Characterization of autonomous families of Tc1/mariner transposons in neoteleost genomes

Characterization of Two Transposable Elements and an Ultra-Conserved Element Isolated in the Genome of Zootoca vivipara (Squamata, Lacertidae)

Transposable elements (TEs) constitute a considerable fraction of eukaryote genomes representing a major source of genetic variability. We describe two DNA sequences isolated in the lizard Zootoca vivipara, here named Zv516 and Zv817. Both sequences are single-copy nuclear sequences, including a truncation of two transposable elements (TEs), SINE Squam1 in Zv516 and a Tc1/Mariner-like DNA transposon in Zv817. FISH analyses with Zv516 showed the occurrence of interspersed signals of the SINE Squam1 sequence on all chromosomes of Z. vivipara and quantitative dot blot indicated that this TE is present with about 4700 copies in the Z. vivipara genome. FISH and dot blot with Zv817 did not produce clear hybridization signals. Bioinformatic analysis showed the presence of active SINE Squam 1 copies in the genome of different lacertids, in different mRNAs, and intronic and coding regions of various genes. The Tc1/Mariner-like DNA transposon occurs in all reptiles, excluding Sphenodon and Archosauria. Zv817 includes a trait of 284 bp, representing an amniote ultra-conserved element (UCE). Using amniote UCE homologous sequences from available whole genome sequences of major amniote taxonomic groups, we performed a phylogenetic analysis which retrieved Prototheria as the sister group of Metatheria and Eutheria. Within diapsids, Testudines are the sister group to Aves + Crocodylia (Archosauria), and Sphenodon is the sister group to Squamata. Furthermore, large trait regions flanking the UCE are conserved at family level.

Horizontal Transfer and Evolutionary Profiles of Two Tc1/DD34E Transposons (ZB and SB) in Vertebrates

Both ZeBrafish (ZB), a recently identified DNA transposon in the zebrafish genome, and SB, a reconstructed transposon originally discovered in several fish species, are known to exhibit high transposition activity in vertebrate cells. Although a similar structural organization was observed for ZB and SB transposons, the evolutionary profiles of their homologs in various species remain unknown. In the present study, we compared their taxonomic ranges, structural arrangements, sequence identities, evolution dynamics, and horizontal transfer occurrences in vertebrates. In total, 629 ZB and 366 SB homologs were obtained and classified into four distinct clades, named ZB, ZB-like, SB, and SB-like. They displayed narrow taxonomic distributions in eukaryotes, and were mostly found in vertebrates, Actinopterygii in particular tended to be the major reservoir hosts of these transposons. Similar structural features and high sequence identities were observed for transposons and transposase, notably homologous to the SB and ZB elements. The genomic sequences that flank the ZB and SB transposons in the genomes revealed highly conserved integration profiles with strong preferential integration into AT repeats. Both SB and ZB transposons experienced horizontal transfer (HT) events, which were most common in Actinopterygii. Our current study helps to increase our understanding of the evolutionary properties and histories of SB and ZB transposon families in animals.

Leafy and weedy seadragon genomes connect genic and repetitive DNA features to the extravagant biology of syngnathid fishes

Seadragons are widely recognized for their derived traits, which include leaf-like appendages and extreme spinal curvature. Efforts to understand the genetic basis of these unique traits and conserve these species and their relatives have been limited by genomic resource gaps. In this paper we present full, annotated genomes of leafy and weedy seadragons, which we use to uncover surprising features of gene family and genome architecture evolution that likely relate to the extravagant phenotypic traits of seadragons and their pipefish and seahorse relatives. These genomes and their analyses are important advances for the study of elaborate vertebrate traits, leveraging this diverse, morphologically exceptional group of fishes.

