Transposable elements and viruses as factors in adaptation and evolution: an expansion and strengthening of the TE-Thrust hypothesis

Austropuccinia licaniae, first congeneric with the myrtle rust pathogen A. psidii

In 1895 and 2001, rust fungi affecting Licania trees (Chrysobalanchaceae) in Brazil were described as Uredo licaniae by Hennings in the state of Goiás and as Phakopsora tomentosae by Ferreira et al. in the state of Amazonas, respectively. Recently, a Licania rust fungus collected close to the Amazonian type location sharing symptoms with the former two species was subjected to morphological examinations and molecular phylogenetic analyses using 28S nuc rDNA (ITS2-28S) and cytochrome c oxidase subunit III (CO3) gene sequences. Since the original type specimen of Ph. tomentosae is considered lost, we carefully reviewed the type description and questioned the identity of the telium, which justified the description of the fungus as a Phakopsora species. Furthermore, the additional revision of the type material described by Hennings revealed that Ph. tomentosae is a synonym of U. licaniae. Based on the morphological examinations, disease symptoms, and shared hosts, we concluded that the newly collected material is conspecific with U. licaniae. However, the phylogenetic analyses rejected allocation in Phakopsora and instead assigned the Licania rust fungus in a sister relationship with Austropuccinia psidii (Sphaerophragmiaceae), the causal agent of the globally invasive myrtle rust pathogen. We therefore favored a recombination of U. licaniae (syn. Ph. tomentosae) into Austropuccinia and proposed the new name Austropuccina licaniae for the second species now identified for this genus. The fungus shares conspicuous symptoms with A. psidii, causing often severe infections of growing leaves and shoots that lead to leaf necrosis, leaf shedding, and eventually to the dieback of entire shoots. In view of the very similar symptoms of its aggressively invasive sister species, we briefly discuss the current state of knowledge about A. licaniae and the potential risks, and the opportunity of its identification.

Adaptive expansion of ERVK solo-LTRs is associated with Passeriformes speciation events

Endogenous retroviruses (ERVs) are ancient retroviral remnants integrated in host genomes, and commonly deleted through unequal homologous recombination, leaving solitary long terminal repeats (solo-LTRs). This study, analysing the genomes of 362 bird species and their reptilian and mammalian outgroups, reveals an unusually higher level of solo-LTRs formation in birds, indicating evolutionary forces might have purged ERVs during evolution. Strikingly in the order Passeriformes, and especially the parvorder Passerida, endogenous retrovirus K (ERVK) solo-LTRs showed bursts of formation and recurrent accumulations coinciding with speciation events over past 22 million years. Moreover, our results indicate that the ongoing expansion of ERVK solo-LTRs in these bird species, marked by high transcriptional activity of ERVK retroviral genes in reproductive organs, caused variation of solo-LTRs between individual zebra finches. We experimentally demonstrated that cis-regulatory activity of recently evolved ERVK solo-LTRs may significantly increase the expression level of ITGA2 in the brain of zebra finches compared to chickens. These findings suggest that ERVK solo-LTRs expansion may introduce novel genomic sequences acting as cis-regulatory elements and contribute to adaptive evolution. Overall, our results underscore that the residual sequences of ancient retroviruses could influence the adaptive diversification of species by regulating host gene expression.

Haplotype-phased and chromosome-level genome assembly of Puccinia polysora, a giga-scale fungal pathogen causing southern corn rust

Rust fungi are characterized by large genomes with high repeat content and have two haploid nuclei in most life stages, which makes achieving high-quality genome assemblies challenging. Here, we described a pipeline using HiFi reads and Hi-C data to assemble a gigabase-sized fungal pathogen, Puccinia polysora f.sp. zeae, to haplotype-phased and chromosome-scale. The final assembled genome is 1.71 Gbp, with ~850 Mbp and 18 chromosomes in each haplotype, being currently one of the two giga-scale fungi assembled to chromosome level. Transcript-based annotation identified 47,512 genes for the dikaryotic genome with a similar number for each haplotype. A high level of interhaplotype variation was found with 10% haplotype-specific BUSCO genes, 5.8 SNPs/kbp, and structural variation accounting for 3% of the genome size. The P. polysora genome displayed over 85% repeat contents, with genome-size expansion and copy number increasing of species-specific orthogroups. Interestingly, these features did not affect overall synteny with other Puccinia species having smaller genomes. Fine-time-point transcriptomics revealed seven clusters of coexpressed secreted proteins that are conserved between two haplotypes. The fact that candidate effectors interspersed with all genes indicated the absence of a "two-speed genome" evolution in P. polysora. Genome resequencing of 79 additional isolates revealed a clonal population structure of P. polysora in China with low geographic differentiation. Nevertheless, a minor population differentiated from the major population by having mutations on secreted proteins including AvrRppC, indicating the ongoing virulence to evade recognition by RppC, a major resistance gene in Chinese corn cultivars. The high-quality assembly provides valuable genomic resources for future studies on disease management and the evolution of P. polysora.

