select the highest score for each protein. Obtained data, including the score, are then stored on a local MYSQL database. Our goal was to define proteins for which an orthologue was present in one group of organisms (e.g. M. kansasii, M. marinum, M. ulcerans) but for which there was no orthologue across all of the remaining organisms (e.g. M. avium-intracellulare, rapid growing mycobacteria and Rhodococcus jostii). Because this analysis was based on these two criteria of orthology (present in some, absent from the rest), we were forced to exclude M. leprae from this analysis because the M. leprae genome is characterized by pseudogenes and deletions  which resulted in an unacceptable degree of error in categorization of orthologues and an uncertain phylogenetic situation.
To focus our analysis on HGT during the evolution of slow growing mycobacteria, we selected these 7 genomes (M. kansasii, M. marinum, M. ulcerans, M. avium subsp. hominissuis, M. avium subsp. paratuberculosis, M. avium subps. avium and M. intracellulare) and the option "compute all combinations for selection" (see Additional file 1 Figure S2-A). Given the selection of these 7 comparisons, the program generated 128 files (27), representing the lists of M. tuberculosis genes that have orthologues in 0 to 7 slow growing mycobacterial genomes and lack orthologues in the remaining 9 to 15 genomes (i.e. proteins with orthologues in n genomes but lacking orthologues in (15- n) genomes). These lists provide the number of genes differentially shared across slow-growing mycobacteria in each of the 128 possible combinations. Subsequently, the program calculates "special lists", representing the chosen sum of the previously calculated 128 lists according to the phylogeny (See additional file 1 – Figure S2-B). These lists are referred to as lists A, B, C, D. As an example, list B will be the sum of lists MM (orthologue in M. marinum absent in all other), MU (orthologue M. ulcerans absent in all other), MM-MK (orthologues in M. marinum, M. kansasii absent in all other), MM-MU, MU-MK and MM-MU-MK.
The mycoHIT program was intensively tested to report potential problems. Furthermore, to ensure that the program sorted genes correctly, we manually performed the analysis twice using two different thresholds ("any hits" and 60% identity). Briefly, using EXCEL we sorted genes and excluded those with hits in at least one of the rapid growing species or R. jostii. In a second phase, we manually grouped genes into lists A, B, C and D following the same strategy explained above. As mycoHIT generated the same genes and numbers at these two thresholds as we obtained in the manual search, this comparison served to validate the mycoHIT program.
Sub-classification and characterization of potential HGT
For further analysis, we began with the most inclusive list obtained through the least restrictive threshold ("any hit") to avoid the risk of overlooking potentially instructive HGT events. For all these genes, we performed a blastP of the predicted encoded protein using the NCBI database composed of 853 bacterial genomes (including 53 Actinobacteria) in order to classify the proteins into four categories 1) proteins with no blast hits at all in the entire NCBI database, potentially indicating false-annotation, 2) proteins that may not represent HGT (putative deletions), based on presence of homologues in at least two different genera of Actinobacteria out of the first five taxonomic classes hits, 3) proteins that provide evidence for HGT but represent vehicles of HGT, such as transposons, phage, toxin-antitoxin genes, and 4) proteins with hits in the NCBI database but one or less hit in the Actinobacteria class out of the first five taxonomic classes presenting hits, most likely to represent the bacterial elements acquired through HGT. Notably, with this approach, we expect that proteins that are part of a conserved family (e.g. ABC transporters) will never be assigned as HGT, due to low- similarity hits. We partially correct for this possibility when such a gene was situated in a HGT cluster. To this end, when a cluster of HGT genes was detected, internal genes not detected through the phylogenetic analysis were added if they met the aforementioned criteria (presence of hits in the NCBI database but one or less hit in Actinobacteria).
For further analysis, we have calculated the proportion of predicted bacterial HGT associated with an annotated vehicle of HGT (transposons, phage, toxin-antitoxin genes) within 5 genes upstream or downstream of the predicted HGT gene or cluster. We enumerated the number of genes that were located in clusters of two or more putative HGT genes. Schematic representation of GC content across a given region was obtained using Bio-Annotator from VectorNTI (Invitrogen) using 2000 bp windows. Finally, we compared this list of genes with already-published genomic databases on M. tuberculosis, specifically: a) essential genes for survival in vitro , in macrophages  and in mice , and b) dispensable genes as shown by their deletion in clinical isolates  c) genes presenting atypical compositional characteristics suggestive of HGT .