Seadragons are a remarkable lineage of teleost fishes in the family Syngnathidae, renowned for having evolved male pregnancy. Comprising three known species, seadragons are widely recognized and admired for their fantastical body forms and coloration, and their specific habitat requirements have made them flagship representatives for marine conservation and natural history interests. Until recently, a gap has been the lack of significant genomic resources for seadragons. We have produced gene-annotated, chromosome-scale genome models for the leafy and weedy seadragon to advance investigations of evolutionary innovation and elaboration of morphological traits in seadragons as well as their pipefish and seahorse relatives. We identified several interesting features specific to seadragon genomes, including divergent noncoding regions near a developmental gene important for integumentary outgrowth, a high genome-wide density of repetitive DNA, and recent expansions of transposable elements and a vesicular trafficking gene family. Surprisingly, comparative analyses leveraging the seadragon genomes and additional syngnathid and outgroup genomes revealed striking, syngnathid-specific losses in the family of fibroblast growth factors (FGFs), which likely involve reorganization of highly conserved gene regulatory networks in ways that have not previously been documented in natural populations. The resources presented here serve as important tools for future evolutionary studies of developmental processes in syngnathids and hold value for conservation of the extravagant seadragons and their relatives.

Revisiting the Tigger Transposon Evolution Revealing Extensive Involvement in the Shaping of Mammal Genomes

Simple Summary

Despite the discovery of the Tigger family of pogo transposons in the mammalian genome, the evolution profile of this family is still incomplete. Here, we conducted a systematic evolution analysis for Tigger in nature. The data revealed that Tigger was found in a broad variety of animals, and extensive invasion of Tigger was observed in mammal genomes. Common horizontal transfer events of Tigger elements were observed across different lineages of animals, including mammals, that may have led to their widespread distribution, while parasites and invasive species may have promoted Tigger HT events. Our results also indicate that the activity of Tigger transposons tends to be low in vertebrates; only one mammalian genome and fish genome may harbor active Tigger.

Abstract

The data of this study revealed that Tigger was found in a wide variety of animal genomes, including 180 species from 36 orders of invertebrates and 145 species from 29 orders of vertebrates. An extensive invasion of Tigger was observed in mammals, with a high copy number. Almost 61% of those species contain more than 50 copies of Tigger; however, 46% harbor intact Tigger elements, although the number of these intact elements is very low. Common HT events of Tigger elements were discovered across different lineages of animals, including mammals, that may have led to their widespread distribution, whereas Helogale parvula and arthropods may have aided Tigger HT incidences. The activity of Tigger seems to be low in the kingdom of animals, most copies were truncated in the mammal genomes and lost their transposition activity, and Tigger transposons only display signs of recent and current activities in a few species of animals. The findings suggest that the Tigger family is important in structuring mammal genomes.

DNA Transposon Expansion is Associated with Genome Size Increase in Mudminnows

Genome sizes of eukaryotic organisms vary substantially, with whole-genome duplications (WGD) and transposable element expansion acting as main drivers for rapid genome size increase. The two North American mudminnows, Umbra limi and Umbra pygmaea, feature genomes about twice the size of their sister lineage Esocidae (e.g., pikes and pickerels). However, it is unknown whether all Umbra species share this genome expansion and which causal mechanisms drive this expansion. Using flow cytometry, we find that the genome of the European mudminnow is expanded similarly to both North American species, ranging between 4.5 and 5.4 pg per diploid nucleus. Observed blocks of interstitially located telomeric repeats in U. limi suggest frequent Robertsonian rearrangements in its history. Comparative analyses of transcriptome and genome assemblies show that the genome expansion in Umbra is driven by the expansion of DNA transposon and unclassified repeat sequences without WGD. Furthermore, we find a substantial ongoing expansion of repeat sequences in the Alaska blackfish Dallia pectoralis, the closest relative to the family Umbridae, which might mark the beginning of a similar genome expansion. Our study suggests that the genome expansion in mudminnows, driven mainly by transposon expansion, but not WGD, occurred before the separation into the American and European lineage.