Transposable Elements in Bats Show Differential Accumulation Patterns Determined by Class and Functionality

Bat genomes are characterized by a diverse transposable element (TE) repertoire. In particular, the genomes of members of the family Vespertilionidae contain both active retrotransposons and active DNA transposons. Each TE type is characterized by a distinct pattern of accumulation over the past ~40 million years. Each also exhibits its own target site preferences (sometimes shared with other TEs) that impact where they are likely to insert when mobilizing. Therefore, bats provide a great resource for understanding the diversity of TE insertion patterns. To gain insight into how these diverse TEs impact genome structure, we performed comparative spatial analyses between different TE classes and genomic features, including genic regions and CpG islands. Our results showed a depletion of all TEs in the coding sequence and revealed patterns of species- and element-specific attraction in the transcript. Trends of attraction in the distance tests also suggested significant TE activity in regions adjacent to genes. In particular, the enrichment of small, non-autonomous TE insertions in introns and near coding regions supports the hypothesis that the genomic distribution of TEs is the product of a balance of the TE insertion preference in open chromatin regions and the purifying selection against TEs within genes.

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.

The Structural, Functional and Evolutionary Impact of Transposable Elements in Eukaryotes

Transposable elements (TEs) are nearly ubiquitous in eukaryotes. The increase in genomic data, as well as progress in genome annotation and molecular biology techniques, have revealed the vast number of ways mobile elements have impacted the evolution of eukaryotes. In addition to being the main cause of difference in haploid genome size, TEs have affected the overall organization of genomes by accumulating preferentially in some genomic regions, by causing structural rearrangements or by modifying the recombination rate. Although the vast majority of insertions is neutral or deleterious, TEs have been an important source of evolutionary novelties and have played a determinant role in the evolution of fundamental biological processes. TEs have been recruited in the regulation of host genes and are implicated in the evolution of regulatory networks. They have also served as a source of protein-coding sequences or even entire genes. The impact of TEs on eukaryotic evolution is only now being fully appreciated and the role they may play in a number of biological processes, such as speciation and adaptation, remains to be deciphered.

Austropuccinia psidii, causing myrtle rust, has a gigabase-sized genome shaped by transposable elements

Austropuccinia psidii, originating in South America, is a globally invasive fungal plant pathogen that causes rust disease on Myrtaceae. Several biotypes are recognized, with the most widely distributed pandemic biotype spreading throughout the Asia-Pacific and Oceania regions over the last decade. Austropuccinia psidii has a broad host range with more than 480 myrtaceous species. Since first detected in Australia in 2010, the pathogen has caused the near extinction of at least three species and negatively affected commercial production of several Myrtaceae. To enable molecular and evolutionary studies into A. psidii pathogenicity, we assembled a highly contiguous genome for the pandemic biotype. With an estimated haploid genome size of just over 1 Gb (gigabases), it is the largest assembled fungal genome to date. The genome has undergone massive expansion via distinct transposable element (TE) bursts. Over 90% of the genome is covered by TEs predominantly belonging to the Gypsy superfamily. These TE bursts have likely been followed by deamination events of methylated cytosines to silence the repetitive elements. This in turn led to the depletion of CpG sites in TEs and a very low overall GC content of 33.8%. Compared to other Pucciniales, the intergenic distances are increased by an order of magnitude indicating a general insertion of TEs between genes. Overall, we show how TEs shaped the genome evolution of A. psidii and provide a greatly needed resource for strategic approaches to combat disease spread.

Transposable elements, contributors in the evolution of organisms (from an arms race to a source of raw materials)

There is a concept proposing that the primitive lineages of prokaryotes, eukaryotes, and viruses emerged from the primordial pool of primitive genetic elements. In this genetic pool, transposable elements (TEs) became a source of raw material for primitive genomes, tools of genetic innovation, and ancestors of modern genes (e.g. ncRNAs, tRNAs, and rRNAs). TEs contributed directly to the genome evolution of three forms of life on the earth. TEs now appear as tools that were used to giving rise to sexual dimorphism and sex determination, lineage-specific expression of genes and tissue differentiation and finally genome stability and lifespan determination.

Transposable Elements and the Evolution of Insects

Insects are major contributors to our understanding of the interaction between transposable elements (TEs) and their hosts, owing to seminal discoveries, as well as to the growing number of sequenced insect genomes and population genomics and functional studies. Insect TE landscapes are highly variable both within and across insect orders, although phylogenetic relatedness appears to correlate with similarity in insect TE content. This correlation is unlikely to be solely due to inheritance of TEs from shared ancestors and may partly reflect preferential horizontal transfer of TEs between closely related species. The influence of insect traits on TE landscapes, however, remains unclear. Recent findings indicate that, in addition to being involved in insect adaptations and aging, TEs are seemingly at the cornerstone of insect antiviral immunity. Thus, TEs are emerging as essential insect symbionts that may have deleterious or beneficial consequences on their hosts, depending on context.

