Telomerase lost?

Extended longevity of termite kings and queens is accompanied by extranuclear localization of telomerase in somatic organs and caste-specific expression of its isoforms

Kings and queens of termites are endowed with an extraordinary longevity coupled with lifelong fecundity. We recently reported that termite kings and queens display a dramatically increased enzymatic activity and abundance of telomerase in their somatic organs when compared to short-lived workers and soldiers. We hypothesized that this telomerase activation may represent a noncanonical pro-longevity function, independent of its canonical role in telomere maintenance. Here, we explore this avenue and investigate whether the presumed noncanonical role of telomerase may be due to alternative splicing of the catalytic telomerase subunit TERT and whether the subcellular localization of TERT isoforms differs among organs and castes in the termite Prorhinotermes simplex. We empirically confirm the expression of four in silico predicted splice variants (psTERT1-A, psTERT1-B, psTERT2-A, psTERT2-B), defined by N-terminal splicing implicating differential localizations, and C-terminal splicing giving rise to full-length and truncated isoforms. We show that the transcript proportions of the psTERT are caste- and tissue-specific and that the extranuclear full-length isoform TERT1-A is relatively enriched in the soma of neotenic kings and queens compared to their gonads and to the soma of workers. We also show that extranuclear TERT protein quantities are significantly higher in the soma of kings and queens compared to workers, namely due to the cytosolic TERT. Independently, we confirm by microscopy the extranuclear TERT localization in somatic organs. We conclude that the presumed pleiotropic action of telomerase combining the canonical nuclear role in telomere maintenance with extranuclear functions is driven by complex TERT splicing.

Telomeres and telomerase: active but complex players in life-history decisions

Studies on human telomeres have established that telomeres exert a significant influence on lifespan and health of organisms. However, recent research has indicated that the original idea that telomeres affect lifespan in a universal and central manner across all eukaryotic species is an oversimplification. Indeed, findings from a variety of animal species revealed that the role of telomere biology in aging is more subtle and intricate than previously recognized. Here, we show how telomere biology varies depending on the taxon. We also show how telomere biology corresponds to basic life history traits and affects the life table of a species and investments in growth, body size, reproduction, and lifespan; telomeres are hypothesized to shape evolutionary perspectives for species in an active but complex manner. Our evaluation is based on telomere biology data from many examples from throughout the animal kingdom that vary according to the degree of organismal complexity and life history strategies.

Unzipped genome assemblies of polyploid root-knot nematodes reveal unusual and clade-specific telomeric repeats

Using long-read sequencing, we assembled and unzipped the polyploid genomes of Meloidogyne incognita, M. javanica and M. arenaria, three of the most devastating plant-parasitic nematodes. We found the canonical nematode telomeric repeat to be missing in these and other Meloidogyne genomes. In addition, we find no evidence for the enzyme telomerase or for orthologs of C. elegans telomere-associated proteins, suggesting alternative lengthening of telomeres. Instead, analyzing our assembled genomes, we identify species-specific composite repeats enriched mostly at one extremity of contigs. These repeats are G-rich, oriented, and transcribed, similarly to canonical telomeric repeats. We confirm them as telomeric using fluorescent in situ hybridization. These repeats are mostly found at one single end of chromosomes in these species. The discovery of unusual and specific complex telomeric repeats opens a plethora of perspectives and highlights the evolutionary diversity of telomeres despite their central roles in senescence, aging, and chromosome integrity.

Retrotransposons and Telomeres

Transposable elements (TEs) comprise a significant part of eukaryotic genomes being a major source of genome instability and mutagenesis. Cellular defense systems suppress the TE expansion at all stages of their life cycle. Piwi proteins and Piwi-interacting RNAs (piRNAs) are key elements of the anti-transposon defense system, which control TE activity in metazoan gonads preventing inheritable transpositions and developmental defects. In this review, we discuss various regulatory mechanisms by which small RNAs combat TE activity. However, active transposons persist, suggesting these powerful anti-transposon defense mechanisms have a limited capacity. A growing body of evidence suggests that increased TE activity coincides with genome reprogramming and telomere lengthening in different species. In the Drosophila fruit fly, whose telomeres consist only of retrotransposons, a piRNA-mediated mechanism is required for telomere maintenance and their length control. Therefore, the efficacy of protective mechanisms must be finely balanced in order not only to suppress the activity of transposons, but also to maintain the proper length and stability of telomeres. Structural and functional relationship between the telomere homeostasis and LINE1 retrotransposon in human cells indicates a close link between selfish TEs and the vital structure of the genome, telomere. This relationship, which permits the retention of active TEs in the genome, is reportedly a legacy of the retrotransposon origin of telomeres. The maintenance of telomeres and the execution of other crucial roles that TEs acquired during the process of their domestication in the genome serve as a type of payment for such a "service."