MultiLocus Sequence Analysis-based phylogeny
Analysis of 20 genes across 16 mycobacterial species, Rhodococcus jostii and Nocardia farcinica revealed a total of 12741 polymorphic sites at the protein level. The number of amino acid differences between mycobacteria strains ranged from 0 (between Mycobacterium sp. KMS and MCS) to 4166 (between M. bovis and R. jostii). The numbers of amino acid differences are tabulated in the Additional file 1 (Figure S3). An un-rooted phylogenetic representation of these differences, using MEGA 4.1, is shown in Figure 1. This phylogeny presents three groups of organisms: the out-group bacteria, the rapid-growing mycobacteria and the slow-growing mycobacteria. Within the latter group, there were four major lineages: 1) M. avium-intracellulare, 2) M. marinum-ulcerans, 3) M. kansasii, and 4) M. tuberculosis and related members of the M. tuberculosis complex. Individual trees based on the selected genes also presented a similar phylogenetic portrait of the genus (data not shown). Notably, the unexpected finding that M. kansasii was the nearest neighbor to the M. tuberculosis complex was supported in the majority of single gene phylogenies.
Genes acquired by HGT in M. tuberculosisdetected by a phylogenetic method
We aimed to describe not only the HGTs specific to M. tuberculosis but also those acquired at each node of separation in the phylogeny (excluding M. leprae). Using the different thresholds for orthology, we obtained lists of M. tuberculosis proteins where an orthologues is present in 0 to 7 slow growing mycobacteria but absent in all other genomes. As seen in Figure 1, in conjunction with the MLSA-based analysis, we have grouped these proteins into the following lists: List A, genes likely acquired by the ancestor of all slow growing mycobacteria, List B, genes likely acquired by the ancestor of the M. marinum-ulcerans/M. kansasii/M. tuberculosis lineage, List C, genes likely acquired by the ancestor of the M. kansasii/M. tuberculosis lineage, and List D, genes likely acquired in the M. tuberculosis lineage. To test for the robustness of these List assignments, we looked for evidence of genes that are discordant with the proposed phylogeny. To test the MLSA-based prediction that M. kansasii is the nearest neighbor among sequenced strains to M. tuberculosis, we determined how many genes are shared between M. tuberculosis and M. kansasii, but no other species, as compared to the number of genes that are shared only between M. tuberculosis and M. marinum. Under a null model, the two comparisons (M. tuberculosis/M. kansasii versus M. tuberculosis/M. marinum) would have an equal number of orthologues. This hypothesis was rejected by observing that there were a total of 72 such orthologous pairs; 42 in the case of M. tuberculosis/M. kansasii compared to 30 in the case of M. tuberculosis/M. marinum. (p < 0.05 by exact Binomial method). Therefore, as orthology searches provided independent support for the MLSA-based phylogeny, we proceeded to enumerate acquired genes at different steps in the evolution of M. tuberculosis (Table 1). When using different thresholds of orthology ("any hit", 95th percentile, 90th percentile or 85th percentile), we obtained slight shifts in these numbers, but overall a stable estimate of around 11–14% of the genome was assigned as potential HGT (versus predicted HGT, based on more stringent criteria, as described below). Notably, about one half of these were specific to M. tuberculosis (List D).
In addition to helping uncover HGT, the mycoHIT program can also be used to detect potential deletion events during the step-wise genesis of M. tuberculosis. This approach, along with associated lists, are provided in Additional file 2. Of note, list Dd comprises 689 genes potentially deleted from the ancestor of M. tuberculosis after its separation with M. kansasii. This list is approximately three times longer than the corresponding list D of potencial HGT. These results emphasize that in parallel with the acquisition of foreign genes, there has been reduction of the M. tuberculosis genome associated with pathogenic specialization, as has been described for other mycobacterial species [6, 12]. Unlike M. marinum and M. kansasii, M. tuberculosis has not been isolated from the environment, therefore the process of reductive genomics was potentially due to lack of selective pressure to retain genes important for environmental survival.
Sub-classification of potential HGT
To determine genes most likely to represent true-positive HGT, we analyzed the total list of genes obtained with the "any hits" threshold to find those that were unlikely to represent HGT, either because they do not code for a protein or because the encoded protein has multiple orthologues in other genera of Actinobacteria. Figure 2 presents the number (2A) and the proportion (2B) of genes from the four lists in each category. The first category represents genes with no blast results across the NCBI database, suggesting that these genetic sequences do not code for a protein product or instead that the encoded proteins are so small that they cannot meet the minimal E-value for an orthologue. This situation applied to 36% of List A proteins, 21% of list B proteins, 9% of list C proteins and 30% of list D proteins. These results were not unexpected. For instance, the high proportion in list A is explained by duplication of small proteins (e.g. 56% of proteins in this category are Esx and PE proteins).