Identification and characterization of microRNAs (miRNAs) and their transposable element origins in the saltwater crocodile, Crocodylus porosus

MicroRNAs (miRNAs) are 18-24 nucleotide regulatory RNAs. They are involved in the regulation of genetic and biological pathways through post transcriptional gene silencing and/or translational repression. Data suggests a slow evolutionary rate for the saltwater crocodile (Crocodylus porosus) over the past several million years when compared to birds, the closest extant relatives of crocodilians. Understanding gene regulation in the saltwater crocodile in the context of relatively slow genomic change thus holds potential for the investigation of genomics, evolution, and adaptation. Utilizing eleven tissue types and sixteen small RNA libraries, we report 644 miRNAs in the saltwater crocodile with >78% of miRNAs being novel to crocodilians. We also identified potential targets for the miRNAs and analyzed the relationship of the miRNA repertoire to transposable elements (TEs). Results suggest an increased association of DNA transposons with miRNAs when compared to retrotransposons. This work reports the first comprehensive analysis of miRNAs in Crocodylus porosus and addresses the potential impacts of miRNAs in regulating the genome in the saltwater crocodile. In addition, the data suggests a supporting role of TEs as a source for miRNAs, adding to the increasing evidence that TEs play a significant role in the evolution of gene regulation.

Reawakening the Developmental Origins of Cancer Through Transposable Elements

Transposable elements (TEs) have an established role as important regulators of early human development, functioning as tissue-specific genes and regulatory elements. Functional TEs are highly active during early development, and interact with important developmental genes, some of which also function as oncogenes. Dedifferentiation is a hallmark of cancer, and is characterized by genetic and epigenetic changes that enable proliferation, self-renewal and a metabolism reminiscent of embryonic stem cells. There is also compelling evidence suggesting that the path to dedifferentiation in cancer can contribute to invasion and metastasis. TEs are frequently expressed in cancer, and recent work has identified a newly proposed mechanism involving extensive recruitment of TE-derived promoters to drive expression of oncogenes and subsequently promote oncogenesis—a process termed onco-exaptation. However, the mechanism by which this phenomenon occurs, and the extent to which it contributes to oncogenesis remains unknown. Initial hypotheses have proposed that onco-exaptation events are cancer-specific and arise randomly due to the dysregulated and hypomethylated state of cancer cells and abundance of TEs across the genome. However, we suspect that exaptation-like events may not just arise due to chance activation of novel regulatory relationships as proposed previously, but as a result of the reestablishment of early developmental regulatory relationships. Dedifferentiation in cancer is well-documented, along with expression of TEs. The known interactions between TEs and pluripotency factors such as NANOG and OCTt4 during early development, along with the expression of some placental-specific TE-derived transcripts in cancer support a possible link between TEs and dedifferentiation of tumor cells. Thus, we hypothesize that onco-exaptation events can be associated with the epigenetic reawakening of early developmental TEs to regulate expression of oncogenes and promote oncogenesis. We also suspect that activation of these early developmental regulatory TEs may promote dedifferentiation, although at this stage it is hard to predict whether TE activation is one of the initial drivers of dedifferentiation. We expect that developmental TE activation occurs as a result of the establishment of an epigenetic landscape in cancer that resembles that of early development and that developmental TE activation may also enable cancers to exploit early developmental pathways, repurposing them to promote malignancy.

Simultaneous TE Analysis of 19 Heliconiine Butterflies Yields Novel Insights into Rapid TE-Based Genome Diversification and Multiple SINE Births and Deaths

Transposable elements (TEs) play major roles in the evolution of genome structure and function. However, because of their repetitive nature, they are difficult to annotate and discovering the specific roles they may play in a lineage can be a daunting task. Heliconiine butterflies are models for the study of multiple evolutionary processes including phenotype evolution and hybridization. We attempted to determine how TEs may play a role in the diversification of genomes within this clade by performing a detailed examination of TE content and accumulation in 19 species whose genomes were recently sequenced. We found that TE content has diverged substantially and rapidly in the time since several subclades shared a common ancestor with each lineage harboring a unique TE repertoire. Several novel SINE lineages have been established that are restricted to a subset of species. Furthermore, the previously described SINE, Metulj, appears to have gone extinct in two subclades while expanding to significant numbers in others. This diversity in TE content and activity has the potential to impact how heliconiine butterflies continue to evolve and diverge.

Hormesis Promotes Evolutionary Change

Exposure to moderate environmental stress is one important source of evolutionary change. This evidence would support the hypothesis that hormesis is an evolutionary expectation. In this short review, I discuss relevant examples of genetic and phenotypic responses to moderate stress exposure that are compatible with hormesis and with paradigms of evolutionary theory such as evolutionary rescue or phenotypic plasticity. Genetic recombination, nonlethal mutations, activity of transposable elements, or gene expression are some of the molecular mechanisms through which hormesis might enable organisms to maintain or even increase evolutionary fitness in stressful environments. These mechanisms span the tree of life from plants to vertebrates.

RNAi-dependent Polycomb repression controls transposable elements in Tetrahymena

RNAi and Polycomb repression play evolutionarily conserved and often coordinated roles in transcriptional silencing. Here, we show that, in the protozoan Tetrahymena thermophila, germline-specific internally eliminated sequences (IESs)-many related to transposable elements (TEs)-become transcriptionally activated in mutants deficient in the RNAi-dependent Polycomb repression pathway. Germline TE mobilization also dramatically increases in these mutants. The transition from noncoding RNA (ncRNA) to mRNA production accompanies transcriptional activation of TE-related sequences and vice versa for transcriptional silencing. The balance between ncRNA and mRNA production is potentially affected by cotranscriptional processing as well as RNAi and Polycomb repression. We posit that interplay between RNAi and Polycomb repression is a widely conserved phenomenon, whose ancestral role is epigenetic silencing of TEs.