Diversity and Evolution of pogo and Tc1/mariner Transposons in the Apoidea Genomes

Bees (Apoidea), the largest and most crucial radiation of pollinators, play a vital role in the ecosystem balance. Transposons are widely distributed in nature and are important drivers of species diversity. However, transposons are rarely reported in important pollinators such as bees. Here, we surveyed 37 bee genomesin Apoidea, annotated the pogo and Tc1/mariner transposons in the genome of each species, and performed a phylogenetic analysis and determined their overall distribution. The pogo and Tc1/mariner families showed high diversity and low abundance in the 37 species, and their proportion was significantly higher in solitary bees than in social bees. DD34D/mariner was found to be distributed in almost all species and was found in Apis mellifera, Apis mellifera carnica, Apis mellifera caucasia, and Apis mellifera mellifera, and Euglossa dilemma may still be active. Using horizontal transfer analysis, we found that DD29-30D/Tigger may have experienced horizontal transfer (HT) events. The current study displayed the evolution profiles (including diversity, activity, and abundance) of the pogo and Tc1/mariner transposons across 37 species of Apoidea. Our data revealed their contributions to the genomic variations across these species and facilitated in understanding of the genome evolution of this lineage.

Transposable Elements and Stress in Vertebrates: An Overview

Since their identification as genomic regulatory elements, Transposable Elements (TEs) were considered, at first, molecular parasites and later as an important source of genetic diversity and regulatory innovations. In vertebrates in particular, TEs have been recognized as playing an important role in major evolutionary transitions and biodiversity. Moreover, in the last decade, a significant number of papers has been published highlighting a correlation between TE activity and exposition to environmental stresses and dietary factors. In this review we present an overview of the impact of TEs in vertebrate genomes, report the silencing mechanisms adopted by host genomes to regulate TE activity, and finally we explore the effects of environmental and dietary factor exposures on TE activity in mammals, which is the most studied group among vertebrates. The studies here reported evidence that several factors can induce changes in the epigenetic status of TEs and silencing mechanisms leading to their activation with consequent effects on the host genome. The study of TE can represent a future challenge for research for developing effective markers able to detect precocious epigenetic changes and prevent human diseases.

Traveler, a New DD35E Family of Tc1/Mariner Transposons, Invaded Vertebrates Very Recently

The discovery of new members of the Tc1/mariner superfamily of transposons is expected based on the increasing availability of genome sequencing data. Here, we identified a new DD35E family termed Traveler (TR). Phylogenetic analyses of its DDE domain and full-length transposase showed that, although TR formed a monophyletic clade, it exhibited the highest sequence identity and closest phylogenetic relationship with DD34E/Tc1. This family displayed a very restricted taxonomic distribution in the animal kingdom and was only detected in ray-finned fish, anura, and squamata, including 91 vertebrate species. The structural organization of TRs was highly conserved across different classes of animals. Most intact TR transposons had a length of ∼1.5 kb (range 1,072-2,191 bp) and harbored a single open reading frame encoding a transposase of ∼340 aa (range 304-350 aa) flanked by two short-terminal inverted repeats (13-68 bp). Several conserved motifs, including two helix-turn-helix motifs, a GRPR motif, a nuclear localization sequence, and a DDE domain, were also identified in TR transposases. This study also demonstrated the presence of horizontal transfer events of TRs in vertebrates, whereas the average sequence identities and the evolutionary dynamics of TR elements across species and clusters strongly indicated that the TR family invaded the vertebrate lineage very recently and that some of these elements may be currently active, combining the intact TR copies in multiple lineages of vertebrates. These data will contribute to the understanding of the evolutionary history of Tc1/mariner transposons and that of their hosts.

A genomic survey of Tc1-mariner transposons in nematodes suggests extensive horizontal transposon transfer events

The number of reports concerning horizontal transposon transfers (HTT) in metazoan species is considerably increased, alongside with the exponential growth of genomic sequence data However, our understanding of the mechanisms of such phenomenon is still at an early stage. Nematodes constitute an animal phylum successfully adapted to almost every ecosystem and for this reason could potentially contribute to spreading the genetic information through horizontal transfer. To date, few studies describe HTT of nematode retrotransposons. This is due to the lack of annotation of transposable elements in the sequenced nematode genomes, especially DNA transposons, which are acknowledged as the best horizontal travelers among mobile sequences. We have therefore started a survey of DNA transposons and their possible involvement in HTT in sequenced nematode genomes. Here, we describe 83 new Tc1/mariner elements distributed in 17 nematode species. Among them, nine families were possibly horizontally transferred between nematodes and the most diverse animal species, including ants as preferred partner of HTT. The results obtained suggest that HTT events involving nematodes Tc1/mariner elements are not uncommon, and that nematodes could have a possible role as transposon reservoir that, in turn, can be redistributed among animal genomes. Overall, this could be relevant to understand how the inter-species genetic flows shape the landscape of genetic variation of organisms inhabiting specific environmental communities.

