The protist Trichomonas vaginalis harbors multiple lineages of transcriptionally active Mutator-like elements

Comparative genomics of Ascetosporea gives new insight into the evolutionary basis for animal parasitism in Rhizaria

BACKGROUND

Ascetosporea (Endomyxa, Rhizaria) is a group of unicellular parasites infecting aquatic invertebrates. They are increasingly being recognized as widespread and important in marine environments, causing large annual losses in invertebrate aquaculture. Despite their importance, little molecular data of Ascetosporea exist, with only two genome assemblies published to date. Accordingly, the evolutionary origin of these parasites is unclear, including their phylogenetic position and the genomic adaptations that accompanied the transition from a free-living lifestyle to parasitism. Here, we sequenced and assembled three new ascetosporean genomes, as well as the genome of a closely related amphizoic species, to investigate the phylogeny, origin, and genomic adaptations to parasitism in Ascetosporea.

RESULTS

Using a phylogenomic approach, we confirm the monophyly of Ascetosporea and show that Paramyxida group with Mikrocytida, with Haplosporida being sister to both groups. We report that the genomes of these parasites are relatively small (12-36 Mb) and gene-sparse (~ 2300-5200 genes), while containing surprisingly high amounts of non-coding sequence (~ 70-90% of the genomes). Performing gene-tree aware ancestral reconstruction of gene families, we demonstrate extensive gene losses at the origin of parasitism in Ascetosporea, primarily of metabolic functions, and little gene gain except on terminal branches. Finally, we highlight some functional gene classes that have undergone expansions during evolution of the group.

CONCLUSIONS

We present important new genomic information from a lineage of enigmatic but important parasites of invertebrates and illuminate some of the genomic innovations accompanying the evolutionary transition to parasitism in this lineage. Our results and data provide a genetic basis for the development of control measures against these parasites.

Cloning of Maize TED Transposon into Escherichia coli Reveals the Polychromatic Sequence Landscape of Refractorily Propagated Plasmids

MuDR, the founder member of the Mutator superfamily and its MURA transcripts, has been identified as toxic sequences to Escherichia coli (E. coli), which heavily hindered the elucidation of the biochemical features of MURA transposase and confined the broader application of the Mutator system in other organisms. To harness less constrained systems as alternatives, we attempted to clone TED and Jittery, two recently isolated autonomous Mutator-like elements (MULEs) from maize, respectively. Their full-length transcripts and genomic copies are successfully cloned when the incubation time for bacteria to recover from heat shock is extended appropriately prior to plating. However, during their proliferation in E. coli, TED transformed plasmids are unstable, as evidenced by derivatives from which frameshift, deletion mutations, or IS transposon insertions are readily detected. Our results suggest that neither leaky expression of the transposase nor the presence of terminal inverse repeats (TIRs) are responsible for the cloning barriers, which were once ascribed to the presence of the Shine–Dalgarno-like sequence. Instead, the internal sequence of TED (from 1250 to 2845 bp), especially the exons in this region, was the most likely causer. The findings provide novel insights into the property and function of the Mutator superfamily and shed light on the dissection of toxic effects on cloning from MULEs.

Small RNAs Are Implicated in Regulation of Gene and Transposable Element Expression in the Protist Trichomonas vaginalis

Trichomoniasis, caused by the protozoan Trichomonas vaginalis, is the most common nonviral sexually transmitted infection in humans. The millions of cases each year have sequelae that may include complications during pregnancy and increased risk of HIV infection.

Trichomonas vaginalis is the causative agent of trichomoniasis, the most prevalent nonviral sexually transmitted infection worldwide. Repetitive elements, including transposable elements (TEs) and virally derived repeats, comprise more than half of the ∼160-Mb T. vaginalis genome. An intriguing question is how the parasite controls its potentially lethal complement of mobile elements, which can disrupt transcription of protein-coding genes and genome functions. In this study, we generated high-throughput RNA sequencing (RNA-Seq) and small RNA-Seq data sets in triplicate for the T. vaginalis G3 reference strain and characterized the mRNA and small RNA populations and their mapping patterns along all six chromosomes. Mapping the RNA-Seq transcripts to the genome revealed that the majority of genes predicted within repetitive elements are not expressed. Interestingly, we identified a novel species of small RNA that maps bidirectionally along the chromosomes and is correlated with reduced protein-coding gene expression and reduced RNA-Seq coverage in repetitive elements. This novel small RNA family may play a regulatory role in gene and repetitive element expression. Our results identify a possible small RNA pathway mechanism by which the parasite regulates expression of genes and TEs and raise intriguing questions as to the role repeats may play in shaping T. vaginalis genome evolution and the diversity of small RNA pathways in general.