Multiple and Independent Phases of Transposable Element Amplification in the Genomes of Piciformes (Woodpeckers and Allies)

The small and conserved genomes of birds are likely a result of flight-related metabolic constraints. Recombination-driven deletions and minimal transposable element (TE) expansions have led to continually shrinking genomes during evolution of many lineages of volant birds. Despite constraints of genome size in birds, we identified multiple waves of amplification of TEs in Piciformes (woodpeckers, honeyguides, toucans, and barbets). Relative to other bird species’ genomic TE abundance (< 10% of genome), we found ∼17–30% TE content in multiple clades within Piciformes. Several families of the retrotransposon superfamily chicken repeat 1 (CR1) expanded in at least three different waves of activity. The most recent CR1 expansions (∼4–7% of genome) preceded bursts of diversification in the woodpecker clade and in the American barbets + toucans clade. Additionally, we identified several thousand polymorphic CR1 insertions (hundreds per individual) in three closely related woodpecker species. Woodpecker CR1 insertion polymorphisms are maintained at lower frequencies than single nucleotide polymorphisms indicating that purifying selection is acting against additional CR1 copies and that these elements impose a fitness cost on their host. These findings provide evidence of large scale and ongoing TE activity in avian genomes despite continual constraint on genome size.

The impact of transposable elements in adaptive evolution

The growing knowledge about the influence of transposable elements (TEs) on (a) long-term genome and transcriptome evolution; (b) genomic, transcriptomic and epigenetic variation within populations; and (c) patterns of somatic genetic differences in individuals continues to spur the interest of evolutionary biologists in the role of TEs in adaptive evolution. As TEs can trigger a broad range of molecular variation in a population with potentially severe fitness and phenotypic consequences for individuals, different mechanisms evolved to keep TE activity in check, allowing for a dynamic interplay between the host, its TEs and the environment in evolution. Here, we review evidence for adaptive phenotypic changes associated with TEs and the basic molecular mechanisms by which the underlying genetic changes arise: (a) domestication, (b) exaptation, (c) host gene regulation, (d) TE-mediated formation of intronless gene copies-so-called retrogenes and (e) overall increased genome plasticity. Furthermore, we review and discuss how the stress-dependent incapacitation of defence mechanisms against the activity of TEs might facilitate adaptive responses to environmental challenges and how such mechanisms might be particularly relevant in species frequently facing novel environments, such as invasive, pathogenic or parasitic species.

Transposable Elements Activity is Positively Related to Rate of Speciation in Mammals

Transposable elements (TEs) play an essential role in shaping eukaryotic genomes and generating variability. Speciation and TE activity bursts could be strongly related in mammals, in which simple gradualistic models of differentiation do not account for the currently observed species variability. In order to test this hypothesis, we designed two parameters: the Density of insertion (DI) and the Relative rate of speciation (RRS). DI is the ratio between the number of TE insertions in a genome and its size, whereas the RRS is a conditional parameter designed to identify potential speciation bursts. Thus, by analyzing TE insertions in mammals, we defined the genomes as “hot” (high DI) and “cold” (low DI). Then, comparing TE activity among 29 taxonomical families of the whole Mammalia class, 16 intra-order pairs of mammalian species, and four superorders of Eutheria, we showed that taxa with high rates of speciation are associated with “hot” genomes, whereas taxa with low ones are associated with “cold” genomes. These results suggest a remarkable correlation between TE activity and speciation, also being consistent with patterns describing variable rates of differentiation and accounting for the different time frames of the speciation bursts.

Cause of Cambrian Explosion - Terrestrial or Cosmic?

We review the salient evidence consistent with or predicted by the Hoyle-Wickramasinghe (H-W) thesis of Cometary (Cosmic) Biology. Much of this physical and biological evidence is multifactorial. One particular focus are the recent studies which date the emergence of the complex retroviruses of vertebrate lines at or just before the Cambrian Explosion of ∼500 Ma. Such viruses are known to be plausibly associated with major evolutionary genomic processes. We believe this coincidence is not fortuitous but is consistent with a key prediction of H-W theory whereby major extinction-diversification evolutionary boundaries coincide with virus-bearing cometary-bolide bombardment events. A second focus is the remarkable evolution of intelligent complexity (Cephalopods) culminating in the emergence of the Octopus. A third focus concerns the micro-organism fossil evidence contained within meteorites as well as the detection in the upper atmosphere of apparent incoming life-bearing particles from space. In our view the totality of the multifactorial data and critical analyses assembled by Fred Hoyle, Chandra Wickramasinghe and their many colleagues since the 1960s leads to a very plausible conclusion - life may have been seeded here on Earth by life-bearing comets as soon as conditions on Earth allowed it to flourish (about or just before 4.1 Billion years ago); and living organisms such as space-resistant and space-hardy bacteria, viruses, more complex eukaryotic cells, fertilised ova and seeds have been continuously delivered ever since to Earth so being one important driver of further terrestrial evolution which has resulted in considerable genetic diversity and which has led to the emergence of mankind.