Intruder (DD38E), a recently evolved sibling family of DD34E/Tc1 transposons in animals

BACKGROUND

A family of Tc1/mariner transposons with a characteristic DD38E triad of catalytic amino acid residues, named Intruder (IT), was previously discovered in sturgeon genomes, but their evolutionary landscapes remain largely unknown.

RESULTS

Here, we comprehensively investigated the evolutionary profiles of ITs, and evaluated their cut-and-paste activities in cells. ITs exhibited a narrow taxonomic distribution pattern in the animal kingdom, with invasions into two invertebrate phyla (Arthropoda and Cnidaria) and three vertebrate lineages (Actinopterygii, Agnatha, and Anura): very similar to that of the DD36E/IC family. Some animal orders and species seem to be more hospitable to Tc1/mariner transposons, one order of Amphibia and seven Actinopterygian orders are the most common orders with horizontal transfer events and have been invaded by all four families (DD38E/IT, DD35E/TR, DD36E/IC and DD37E/TRT) of Tc1/mariner transposons, and eight Actinopterygii species were identified as the major hosts of these families. Intact ITs have a total length of 1.5-1.7 kb containing a transposase gene flanked by terminal inverted repeats (TIRs). The phylogenetic tree and sequence identity showed that IT transposases were most closely related to DD34E/Tc1. ITs have been involved in multiple events of horizontal transfer in vertebrates and have invaded most lineages recently (< 5 million years ago) based on insertion age analysis. Accordingly, ITs presented high average sequence identity (86-95%) across most vertebrate species, suggesting that some are putatively active. ITs can transpose in human HeLa cells, and the transposition efficiency of consensus TIRs was higher than that of the TIRs of natural isolates.

CONCLUSIONS

We conclude that DD38E/IT originated from DD34E/Tc1 and can be detected in two invertebrate phyla (Arthropoda and Cnidaria), and in three vertebrate lineages (Actinopterygii, Agnatha and Anura). IT has experienced multiple HT events in animals, dominated by recent amplifications in most species and has high identity among vertebrate taxa. Our reconstructed IT transposon vector designed according to the sequence from the "cat" genome showed high cut-and-paste activity. The data suggest that IT has been acquired recently and is active in many species. This study is meaningful for understanding the evolution of the Tc1/mariner superfamily members and their hosts.

Mobile Elements in Ray-Finned Fish Genomes

Ray-finned fishes (Actinopterygii) are a very diverse group of vertebrates, encompassing species adapted to live in freshwater and marine environments, from the deep sea to high mountain streams. Genome sequencing offers a genetic resource for investigating the molecular bases of this phenotypic diversity and these adaptations to various habitats. The wide range of genome sizes observed in fishes is due to the role of transposable elements (TEs), which are powerful drivers of species diversity. Analyses performed to date provide evidence that class II DNA transposons are the most abundant component in most fish genomes and that compared to other vertebrate genomes, many TE superfamilies are present in actinopterygians. Moreover, specific TEs have been reported in ray-finned fishes as a possible result of an intricate relationship between TE evolution and the environment. The data summarized here underline the biological interest in Actinopterygii as a model group to investigate the mechanisms responsible for the high biodiversity observed in this taxon.

Evolution of pogo, a separate superfamily of IS630-Tc1-mariner transposons, revealing recurrent domestication events in vertebrates

BACKGROUND

Tc1/mariner and Zator, as two superfamilies of IS630-Tc1-mariner (ITm) group, have been well-defined. However, the molecular evolution and domestication of pogo transposons, once designated as an important family of the Tc1/mariner superfamily, are still poorly understood.