IMPORTANCE Trichomoniasis, caused by the protozoan Trichomonas vaginalis, is the most common nonviral sexually transmitted infection in humans. The millions of cases each year have sequelae that may include complications during pregnancy and increased risk of HIV infection. Given its evident success in this niche, it is paradoxical that T. vaginalis harbors in its genome thousands of transposable elements that have the potential to be extremely detrimental to normal genomic function. In many organisms, transposon expression is regulated by the activity of endogenously expressed short (∼21 to 35 nucleotides [nt]) small RNA molecules that effect gene silencing by targeting mRNAs for degradation or by recruiting epigenetic silencing machinery to locations in the genome. Our research has identified small RNA molecules correlated with reduced expression of T. vaginalis genes and transposons. This suggests that a small RNA pathway is a major contributor to gene expression patterns in the parasite and opens up new avenues for investigation into small RNA biogenesis, function, and diversity.

Diverse transposable element landscapes in pathogenic and nonpathogenic yeast models: the value of a comparative perspective

Genomics and other large-scale analyses have drawn increasing attention to the potential impacts of transposable elements (TEs) on their host genomes. However, it remains challenging to transition from identifying potential roles to clearly demonstrating the level of impact TEs have on genome evolution and possible functions that they contribute to their host organisms. I summarize TE content and distribution in four well-characterized yeast model systems in this review: the pathogens Candida albicans and Cryptococcus neoformans, and the nonpathogenic species Saccharomyces cerevisiae and Schizosaccharomyces pombe. I compare and contrast their TE landscapes to their lifecycles, genomic features, as well as the presence and nature of RNA interference pathways in each species to highlight the valuable diversity represented by these models for functional studies of TEs. I then review the regulation and impacts of the Ty1 and Ty3 retrotransposons from Saccharomyces cerevisiae and Tf1 and Tf2 retrotransposons from Schizosaccharomyces pombe to emphasize parallels and distinctions between these well-studied elements. I propose that further characterization of TEs in the pathogenic yeasts would enable this set of four yeast species to become an excellent set of models for comparative functional studies to address outstanding questions about TE-host relationships.

Evolution of Mutator transposable elements across eukaryotic diversity

Background

Mutator-like elements (MULEs) are a significant superfamily of DNA transposons on account of their: (i) great transpositional activity and propensity for insertion in or near gene sequences, (ii) their consequent high mutagenic capacity, and, (iii) their tendency to acquire host gene fragments. Consequently, MULEs are important genetic tools and represent a key study system for research into host-transposon interactions. Yet, while several studies have focused on the impacts of MULEs on crop and fungus genomes, their evolution remains poorly explored.

Results

We perform comprehensive bioinformatic and phylogenetic analyses to address currently available MULE diversity and reconstruct evolution for the group. For this, we mine MULEs from online databases, and combine search results with available transposase sequences retrieved from previously published studies. Our analyses uncover two entirely new MULE clades that contain elements almost entirely restricted to arthropod hosts, considerably expanding the set of MULEs known from this group, suggesting that many additional MULEs may await discovery from further arthropod genomes. In several cases, close relationships occur between MULEs recovered from distantly related host organisms, suggesting that horizontal transfer events may have played an important role in the evolution of the group. However, it is apparent that MULEs from plants remain separate from MULEs identified from other host groups. MULE structure varies considerably across phylogeny, and TIR length is shown to vary greatly both within and between MULE groups. Our phylogeny suggests that MULE diversity is clustered in well-supported groups, typically according to host taxonomy. With reference to this, we make suggestions on how MULE diversity can be partitioned to provide a robust taxonomic framework.

Conclusions

Our study represents a considerable advance in the understanding of MULE diversity, host range and evolution, and provides a taxonomic framework for the classification of further MULE elements that await discovery. Our findings also raise a number of questions relating to MULE biology, suggesting that this group will provide a rich avenue for future study.

Electronic supplementary material

The online version of this article (10.1186/s13100-019-0153-8) contains supplementary material, which is available to authorized users.

Transposition of Mutator-like transposable elements (MULEs) resembles hAT and Transib elements and V(D)J recombination

Mutator-like transposable elements (MULEs) are widespread across fungal, plant and animal species. Despite their abundance and importance as genetic tools in plants, the transposition mechanism of the MULE superfamily was previously unknown. Discovery of the Muta1 element from Aedes aegypti and its successful transposition in yeast facilitated the characterization of key steps in Muta1 transposition. Here we show that purified transposase binds specifically to the Muta1 ends and catalyzes excision through double strand breaks (DSB) and the joining of newly excised transposon ends with target DNA. In the process, the DSB forms hairpin intermediates on the flanking DNA side. Analysis of transposase proteins containing site-directed mutations revealed the importance of the conserved DDE motif and a W residue. The transposition pathway resembles that of the V(D)J recombination reaction and the mechanism of hAT and Transib transposases including the importance of the conserved W residue in both MULEs and hATs. In addition, yeast transposition and in vitro assays demonstrated that the terminal motif and subterminal repeats of the Muta1 terminal inverted repeat also influence Muta1 transposition. Collectively, our data provides new insights to understand the evolutionary relationships between MULE, hAT and Transib elements and the V(D)J recombinase.