Horizontal acquisition of transposable elements and viral sequences: patterns and consequences

It is becoming clear that most eukaryotic transposable elements (TEs) owe their evolutionary success in part to horizontal transfer events, which enable them to invade new species. Recent large-scale studies are beginning to unravel the mechanisms and ecological factors underlying this mode of transmission. Viruses are increasingly recognized as vectors in the process but also as a direct source of genetic material horizontally acquired by eukaryotic organisms. Because TEs and endogenous viruses are major catalysts of variation and innovation in genomes, we argue that horizontal inheritance has had a more profound impact in eukaryotic evolution than is commonly appreciated. To support this proposal, we compile a list of examples, including some previously unrecognized, whereby new host functions and phenotypes can be directly attributed to horizontally acquired TE or viral sequences. We predict that the number of examples will rapidly grow in the future as the prevalence of horizontal transfer in the life cycle of TEs becomes even more apparent, firmly establishing this form of non-Mendelian inheritance as a consequential facet of eukaryotic evolution.

Phenotype diffusion and one health: A proposed framework for investigating the plurality of obesity epidemics across many species

We propose the idea of "phenotype diffusion," which is a rapid convergence of an observed trait in some human and animal populations. The words phenotype and diffusion both imply observations independent of mechanism as phenotypes are observed traits with multiple possible genetic mechanisms and diffusion is an observed state of being widely distributed. Recognizing shared changes in phenotype in multiple species does not by itself reveal a particular mechanism such as a shared exposure, shared adaptive need, particular stochastic process or a transmission pathway. Instead, identifying phenotype diffusion suggests the mechanism should be explored to help illuminate the ways human and animal health are connected and new opportunities for optimizing these links. Using the plurality of obesity epidemics across multiple species as a prototype for shared changes in phenotype, the goal of this review was to explore eco-evolutionary theories that could inform further investigation. First, evolutionary changes described by hologenome evolution, pawnobe evolution, transposable element (TE) thrust and the drifty gene hypothesis will be discussed within the context of the selection asymmetries among human and animal populations. Secondly, the ecology of common source exposures (bovine milk, xenohormesis and "obesogens"), niche evolution and the hygiene hypothesis will be summarized. Finally, we synthesize these considerations. For example, many agricultural breeds have been aggressively selected for weight gain, microbiota (e.g., adenovirus 36, toxoplasmosis) associated with (or infecting) these breeds cause experimental weight gain in other animals, and these same microbes are associated with human obesity. We propose applications of phenotype diffusion could include zoonotic biosurveillance, biocontainment, antibiotic stewardship and environmental priorities. The One Health field is focused on the connections between the health of humans, animals and the environment, and so identification of phenotype diffusion is highly relevant for practitioners (public health officials, physicians and veterinarians) in this field.

Accumulation of transposable elements in Hox gene clusters during adaptive radiation of Anolis lizards

Transposable elements (TEs) are DNA sequences that can insert elsewhere in the genome and modify genome structure and gene regulation. The role of TEs in evolution is contentious. One hypothesis posits that TE activity generates genomic incompatibilities that can cause reproductive isolation between incipient species. This predicts that TEs will accumulate during speciation events. Here, I tested the prediction that extant lineages with a relatively high rate of speciation have a high number of TEs in their genomes. I sequenced and analysed the TE content of a marker genomic region (Hox clusters) in Anolis lizards, a classic case of an adaptive radiation. Unlike other vertebrates, including closely related lizards, Anolis lizards have high numbers of TEs in their Hox clusters, genomic regions that regulate development of the morphological adaptations that characterize habitat specialists in these lizards. Following a burst of TE activity in the lineage leading to extant Anolis, TEs have continued to accumulate during or after speciation events, resulting in a positive relationship between TE density and lineage speciation rate. These results are consistent with the prediction that TE activity contributes to adaptive radiation by promoting speciation. Although there was no evidence that TE density per se is associated with ecological morphology, the activity of TEs in Hox clusters could have been a rich source for phenotypic variation that may have facilitated the rapid parallel morphological adaptation to microhabitats seen in extant Anolis lizards.

Evolution and Diversity of Transposable Elements in Vertebrate Genomes

Transposable elements (TEs) are selfish genetic elements that mobilize in genomes via transposition or retrotransposition and often make up large fractions of vertebrate genomes. Here, we review the current understanding of vertebrate TE diversity and evolution in the context of recent advances in genome sequencing and assembly techniques. TEs make up 4-60% of assembled vertebrate genomes, and deeply branching lineages such as ray-finned fishes and amphibians generally exhibit a higher TE diversity than the more recent radiations of birds and mammals. Furthermore, the list of taxa with exceptional TE landscapes is growing. We emphasize that the current bottleneck in genome analyses lies in the proper annotation of TEs and provide examples where superficial analyses led to misleading conclusions about genome evolution. Finally, recent advances in long-read sequencing will soon permit access to TE-rich genomic regions that previously resisted assembly including the gigantic, TE-rich genomes of salamanders and lungfishes.