RESULTS

Here, phylogenetic analysis show that pogo transposases, together with Tc1/mariner, DD34E/Gambol, and Zator transposases form four distinct monophyletic clades with high bootstrap supports (> = 74%), suggesting that they are separate superfamilies of ITm group. The pogo superfamily represents high diversity with six distinct families (Passer, Tigger, pogoR, Lemi, Mover, and Fot/Fot-like) and wide distribution with an expansion spanning across all the kingdoms of eukaryotes. It shows widespread occurrences in animals and fungi, but restricted taxonomic distribution in land plants. It has invaded almost all lineages of animals-even mammals-and has been domesticated repeatedly in vertebrates, with 12 genes, including centromere-associated protein B (CENPB), CENPB DNA-binding domain containing 1 (CENPBD1), Jrk helix-turn-helix protein (JRK), JRK like (JRKL), pogo transposable element derived with KRAB domain (POGK), and with ZNF domain (POGZ), and Tigger transposable element-derived 2 to 7 (TIGD2-7), deduced as originating from this superfamily. Two of them (JRKL and TIGD2) seem to have been co-domesticated, and the others represent independent domestication events. Four genes (TIGD3, TIGD4, TIGD5, and POGZ) tend to represent ancient domestications in vertebrates, while the others only emerge in mammals and seem to be domesticated recently. Significant structural variations including target site duplication (TSD) types and the DDE triad signatures (DD29-56D) were observed for pogo transposons. Most domesticated genes are derived from the complete transposase genes; but CENPB, POGK, and POGZ are chimeric genes fused with additional functional domains.

CONCLUSIONS

This is the first report to systematically reveal the evolutionary profiles of the pogo transposons, suggesting that pogo and Tc1/Mariner are two separate superfamilies of ITm group, and demonstrating the repeated domestications of pogo in vertebrates. These data indicate that pogo transposons have played important roles in shaping the genome and gene evolution of fungi and animals. This study expands our understanding of the diversity of pogo transposons and updates the classification of ITm group.

Phylogenetic analysis of the Tc1/mariner superfamily reveals the unexplored diversity of pogo-like elements

Background

Tc1/mariner transposons are widespread DNA transposable elements (TEs) that have made important contributions to the evolution of host genomic complexity in metazoans. However, the evolution and diversity of the Tc1/mariner superfamily remains poorly understood. Following recent developments in genome sequencing and the availability of a wealth of new genomes, Tc1/mariner TEs have been identified in many new taxa across the eukaryotic tree of life. To date, the majority of studies focussing on Tc1/mariner elements have considered only a single host lineage or just a small number of host lineages. Thus, much remains to be learnt about the evolution of Tc1/mariner TEs by performing analyses that consider elements that originate from across host diversity.

Results

We mined the non-redundant database of NCBI using BLASTp searches, with transposase sequences from a diverse set of reference Tc1/mariner elements as queries. A total of 5158 Tc1/mariner elements were retrieved and used to reconstruct evolutionary relationships within the superfamily. The resulting phylogeny is well resolved and includes several new groups of Tc1/mariner elements. In particular, we identify a new family of plant-genome restricted Tc1/mariner elements, which we call PlantMar. We also show that the pogo family is much larger and more diverse than previously appreciated, and we review evidence for a potential revision of its status to become a separate superfamily.

Conclusions

Our study provides an overview of Tc1-mariner phylogeny and summarises the impressive diversity of Tc1-mariner TEs among sequenced eukaryotes. Tc1/mariner TEs are successful in a wide range of eukaryotes, especially unikonts (the taxonomic supergroup containing Amoebozoa, Opisthokonta, Breviatea, and Apusomonadida). In particular, ecdysozoa, and especially arthropods, emerge as important hosts for Tc1/mariner elements (except the PlantMar family). Meanwhile, the pogo family, which is by far the largest Tc1/mariner family, also includes many elements from fungal and chordate genomes. Moreover, there is evidence of the repeated exaptation of pogo elements in vertebrates, including humans, in addition to the well-known example of CENP-B. Collectively, our findings provide a considerable advancement in understanding of Tc1/mariner elements, and more generally they suggest that much work remains to improve understanding of the diversity and evolution of DNA TEs.