Unveiling metabolic integration in psyllids and their nutritional endosymbionts through comparative transcriptomics analysis

Psyllids, a group of insects that feed on plant sap, have a symbiotic relationship with an endosymbiont called Carsonella. Carsonella synthesizes essential amino acids and vitamins for its psyllid host, but lacks certain genes required for this process, suggesting a compensatory role of psyllid host genes. To investigate this, gene expression was compared between two psyllid species, Bactericera cockerelli and Diaphorina citri, in specialized cells where Carsonella resides (bacteriomes). Collaborative psyllid genes, including horizontally transferred genes, showed patterns of conserved gene expression; however, species-specific patterns were also observed, suggesting differences in the nutritional metabolism between psyllid species. Also, the recycling of nitrogen in bacteriomes may primarily rely on glutamate dehydrogenase (GDH). Additionally, lineage-specific gene clusters were differentially expressed in B. cockerelli and D. citri bacteriomes and are highlighted here. These findings shed light on potential host adaptations for the regulation of this symbiosis due to host, microbiome, and environmental differences.

Expanding the Chromosomal Evolution Understanding of Lygaeioid True Bugs (Lygaeoidea, Pentatomomorpha, Heteroptera) by Classical and Molecular Cytogenetic Analysis

The Lygaeoidea comprise about 4660 in 790 genera and 16 families. Using standard chromosome staining and FISH with 18S rDNA and telomeric (TTAGG)n probes, we studied male karyotypes and meiosis in 10 species of Lygaeoidea belonging to eight genera of the families Blissidae, Cymidae, Heterogastridae, Lygaeidae, and Rhyparochromidae. Chromosome numbers were shown to range from 12 to 28, with 2n = 14 being predominant. All species have an XY system and all but one has a pair of m-chromosomes. The exception is Spilostethus saxatilis (Lygaeidae: Lygaeinae); in another species of Lygaeinae, Thunbergia floridulus, m-chromosomes were present, which represent the first finding for this subfamily. All species have an inverted sequence of sex chromosome divisions (“post-reduction”). The 18S rDNA loci were observed on one or both sex chromosomes in Kleidocerys resedae and Th. floridulus, respectively (Lygaeidae), while on an autosomal bivalent in all other species. The rDNA loci tended to be close to the end of the chromosome. Using (TTAGG)n—FISH, we were able to show for the first time that the Lygaeoidea lack the canonical “insect” telomere motif (TTAGG)n. We speculate that this ancestral motif is absent from the entire infraorder Pentatomomorpha being replaced by some other telomere repeat motif sequences.

Incompatibility and Interchangeability in Molecular Evolution

There is remarkable variation in the rate at which genetic incompatibilities in molecular interactions accumulate. In some cases, minor changes-even single-nucleotide substitutions-create major incompatibilities when hybridization forces new variants to function in a novel genetic background from an isolated population. In other cases, genes or even entire functional pathways can be horizontally transferred between anciently divergent evolutionary lineages that span the tree of life with little evidence of incompatibilities. In this review, we explore whether there are general principles that can explain why certain genes are prone to incompatibilities while others maintain interchangeability. We summarize evidence pointing to four genetic features that may contribute to greater resistance to functional replacement: (1) function in multisubunit enzyme complexes and protein-protein interactions, (2) sensitivity to changes in gene dosage, (3) rapid rate of sequence evolution, and (4) overall importance to cell viability, which creates sensitivity to small perturbations in molecular function. We discuss the relative levels of support for these different hypotheses and lay out future directions that may help explain the striking contrasts in patterns of incompatibility and interchangeability throughout the history of molecular evolution.

Telomerase RNA in Hymenoptera (Insecta) switched to plant/ciliate-like biogenesis

In contrast to the catalytic subunit of telomerase, its RNA subunit (TR) is highly divergent in size, sequence and biogenesis pathways across eukaryotes. Current views on TR evolution assume a common origin of TRs transcribed with RNA polymerase II in Opisthokonta (the supergroup including Animalia and Fungi) and Trypanosomida on one hand, and TRs transcribed with RNA polymerase III under the control of type 3 promoter, found in TSAR and Archaeplastida supergroups (including e.g. ciliates and Viridiplantae taxa, respectively). Here, we focus on unknown TRs in one of the largest Animalia order - Hymenoptera (Arthropoda) with more than 300 available representative genomes. Using a combination of bioinformatic and experimental approaches, we identify their TRs. In contrast to the presumed type of TRs (H/ACA box snoRNAs transcribed with RNA Polymerase II) corresponding to their phylogenetic position, we find here short TRs of the snRNA type, likely transcribed with RNA polymerase III under the control of the type 3 promoter. The newly described insect TRs thus question the hitherto assumed monophyletic origin of TRs across Animalia and point to an evolutionary switch in TR type and biogenesis that was associated with the divergence of Arthropods.