Novel Role of 3'UTR-Embedded Alu Elements as Facilitators of Processed Pseudogene Genesis and Host Gene Capture by Viral Genomes

Since the discovery of the high abundance of Alu elements in the human genome, the interest for the functional significance of these retrotransposons has been increasing. Primate Alu and rodent Alu-like elements are retrotransposed by a mechanism driven by the LINE1 (L1) encoded proteins, the same machinery that generates the L1 repeats, the processed pseudogenes (PPs), and other retroelements. Apart from free Alu RNAs, Alus are also transcribed and retrotranscribed as part of cellular gene transcripts, generally embedded inside 3' untranslated regions (UTRs). Despite different proposed hypotheses, the functional implication of the presence of Alus inside 3'UTRs remains elusive. In this study we hypothesized that Alu elements in 3'UTRs could be involved in the genesis of PPs. By analyzing human genome data we discovered that the existence of 3'UTR-embedded Alu elements is overrepresented in genes source of PPs. In contrast, the presence of other retrotransposable elements in 3'UTRs does not show this PP linked overrepresentation. This research was extended to mouse and rat genomes and the results accordingly reveal overrepresentation of 3'UTR-embedded B1 (Alu-like) elements in PP parent genes. Interestingly, we also demonstrated that the overrepresentation of 3'UTR-embedded Alus is particularly significant in PP parent genes with low germline gene expression level. Finally, we provide data that support the hypothesis that the L1 machinery is also the system that herpesviruses, and possibly other large DNA viruses, use to capture host genes expressed in germline or somatic cells. Altogether our results suggest a novel role for Alu or Alu-like elements inside 3'UTRs as facilitators of the genesis of PPs, particularly in lowly expressed genes. Moreover, we propose that this L1-driven mechanism, aided by the presence of 3'UTR-embedded Alus, may also be exploited by DNA viruses to incorporate host genes to their viral genomes.

DNA Transposition at Work

DNA transposons are defined segments of DNA that are able to move from one genomic location to another. Movement is facilitated by one or more proteins, called the transposase, typically encoded by the mobile element itself. Here, we first provide an overview of the classification of such mobile elements in a variety of organisms. From a mechanistic perspective, we have focused on one particular group of DNA transposons that encode a transposase with a DD(E/D) catalytic domain that is topologically similar to RNase H. For these, a number of three-dimensional structures of transpososomes (transposase-nucleic acid complexes) are available, and we use these to describe the basics of their mechanisms. The DD(E/D) group, in addition to being the largest and most common among all DNA transposases, is the one whose members have been used for a wide variety of genomic applications. Therefore, a second focus of the article is to provide a nonexhaustive overview of transposon applications. Although several non-transposon-based approaches to site-directed genome modifications have emerged in the past decade, transposon-based applications are highly relevant when integration specificity is not sought. In fact, for many applications, the almost-perfect randomness and high frequency of integration make transposon-based approaches indispensable.

[STRUCTURE AND DISTRIBUTION OF THE RETROTRANSPOSON BOV-B LINE]

The classification of mobile elements was discussed. Special attention was devoted to the retroelement of the LINE group: retrotransposon Bov-B LINE. The history of its origin and distribution in the nature was considered. The results of the phenomenon of horizontal transition of the retrotransposon Bov-B LINE between evolutionally distant classes were discussed.

Cognition, Information Fields and Hologenomic Entanglement: Evolution in Light and Shadow

As the prime unification of Darwinism and genetics, the Modern Synthesis continues to epitomize mainstay evolutionary theory. Many decades after its formulation, its anchor assumptions remain fixed: conflict between macro organic organisms and selection at that level represent the near totality of any evolutionary narrative. However, intervening research has revealed a less easily appraised cellular and microbial focus for eukaryotic existence. It is now established that all multicellular eukaryotic organisms are holobionts representing complex collaborations between the co-aligned microbiome of each eukaryote and its innate cells into extensive mixed cellular ecologies. Each of these ecological constituents has demonstrated faculties consistent with basal cognition. Consequently, an alternative hologenomic entanglement model is proposed with cognition at its center and conceptualized as Pervasive Information Fields within a quantum framework. Evolutionary development can then be reconsidered as being continuously based upon communication between self-referential constituencies reiterated at every scope and scale. Immunological reactions support and reinforce self-recognition juxtaposed against external environmental stresses.

Microbial Consortium Associated with the Antarctic Marine Ciliate Euplotes focardii: An Investigation from Genomic Sequences

We report the characterization of the bacterial consortium associated to Euplotes focardii, a strictly psychrophilic marine ciliate that was maintained in laboratory cultures at 4 °C after its first isolation from Terra Nova Bay, in Antarctica. By Illumina genome analyser, we obtained 11,179 contigs of potential prokaryotic origin and classified them according to the NCBI's prokaryotic attributes table. The majority of these sequences correspond to either Bacteroidetes (16 %) or Proteobacteria (78 %). The latter were dominated by gamma- (39 %, including sequences related to the pathogenic genus Francisella), and alpha-proteobacterial (30 %) sequences. Analysis of the Pfam domain family and Gene Ontology term variation revealed that the most frequent terms that appear unique to this consortium correspond to proteins involved in "transmembrane transporter activity" and "oxidoreductase activity". Furthermore, we identified genes that encode for enzymes involved in the catabolism of complex substance for energy reserves. We also characterized members of the transposase and integrase superfamilies, whose role in bacterial evolution is well documented, as well as putative antifreeze proteins. Antibiotic treatments of E. focardii cultures delayed the cell division of the ciliate. To conclude, our results indicate that this consortium is largely represented by bacteria derived from the original Antarctic sample and may contribute to the survival of E. focardii in laboratory condition. Furthermore, our results suggest that these bacteria may have a more general role in E. focardii survival in its natural cold and oxidative environment.