The next category of potentially false-positive HGT genes was those for which we observed orthologues in at least two different genera from the Actinobacteria phylum. Although some of these genes could represent HGT events within the family or multiple transfers, to be conservative, we assumed that these may represent ancestral genes that have been deleted in certain lineages, resulting in a restricted distribution (e.g. deleted from the rapid-growing mycobacteria lineage and Rhodococcus jostii. but present in all slow-growing mycobacteria). The proportion of proteins classified in this category represents 31%, 31%, 16% and 16% respectively from lists A, B, C and D.
Of genes most likely to represent HGT, we divided these between the process of HGT and the accompanying bacterial genes. Genes with homology to transposons, phage and toxin-antitoxin comprised 13% of proteins from the list A, 5% of proteins from list B, 43% of proteins from list C and 34% of proteins from list D. The lower proportion of HGT vehicles in the earlier lists is consistent with the expectation that these genes may be eliminated with greater time due to a lack of selective pressure for their maintenance.
Finally, the remaining proteins (listed in table 2) on each of Lists A-D were considered to represent bacterial genes acquired through the process of HGT. Based on these Lists, we obtained 28 M. tuberculosis genes that were acquired by the ancestor of all slow growing bacteria, 40 that were acquired by the ancestor of the M. marinum-ulcerans/M. kansasii/M. tuberculosis lineage, 14 that were acquired by the ancestor of the M. kansasii/M. tuberculosis lineage and finally 55 that were specifically acquired by the ancestor of the M. tuberculosis complex. Together, this makes a most conservative estimate that in addition to vehicles of HGT, 137 genes (28 + 40 + 14 + 55) in M. tuberculosis were acquired through the process of HGT. As a note, we also performed a PSIBLAST (Position-Specific iterated BLAST) on the 55 proteins from ListD (data not shown). Only two proteins (both regulators) have been found to have a hit in other Actinobacteria. This argues that the HGT estimate is not biased by high sequence variability within a vertical transmitted protein.
Predicted HGT reveal characteristics other than abnormal phylogeny
When looking at the location of HGT in the genome, we observed that bacterial genes acquired horizontally appeared in clusters, often located in the same region as an HGT vehicle (see Additional file 1 – Figure S4). 99 of the 137 genes (72%) were associated with another horizontally acquired gene, with the highest proportion seen in list D (87%). Of the 137 genes, 82 (60%) were associated with a HGT vehicle, with the highest proportion again observed with list D (76%). To illustrate compositional signs of HGT (presence of vehicle, clustering or skewed GC%), we graphically present four examples: the lipo-ligosaccharide (LOS) locus (Figure 3A), the phenolic glycolipid (PGL) locus (Figure 3B), the hyc locus (Figure 4) and the Rv3775 locus (Figure 5). These examples were chosen for their pertinence in the M. tuberculosis virulence as discussed below.
For the LOS locus, one observes potential vehicles of HGT (Toxin-Antitoxin genes), two clusters (one a set derived from List B, the other comprising mostly List A genes) and altered GC%. For the List A cluster, the GC% was near 65%, which is the average GC% for the M. tuberculosis genome. In contrast, the cluster of List B genes was associated with a drop in GC%, as low as 53% over a calculated interval of 3000 bp.
In the PGL locus, we detect transposases and again these genes are in clusters. List A genes were found at each extremity, whereas genes acquired more recently (Lists B-D) were detected in a more central situation. The GC content of these gene clusters was associated with their assigned list; genes from Lists D and B had no GC% skew whereas genes from lists C and A had decreased GC%.
The third example is the hyc operon (Figure 4). Among Actinobacteria, the hyc locus is found in three distinct forms. In Nocardiodes sp. the hyc locus features neither a transposase nor a split gene (Figure 4A). Rather, one observes at each side of the hyc locus a duplication of a part of a conserved gene (Noca_4472), suggestive of insertion by homologous recombination. In M. avium, this locus is part of a large island, called LSPA6 , that is found in M. avium subsp. hominissuis but not other M. avium subspecies. This island includes one transposase and has split radC into two pieces, suggestive of insertion by transposition (Figure 4B). Finally, there is a hyc locus in M. tuberculosis, M. kansasii, M. marinum and M. ulcerans without features of transposition or homologous recombination. Nonetheless, this locus has increased GC content, in contrast to other HGTs in the same region (acpA/bioF, Rv0059/60, Rv0113/14/15) where GC% is decreased (Figure 4C). These three variants of the hyc locus suggest transfer across bacteria and provide mechanistic clues regarding the mode of acquisition in two of three lineages.