The Epigenetic Control of the Transposable Element Life Cycle in Plant Genomes and Beyond

Within the life cycle of a living organism, another life cycle exists for the selfish genome inhabitants, which are called transposable elements (TEs). These mobile sequences invade, duplicate, amplify, and diversify within a genome, increasing the genome's size and generating new mutations. Cells act to defend their genome, but rather than permanently destroying TEs, they use chromatin-level repression and epigenetic inheritance to silence TE activity. This level of silencing is ephemeral and reversible, leading to a dynamic equilibrium between TE suppression and reactivation within a host genome. The coexistence of the TE and host genome can also lead to the domestication of the TE to serve in host genome evolution and function. In this review, we describe the life cycle of a TE, with emphasis on how epigenetic regulation is harnessed to control TEs for host genome stability and innovation.

Telomeres and Their Neighbors

Telomeres are essential structures formed from satellite DNA repeats at the ends of chromosomes in most eukaryotes. Satellite DNA repeat sequences are useful markers for karyotyping, but have a more enigmatic role in the eukaryotic cell. Much work has been done to investigate the structure and arrangement of repetitive DNA elements in classical models with implications for species evolution. Still more is needed until there is a complete picture of the biological function of DNA satellite sequences, particularly when considering non-model organisms. Celebrating Gregor Mendel’s anniversary by going to the roots, this review is designed to inspire and aid new research into telomeres and satellites with a particular focus on non-model organisms and accessible experimental and in silico methods that do not require specialized equipment or expensive materials. We describe how to identify telomere (and satellite) repeats giving many examples of published (and some unpublished) data from these techniques to illustrate the principles behind the experiments. We also present advice on how to perform and analyse such experiments, including details of common pitfalls. Our examples are a selection of recent developments and underexplored areas of research from the past. As a nod to Mendel’s early work, we use many examples from plants and insects, especially as much recent work has expanded beyond the human and yeast models traditional in telomere research. We give a general introduction to the accepted knowledge of telomere and satellite systems and include references to specialized reviews for the interested reader.

The nanoCUT&RUN technique visualizes telomeric chromatin in Drosophila

Advances in genomic technology led to a more focused pattern for the distribution of chromosomal proteins and a better understanding of their functions. The recent development of the CUT&RUN technique marks one of the important such advances. Here we develop a modified CUT&RUN technique that we termed nanoCUT&RUN, in which a high affinity nanobody to GFP is used to bring micrococcal nuclease to the binding sites of GFP-tagged chromatin proteins. Subsequent activation of the nuclease cleaves the chromatin, and sequencing of released DNA identifies binding sites. We show that nanoCUT&RUN efficiently produces high quality data for the TRL transcription factor in Drosophila embryos, and distinguishes binding sites specific between two TRL isoforms. We further show that nanoCUT&RUN dissects the distributions of the HipHop and HOAP telomere capping proteins, and uncovers unexpected binding of telomeric proteins at centromeres. nanoCUT&RUN can be readily applied to any system in which a chromatin protein of interest, or its isoforms, carries the GFP tag.

The method of chromatin immunoprecipitation followed by genomic sequencing (ChIP-seq) has been employed to study the distribution of chromatin binding proteins genome-wide. Such studies have greatly enhanced our understanding of the function of the target proteins. However, the uses of chemical crosslinking combined with the procedure of antibody-medicated precipitation of the protein-DNA complex have limited the efficiency of ChIP-seq. The recently developed CUT&RUN method has greatly improved that efficiency. We here developed the “nanoCUT&RUN” extension of CUT&RUN, which can be readily applied to any target protein with a GFP tag. Using nanoCUT&RUN, we profiled the HipHop and HOAP proteins that protect telomeric chromatin in Drosophila. We uncovered sequence-independent binding of both proteins predominantly to telomeres. Interestingly, HipHop binding can also be detected at centromeric chromatin suggestive of a novel function of a telomere capping protein.