Transposable elements and small RNAs: Genomic fuel for species diversity

While transposable elements (TE) have long been suspected of involvement in species diversification, identifying specific roles has been difficult. We recently found evidence of TE-derived regulatory RNAs in a species-rich family of bats. The TE-derived small RNAs are temporally associated with the burst of species diversification, suggesting that they may have been involved in the processes that led to the diversification. In this commentary, we expand on the ideas that were briefly touched upon in that manuscript. Specifically, we suggest avenues of research that may help to identify the roles that TEs may play in perturbing regulatory pathways. Such research endeavors may serve to inform evolutionary biologists of the ways that TEs have influenced the genomic and taxonomic diversity around us.

Regulatory roles of LINE-1-encoded reverse transcriptase in cancer onset and progression

LINE-1 retrotransposons encode the reverse transcriptase (RT) enzyme, required for their own mobility, the expression of which is inhibited in differentiated tissues while being active in tumors. Experimental evidence indicate that the inhibition of LINE-1-derived RT restores differentiation in cancer cells, inhibits tumor progression and yields globally reprogrammed transcription profiles. Newly emerging data suggest that LINE-1-encoded RT modulates the biogenesis of miRNAs, by governing the balance between the production of regulatory double-stranded RNAs and RNA:DNA hybrid molecules, with a direct impact on global gene expression. Abnormally high RT activity unbalances the transcriptome in cancer cells, while RT inhibition restores ‘normal’ miRNA profiles and their regulatory networks. This RT-dependent mechanism can target the myriad of transcripts - both coding and non-coding, sense and antisense - in eukaryotic transcriptomes, with a profound impact on cell fates. LINE-1-encoded RT emerges therefore as a key regulator of a previously unrecognized mechanism in tumorigenesis

When Competing Viruses Unify: Evolution, Conservation, and Plasticity of Genetic Identities

In the early 1970s, Manfred Eigen and colleagues developed the quasispecies model (qs) for the population-based origin of RNAs representing the early genetic code. The Eigen idea is basically that a halo of mutants is generated by error-prone replication around the master fittest type which will behave similarly as a biological population. But almost from the start, very interesting and unexpected observations were made regarding competition versus co-operation which suggested more complex interactions. It thus became increasingly clear that although viruses functioned similar to biological species, their behavior was much more complex than the original theory could explain, especially adaptation without changing the consensus involving minority populations. With respect to the origin of natural codes, meaning, and code-use in interactions (communication), it also became clear that individual fittest type-based mechanisms were likewise unable to explain the origin of natural codes such as the genetic code with their context- and consortia-dependence (pragmatic nature). This, instead, required the participation of groups of agents competent in the code and able to edit code because natural codes do not code themselves. Three lines of inquiry, experimental virology, quasispecies theory, and the study of natural codes converged to indicate that consortia of co-operative RNA agents such as viruses must be involved in the fitness of RNA and its involvement in communication, i.e., code-competent interactions. We called this co-operative form quasispecies consortia (qs-c). They are the essential agents that constitute the possibility of evolution of biological group identity. Finally, the basic interactional motifs for the emergence of group identity, communication, and co-operation-together with its opposing functions-are explained by the "Gangen" hypothesis.

Using archaeogenomic and computational approaches to unravel the history of local adaptation in crops

Our understanding of the evolution of domestication has changed radically in the past 10 years, from a relatively simplistic rapid origin scenario to a protracted complex process in which plants adapted to the human environment. The adaptation of plants continued as the human environment changed with the expansion of agriculture from its centres of origin. Using archaeogenomics and computational models, we can observe genome evolution directly and understand how plants adapted to the human environment and the regional conditions to which agriculture expanded. We have applied various archaeogenomics approaches as exemplars to study local adaptation of barley to drought resistance at Qasr Ibrim, Egypt. We show the utility of DNA capture, ancient RNA, methylation patterns and DNA from charred remains of archaeobotanical samples from low latitudes where preservation conditions restrict ancient DNA research to within a Holocene timescale. The genomic level of analyses that is now possible, and the complexity of the evolutionary process of local adaptation means that plant studies are set to move to the genome level, and account for the interaction of genes under selection in systems-level approaches. This way we can understand how plants adapted during the expansion of agriculture across many latitudes with rapidity.

Archaeogenomic insights into the adaptation of plants to the human environment: pushing plant-hominin co-evolution back to the Pliocene

The colonization of the human environment by plants, and the consequent evolution of domesticated forms is increasingly being viewed as a co-evolutionary plant-human process that occurred over a long time period, with evidence for the co-evolutionary relationship between plants and humans reaching ever deeper into the hominin past. This developing view is characterized by a change in emphasis on the drivers of evolution in the case of plants. Rather than individual species being passive recipients of artificial selection pressures and ultimately becoming domesticates, entire plant communities adapted to the human environment. This evolutionary scenario leads to systems level genetic expectations from models that can be explored through ancient DNA and Next Generation Sequencing approaches. Emerging evidence suggests that domesticated genomes fit well with these expectations, with periods of stable complex evolution characterized by large amounts of change associated with relatively small selective value, punctuated by periods in which changes in one-half of the plant-hominin relationship cause rapid, low-complexity adaptation in the other. A corollary of a single plant-hominin co-evolutionary process is that clues about the initiation of the domestication process may well lie deep within the hominin lineage.