The last example presented in Figure 5 is the Rv3375 locus, which comprises the insertion of two clusters of foreign genes (two genes acquired in list C and three genes acquired in List D). Again, there are transposase elements close to the locus and a GC% drop depending on which list the genes derived from: genes from List C are not associated with an evident GC % change, unlike the decrease seen for List D genes.
A recent study looked for HGT in M. tuberculosis using altered GC% at the first and third codon positions or the codon usage as the search criteria . We note that 48% of genes reported in this paper were also detected in our study. More interestingly, 30% of the HGT detected in our study were also detected in this other report, with a higher proportion among our List D genes (57%). This enrichment of compositional features of HGT in list D (the most evolutionarily recent) is expected, as characteristics of HGT tend to homogenize with the rest of the genome and dispensable HGT (e.g. phage proteins) tend to be deleted with time . Moreover, these findings highlight the complementarity of the two approaches. While compositional methods may predict certain HGT that would be overlooked by phylogenetic methods (e.g. between bacteria of the same phylum), the use of our phylogenetic approach has permitted detection of instances of HGT where the genomic signature is unremarkable based on compositional considerations, such as more ancestral events.
Role of HGT in the emergence of M. tuberculosis
Upon analysis of the predicted function of the 137 proteins, an evident increase in transferases (e.g. glycosyl-, formyl-, aminotransferases) was observed (30/137 = 22%). These enzymes transfer a functional group (e.g. methyl) from one molecule to another. Two regions that have been particularly affected by acquisition of foreign transferases are the LOS locus and the PGL locus. Apart from transferases, the HGT lists also contain genes coding for proteins involved in metabolic functions related to adaptation to anaerobic conditions. We detected two such regions: the hyc (formate hydrogenlyase operon) and the frd (fumarate reductase operon).
Given the evidence in favor of HGT, we evaluated the genes so acquired for clues regarding their potential role in the genesis of M. tuberculosis as a specialized pathogen. By comparing the 137 genes against available databases, we assessed for whether these genes were predicted to be essential for growth in vitro , in macrophages , or during experimental in vivo infection using the mouse model . As indicated in Additional file 1 (Figure S5) and highlighted in Table 2, we found that 18 genes out of 137 were deemed essential by one of these screens, some clustered in the same region (e.g. two genes essential in vivo were in the Rv3375 locus whereas the hyc operon contained two genes essential in vitro and 1 gene essential during macrophage infection). We also compared against genes deleted in clinical isolates of M. tuberculosis  as evidence of their dispensability for full virulence in the human host. In this case, 21 of the 137 genes (primarily from list B) have been deleted in at least one clinical isolate. These results suggest that while some genes acquired by horizontal transfer are necessary for full virulence, others may represent genes that were important at earlier stages in the evolution of slow-growing mycobacterial pathogens but are apparently dispensable for the specific pathogenicity of M. tuberculosis.
Previous genomic study on M. tuberculosis, using sequence level comparisons and post-genomic analysis, has shown that this organism has evolved through clonal evolution, mutation and gene deletion [27, 28]. Importantly, these studies have largely concentrated on within-species variability, and therefore have emphasized devolution once an organism is already M. tuberculosis. Our study indicates that ancestors of M. tuberculosis complex organisms acquired at least 137 coding genes in a step-wise manner through the process of HGT. More specifically, the emergence of the epidemic pathogen, M. tuberculosis, from an environmental ancestor has coincided with 'infection' by phages and/or transposable elements after the common ancestor with M. kansasii. This scenario is supported by the clinical and epidemiologic observations that M. kansasii can cause TB-like pulmonary disease, without evidence of person-to-person spread.