Functional characterization of the active Mutator-like transposable element, Muta1 from the mosquito Aedes aegypti

BACKGROUND

Mutator-like transposable elements (MULEs) are widespread with members in fungi, plants, and animals. Most of the research on the MULE superfamily has focused on plant MULEs where they were discovered and where some are extremely active and have significant impact on genome structure. The maize MuDR element has been widely used as a tool for both forward and reverse genetic studies because of its high transposition rate and preference for targeting genic regions. However, despite being widespread, only a few active MULEs have been identified, and only one, the rice Os3378, has demonstrated activity in a non-host organism.

RESULTS

Here we report the identification of potentially active MULEs in the mosquito Aedes aegypti. We demonstrate that one of these, Muta1, is capable of excision and reinsertion in a yeast transposition assay. Element reinsertion generated either 8 bp or 9 bp target site duplications (TSDs) with no apparent sequence preference. Mutagenesis analysis of donor site TSDs in the yeast assay indicates that their presence is important for precise excision and enhanced transposition. Site directed mutagenesis of the putative DDE catalytic motif and other conserved residues in the transposase protein abolished transposition activity.

CONCLUSIONS

Collectively, our data indicates that the Muta1 transposase of Ae. aegypti can efficiently catalyze both excision and reinsertion reactions in yeast. Mutagenesis analysis reveals that several conserved amino acids, including the DDE triad, play important roles in transposase function. In addition, donor site TSD also impacts the transposition of Muta1.

Bulky Trichomonad Genomes: Encoding a Swiss Army Knife

The trichomonads are a remarkably successful lineage of ancient, predominantly parasitic protozoa. Recent molecular analyses have revealed extensive duplication of certain genetic loci in trichomonads. Consequently, their genomes are exceptionally large compared to other parasitic protozoa. Retention of these large gene expansions across different trichomonad families raises the question: do these duplications afford an advantage? Many duplicated genes are linked to the parasitic lifestyle and some are regulated differently to their paralogues, suggesting they have acquired new functions. It is proposed that these large genomes encode a Swiss army knife of sorts, packed with a multitude of tools for use in many different circumstances. This may have bestowed trichomonads with the extraordinary versatility that has undoubtedly contributed to their success.

MuTAnT: a family of Mutator-like transposable elements targeting TA microsatellites in Medicago truncatula

Transposable elements (TEs) are mobile DNA segments, abundant and dynamic in plant genomes. Because their mobility can be potentially deleterious to the host, a variety of mechanisms evolved limiting that negative impact, one of them being preference for a specific target insertion site. Here, we describe a family of Mutator-like DNA transposons in Medicago truncatula targeting TA microsatellites. We identified 218 copies of MuTAnTs and an element carrying a complete ORF encoding a mudrA-like transposase. Most insertion sites are flanked by a variable number of TA tandem repeats, indicating that MuTAnTs are specifically targeting TA microsatellites. Other TE families flanked by TA repeats (e.g. TAFT elements in maize) were described previously, however we identified the first putative autonomous element sharing that characteristics with a related group of short non-autonomous transposons.

Functional impacts of the intestinal microbiome in the pathogenesis of inflammatory bowel disease

: The human intestinal microbiome plays a critical role in human health and disease, including the pathogenesis of inflammatory bowel disease (IBD). Numerous studies have identified altered bacterial diversity and abundance at varying taxonomic levels through biopsies and fecal samples of patients with IBD and diseased model animals. However, inconsistent observations regarding the microbial compositions of such patients have hindered the efforts in assessing the etiological role of specific bacterial species in the pathophysiology of IBD. These observations highlight the importance of minimizing the confounding factors associated with IBD and the need for a standardized methodology to analyze well-defined microbial sampling sources in early IBD diagnosis. Furthermore, establishing the linkage between microbiota compositions with their function within the host system can provide new insights on the pathogenesis of IBD. Such research has been greatly facilitated by technological advances that include functional metagenomics coupled with proteomic and metabolomic profiling. This review provides updates on the composition of the microbiome in IBD and emphasizes microbiota dysbiosis-involved mechanisms. We highlight functional roles of specific bacterial groups in the development and management of IBD. Functional analyses of the microbiome may be the key to understanding the role of microbiota in the development and chronicity of IBD and reveal new strategies for therapeutic intervention.