Evolution and domestication of Tc1/mariner transposons in the genome of African coelacanth (Latimeria chalumnae)

Here, we comprehensively analysed the abundance, diversity, and activity of Tc1/mariner transposons in African coelacanth (Latimeria chalumnae). Fifteen Tc1/mariner autonomous transposons were identified and grouped into six clades: DD34E/Tc1, DD34D/mariner, DD35D/Fot, DD31D/pogo, DD30-31D/pogo-like, and DD32-36D/Tigger, belonging to three known families: DD34E/Tc1, DD34D/mariner, and DD×D/pogo (DD35D/Fot, DD31D/pogo, DD30-31D/pogo-like, and DD32-36D/Tigger). Thirty-one miniature inverted-repeat transposable element (MITE) transposons of Tc1/mariner were also identified, and 20 of them display similarity to the identified autonomous transposons. The structural organization of these full Tc1/mariner elements includes a transposase gene flanked by terminal inverted repeats (TIRs) with TA dinucleotides. The transposases contain N-terminal DNA binding domain and a C-terminal catalytic domain characterized by the presence of a conservative D(Asp)DE(Glu)/D triad that is essential for transposase activity. The Tc1/mariner superfamily in coelacanth exhibited very low genome coverage (0.3%), but it experienced an extraordinary difference of proliferation dynamics among the six clades identified; moreover, most of them exhibited a very recent and current proliferation, suggesting that some copies of these transposons are putatively active. Additionally, at least four functional genes derived from Tc1/mariner transposons were found. We provide an up-to-date overview of Tc1/mariner in coelacanth, which may be helpful in determining genome and gene evolution in this living fossil.

Incomer, a DD36E family of Tc1/mariner transposons newly discovered in animals

Background

The Tc1/mariner superfamily might represent the most diverse and widely distributed group of DNA transposons. Several families have been identified; however, exploring the diversity of this superfamily and updating its classification is still ongoing in the life sciences.

Results

Here we identified a new family of Tc1/mariner transposons, named Incomer (IC), which is close to, but distinct from the known family DD34E/Tc1. ICs have a total length of about 1.2 kb, and harbor a single open reading frame encoding a ~ 346 amino acid transposase with a DD36E motif and flanked by short terminal inverted repeats (TIRs) (22–32 base pairs, bp). This family is absent from prokaryotes, and is mainly distributed among vertebrates (141 species of four classes), including Agnatha (one species of jawless fish), Actinopterygii (132 species of ray-finned fish), Amphibia (four species of frogs), and Mammalia (four species of bats), but have a restricted distribution in invertebrates (four species in Insecta and nine in Arachnida). All ICs in bats (Myotis lucifugus, Eptesicus fuscus, Myotis davidii, and Myotis brandtii) are present as truncated copies in these genomes, and most of them are flanked by relatively long TIRs (51–126 bp). High copy numbers of miniature inverted-repeat transposable elements (MITEs) derived from ICs were also identified in bat genomes. Phylogenetic analysis revealed that ICs are more closely related to DD34E/Tc1 than to other families of Tc1/mariner (e.g., DD34D/mariner and DD × D/pogo), and can be classified into four distinct clusters. The host and IC phylogenies and pairwise distance comparisons between RAG1 genes and all consensus sequences of ICs support the idea that multiple episodes of horizontal transfer (HT) of ICs have occurred in vertebrates. In addition, the discovery of intact transposases, perfect TIRs and target site duplications of ICs suggests that this family may still be active in Insecta, Arachnida, frogs, and fish.

Conclusions

Exploring the diversity of Tc1/mariner transposons and revealing their evolutionary profiles will help provide a better understanding of the evolution of DNA transposons and their impact on genomic evolution. Here, a newly discovered family (DD36E/Incomer) of Tc1/mariner transposons is described in animals. It displays a similar structural organization and close relationship with the known DD34E/Tc1 elements, but has a relatively narrow distribution, indicating that DD36E/IC might have originated from the DD34E/Tc1 family. Our data also support the hypothesis of horizontal transfer of IC in vertebrates, even invading one lineage of mammals (bats). This study expands our understanding of the diversity of Tc1/mariner transposons and updates the classification of this superfamily.