Force for ancient and recent life: viral and stem-loop RNA consortia promote life

Lytic viruses were thought to kill the most numerous host (i.e., kill the winner). But persisting viruses/defectives can also protect against viruses, especially in a ubiquitous virosphere. In 1991, Yarmolinsky et al. discovered the addiction modules of P1 phage, in which opposing toxic and protective functions stabilize persistence. Subsequently, I proposed that lytic and persisting cryptic virus also provide addiction modules that promote group identity. In eukaryotes (and the RNA world), a distinct RNA virus-host relationship exists. Retrovirurses/retroposons are major contributors to eukaryotic genomes. Eukaryotic complexity appears to be mostly mediated by regulatory complexity involving noncoding retroposon-derived RNA. RNA viruses evolve via quasispecies, which contain cooperating, minority, and even opposing RNA types. Quasispecies can also demonstrate group preclusion (e.g., hepatitis C). Stem-loop RNA domains are found in long terminal repeats (and viral RNA) and mediate viral regulation/identity. Thus, stem-loop RNAs may be ancestral regulators. I consider the RNA (ribozyme) world scenario from the perspective of addiction modules and cooperating quasispecies (i.e., subfunctional agents that establish group identity). Such an RNA collective resembles a "gang" but requires the simultaneous emergence of endonuclease, ligase, cooperative catalysis, group identity, and history markers (RNA). I call such a collective a gangen (pathway to gang) needed for life to emerge.

LTR retroelements are intrinsic components of transcriptional networks in frogs

Background

LTR retroelements (LTR REs) constitute a major group of transposable elements widely distributed in eukaryotic genomes. Through their own mechanism of retrotranscription LTR REs enrich the genomic landscape by providing genetic variability, thus contributing to genome structure and organization. Nonetheless, transcriptomic activity of LTR REs still remains an obscure domain within cell, developmental, and organism biology.

Results

Here we present a first comparative analysis of LTR REs for anuran amphibians based on a full depth coverage transcriptome of the European pool frog, Pelophylax lessonae, the genome of the African clawed frog, Silurana tropicalis (release v7.1), and additional transcriptomes of S. tropicalis and Cyclorana alboguttata. We identified over 1000 copies of LTR REs from all four families (Bel/Pao, Ty1/Copia, Ty3/Gypsy, Retroviridae) in the genome of S. tropicalis and discovered transcripts of several of these elements in all RNA-seq datasets analyzed. Elements of the Ty3/Gypsy family were most active, especially Amn-san elements, which accounted for approximately 0.27% of the genome in Silurana. Some elements exhibited tissue specific expression patterns, for example Hydra1.1 and MuERV-like elements in Pelophylax. In S. tropicalis considerable transcription of LTR REs was observed during embryogenesis as soon as the embryonic genome became activated, i.e. at midblastula transition. In the course of embryonic development the spectrum of transcribed LTR REs changed; during gastrulation and neurulation MuERV-like and SnRV like retroviruses were abundantly transcribed while during organogenesis transcripts of the XEN1 retroviruses became much more active.

Conclusions

The differential expression of LTR REs during embryogenesis in concert with their tissue-specificity and the protein domains they encode are evidence for the functional roles these elements play as integrative parts of complex regulatory networks. Our results support the meanwhile widely accepted concept that retroelements are not simple “junk DNA” or “harmful genomic parasites” but essential components of the transcriptomic machinery in vertebrates.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-626) contains supplementary material, which is available to authorized users.

Transposable elements: powerful contributors to angiosperm evolution and diversity

Transposable elements (TEs) are a dominant feature of most flowering plant genomes. Together with other accepted facilitators of evolution, accumulating data indicate that TEs can explain much about their rapid evolution and diversification. Genome size in angiosperms is highly correlated with TE content and the overwhelming bulk (>80%) of large genomes can be composed of TEs. Among retro-TEs, long terminal repeats (LTRs) are abundant, whereas DNA-TEs, which are often less abundant than retro-TEs, are more active. Much adaptive or evolutionary potential in angiosperms is due to the activity of TEs (active TE-Thrust), resulting in an extraordinary array of genetic changes, including gene modifications, duplications, altered expression patterns, and exaptation to create novel genes, with occasional gene disruption. TEs implicated in the earliest origins of the angiosperms include the exapted Mustang, Sleeper, and Fhy3/Far1 gene families. Passive TE-Thrust can create a high degree of adaptive or evolutionary potential by engendering ectopic recombination events resulting in deletions, duplications, and karyotypic changes. TE activity can also alter epigenetic patterning, including that governing endosperm development, thus promoting reproductive isolation. Continuing evolution of long-lived resprouter angiosperms, together with genetic variation in their multiple meristems, indicates that TEs can facilitate somatic evolution in addition to germ line evolution. Critical to their success, angiosperms have a high frequency of polyploidy and hybridization, with resultant increased TE activity and introgression, and beneficial gene duplication. Together with traditional explanations, the enhanced genomic plasticity facilitated by TE-Thrust, suggests a more complete and satisfactory explanation for Darwin's "abominable mystery": the spectacular success of the angiosperms.