Interestingly, we have detected certain 'hotspots' for HGT, where one observes genes from different lists, indicative of distinct HGT events. This may be due to the presence at these sites of DNA sequences with characteristics (e.g. transposase insertion sequence, homologous DNA to phage DNA) that permit independent HGT. Alternatively, features acquired in a first HGT event may permit secondary insertions due to sequence similarity with DNA of a potential donor. In two 'hotspots', acquisition of a high number of transferases is clearly linked to lineage-specific biochemical properties. Phenolic glycolipids (PGLs) are synthesized on the backbone phthiocerol dimycocerosate (DIM), which is common to all mycobacteria, by adding a phenol ring and methylated sugars (reviewed in ). PGLs are produced exclusively by some slow growing mycobacteria (e.g. M. tuberculosis, M. kansasii, M. marinum, M. ulcerans and M. leprae) . Our phylogenetic screen detected Rv2949c, the chorismate-formate lyase responsible for production of the phenol ring , as an HGT in the ancestor of these bacteria (List B). We also detected other HGT in this locus, notably Rv2962c and Rv2958c (List B) and Rv2959c (List C), responsible for the addition of different sugars to produce the M. tuberculosis form of PGL . Lastly, the methyltransferase Rv2959c, implicated in the O-methylation of one of the hydroxyl groups of the rhamnosyl residue of M. tuberculosis PGL , was also detected as a foreign gene (List C). As PGL is an important M. tuberculosis virulence factor  and the enzymes responsible for building from DIM to PGL to the M. tuberculosis form of PGL have been horizontally acquired, this locus provides a clear illustration of how HGT has contributed to the step-wise evolution of virulent M. tuberculosis. A similar scenario applies for the LOS locus. This locus has been implicated in the production of different variants of these highly polar glycolipids in M. marinum , although the complete locus is not conserved in all M. tuberculosis strains . Again, our phylogenetic analysis uncovered evidence of HGT in the form of transferases that affect the species-specificities of LOS products. Moreover, our study has also shown a number of horizontally acquired transferases outside of these two loci. As the acquisition of such enzymes can serve to modify ancestral compounds, a future challenge will be to identify the substrates of these enzymes and determine the role of the newly synthesized compounds in M. tuberculosis pathogenicity.
Beyond biochemical modification of mycobacterial virulence factors, genes acquired through HGT have potentially mediated the transition from an environmental niche to intracellular life within a complex eukaryotic host. As a first example, two loci are predicted to mediate adaptation to anaerobic conditions. The hyc operon shows homology with the formate hydrogenlyase operon from enterobacteria, implicated in formate fermentation by transforming formate into CO2 and H2 . Three genes of this operon were predicted to be essential by TraSH analysis (2 in vitro and one for macrophage infection), therefore this operon has an important role in M. tuberculosis metabolism that merits further study. The second locus shows homology with the fumarate reductase operon, which is required for anaerobic respiration using fumarate as the terminal electron acceptor . As both of these operons are up-regulated during oxygen starvation [38, 39], it can be hypothesized that the acquisition of these genes provided a selective advantage for residence in anaerobic conditions, as encountered within a granuloma . A second example is the acquisition of genes Rv3373 to Rv3378c. Two of these genes have been shown to be essential during infection of mice (Rv3375 and Rv3376) . Curiously, Rv3377c and Rv3378c encode proteins implicated in the production of a compound (tuberculosinol) with a undefined function, never documented in bacteria . These two genes were also identified in a genetic screen for inhibitors of macrophage phagosome acidification , suggesting a role for this compound in modulating the intracellular environment.
We have demonstrated step-wise HGT during the genesis of M. tuberculosis with evidence that this process has mediated the acquisition of established virulence factors. Our original approach has permitted the detection of 'old' HGT which may have increased the pathogenic potential of slow-growing mycobacteria along with M. tuberculosis-specific HGT which may have permitted its adaptation to the human lung. The examples provided in this manuscript, along with other published examples of HGT [8, 43, 44] present opportunities to formally test this, for example, by analyzing for gain-of-function when genetically engineering insertion events into non-tuberculous mycobacteria. Additionally, as our model and software permit the genomic analysis of different bacteria, it will also be possible to determine the degree to which HGT has contributed to the step-wise evolution of other mycobacterial and non-mycobacterial pathogens.
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The authors thank members of the Behr, Reed and Schurr laboratories for their input. The authors thank Dr. Michael Reed for input on mycobacterial lipid synthesis. F.V. was supported through a MUHC studentship. M.B. is Chercheur Boursier Senior of the FRSQ and a William Dawson Scholar of McGill University. Work was funded by an operating grant from the Canadian Institutes for Health Research, MOP- 79309.
FV participated in the conception of the study, in the program's designing and testing, carried out analyzes of the data and drafted the manuscript. DP participated to the program's designing and testing. CT carried out the MLSA analyzes. MB conceived of the study, and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.