Curupira-1 and Curupira-2, two novel Mutator-like DNA transposons from the genomes of human parasites Schistosoma mansoni and Schistosoma japonicum

Transposons of the Mutator superfamily have been widely described in plants, but only recently have metazoan organisms been shown to harbour them. In this work we describe novel Mutator superfamily transposons from the genomes of the human parasites Schistosoma mansoni and S. japonicum, which we name Curupira-1 and Curupira-2. Curupira elements do not have Terminal Inverted Repeats (TIRs) at their extremities and generate Target Site Duplications (TSDs) of 9 base pairs. Curupira-2 transposons code for a conserved transposase and SWIM zinc finger domains, while Curupira-1 elements comprise these same domains plus a WRKY zinc finger. Alignment of transcript sequences from both elements back to the genomes indicates that they are subject to splicing to produce mature transcripts. Phylogenetic analyses indicate that these transposons represent a new lineage of metazoan Mutator-like elements with characteristics that are distinct from the recently described Phantom elements. Description of these novel schistosome transposons provides new insights in the evolution of transposable elements in schistosomes.

Phylogeny of parasitic parabasalia and free-living relatives inferred from conventional markers vs. Rpb1, a single-copy gene

BACKGROUND

Parabasalia are single-celled eukaryotes (protists) that are mainly comprised of endosymbionts of termites and wood roaches, intestinal commensals, human or veterinary parasites, and free-living species. Phylogenetic comparisons of parabasalids are typically based upon morphological characters and 18S ribosomal RNA gene sequence data (rDNA), while biochemical or molecular studies of parabasalids are limited to a few axenically cultivable parasites. These previous analyses and other studies based on PCR amplification of duplicated protein-coding genes are unable to fully resolve the evolutionary relationships of parabasalids. As a result, genetic studies of Parabasalia lag behind other organisms.

PRINCIPAL FINDINGS

Comparing parabasalid EF1α, α-tubulin, enolase and MDH protein-coding genes with information from the Trichomonas vaginalis genome reveals difficulty in resolving the history of species or isolates apart from duplicated genes. A conserved single-copy gene encodes the largest subunit of RNA polymerase II (Rpb1) in T. vaginalis and other eukaryotes. Here we directly sequenced Rpb1 degenerate PCR products from 10 parabasalid genera, including several T. vaginalis isolates and avian isolates, and compared these data by phylogenetic analyses. Rpb1 genes from parabasalids, diplomonads, Parabodo, Diplonema and Percolomonas were all intronless, unlike intron-rich homologs in Naegleria, Jakoba and Malawimonas.

CONCLUSIONS/SIGNIFICANCE

The phylogeny of Rpb1 from parasitic and free-living parabasalids, and conserved Rpb1 insertions, support Trichomonadea, Tritrichomonadea, and Hypotrichomonadea as monophyletic groups. These results are consistent with prior analyses of rDNA and GAPDH sequences and ultrastructural data. The Rpb1 phylogenetic tree also resolves species- and isolate-level relationships. These findings, together with the relative ease of Rpb1 isolation, make it an attractive tool for evaluating more extensive relationships within Parabasalia.

Phantom, a new subclass of Mutator DNA transposons found in insect viruses and widely distributed in animals

Transposons of the Mutator (Mu) superfamily have been shown to play a critical role in the evolution of plant genomes. However, the identification of Mutator transposons in other eukaryotes has been quite limited. Here we describe a previously uncharacterized group of DNA transposons designated Phantom identified in the genomes of a wide range of eukaryotic taxa, including many animals, and provide evidence for its inclusion within the Mutator superfamily. Interestingly three Phantom proteins were also identified in two insect viruses and phylogenetic analysis suggests horizontal movement from insect to virus, providing a new line of evidence for the role of viruses in the horizontal transfer of DNA transposons in animals. Many of the Phantom transposases are predicted to harbor a FLYWCH domain in the amino terminus, which displays a WRKY-GCM1 fold characteristic of the DNA binding domain (DBD) of Mutator transposases and of several transcription factors. While some Phantom elements have terminal inverted repeats similar in length and structure to Mutator elements, some display subterminal inverted repeats (sub-TIRs) and others have more complex termini reminiscent of so-called Foldback (FB) transposons. The structural plasticity of Phantom and the distant relationship of its encoded protein to known transposases may have impeded the discovery of this group of transposons and it suggests that structure in itself is not a reliable character for transposon classification.

Repetitive elements in parasitic protozoa

A recent paper published in BMC Genomics suggests that retrotransposition may be active in the human gut parasite Entamoeba histolytica. This adds to our knowledge of the various types of repetitive elements in parasitic protists and the potential influence of such elements on pathogenicity.

See research article http://www.biomedcentral.com/1471-2164/11/321