The Role of Transposable Elements of the Human Genome in Neuronal Function and Pathology

Transposable elements (TEs) have been extensively studied for decades. In recent years, the introduction of whole-genome and whole-transcriptome approaches, as well as single-cell resolution techniques, provided a breakthrough that uncovered TE involvement in host gene expression regulation underlying multiple normal and pathological processes. Of particular interest is increased TE activity in neuronal tissue, and specifically in the hippocampus, that was repeatedly demonstrated in multiple experiments. On the other hand, numerous neuropathologies are associated with TE dysregulation. Here, we provide a comprehensive review of literature about the role of TEs in neurons published over the last three decades. The first chapter of the present review describes known mechanisms of TE interaction with host genomes in general, with the focus on mammalian and human TEs; the second chapter provides examples of TE exaptation in normal neuronal tissue, including TE involvement in neuronal differentiation and plasticity; and the last chapter lists TE-related neuropathologies. We sought to provide specific molecular mechanisms of TE involvement in neuron-specific processes whenever possible; however, in many cases, only phenomenological reports were available. This underscores the importance of further studies in this area.

Profiles of telomeric repeats in Insecta reveal diverse forms of telomeric motifs in Hymenopterans

Telomeres consist of highly conserved simple tandem telomeric repeat motif (TRM): (TTAGG)n in arthropods, (TTAGGG)n in vertebrates, and (TTTAGGG)n in most plants. TRM can be detected from chromosome-level assembly, which typically requires long-read sequencing data. To take advantage of short-read data, we developed an ultra-fast Telomeric Repeats Identification Pipeline and evaluated its performance on 91 species. With proven accuracy, we applied Telomeric Repeats Identification Pipeline in 129 insect species, using 7 Tbp of short-read sequences. We confirmed (TTAGG)n as the TRM in 19 orders, suggesting it is the ancestral form in insects. Systematic profiling in Hymenopterans revealed a diverse range of TRMs, including the canonical 5-bp TTAGG (bees, ants, and basal sawflies), three independent losses of tandem repeat form TRM (Ichneumonoids, hunting wasps, and gall-forming wasps), and most interestingly, a common 8-bp (TTATTGGG)n in Chalcid wasps with two 9-bp variants in the miniature wasp (TTACTTGGG) and fig wasps (TTATTGGGG). Our results identified extraordinary evolutionary fluidity of Hymenopteran TRMs, and rapid evolution of TRM and repeat abundance at all evolutionary scales, providing novel insights into telomere evolution.

Effect of açai supplementation (Euterpe Oleracea Mart.) associated with exercise in animals and human: a scoping review

ABSTRACT Objective This scoping review aimed to map evidence on açai supplementation combined with exercise in animal and/or human experimental studies. Methods The search considered six electronic databases and screening of relevant references. The selection process and data extraction were performed by two independent authors. The study characteristics, and AS (e.g., form, intervention time, amount ingested) and exercise (e.g., types, intensity, and duration) strategies were summarized, as well as their reported results. Results From an initial total of 342 studies identified; 11 (5 with animal and 6 with human models) were eligible. In animals, açai supplementation and exercise led to benefits in exercise tolerance and improvements in several hemodynamic parameters, as well as significant improvements in liver markers and glucose metabolism. In humans, açai supplementation indicated positive results in increasing exhaustion time to 90% of VO2max and increasing intensity at the anaerobic threshold. Conclusion We conclude that future research involving animals and humans should examine açai supplementation and exercise with (a) obesity models to test the effect of adiponectin on body composition with analysis of histological and histochemical parameters; (b) eccentric injury protocols with the incorporation of muscle quality variables to assess recovery; (c) chronic açai supplementation and strength training; (d) comparison of different forms of açai supplementation in exercise protocols.

Chromosome-level genome assembly reveals homologous chromosomes and recombination in asexual rotifer Adineta vaga

Bdelloid rotifers are notorious as a speciose ancient clade comprising only asexual lineages. Thanks to their ability to repair highly fragmented DNA, most bdelloid species also withstand complete desiccation and ionizing radiation. Producing a well-assembled reference genome is a critical step to developing an understanding of the effects of long-term asexuality and DNA breakage on genome evolution. To this end, we present the first high-quality chromosome-level genome assemblies for the bdelloid Adineta vaga, composed of six pairs of homologous (diploid) chromosomes with a footprint of paleotetraploidy. The observed large-scale losses of heterozygosity are signatures of recombination between homologous chromosomes, either during mitotic DNA double-strand break repair or when resolving programmed DNA breaks during a modified meiosis. Dynamic subtelomeric regions harbor more structural diversity (e.g., chromosome rearrangements, transposable elements, and haplotypic divergence). Our results trigger the reappraisal of potential meiotic processes in bdelloid rotifers and help unravel the factors underlying their long-term asexual evolutionary success.

Telomeric DNA sequences in beetle taxa vary with species richness

Telomeres are protective structures at the ends of eukaryotic chromosomes, and disruption of their nucleoprotein composition usually results in genome instability and cell death. Telomeric DNA sequences have generally been found to be exceptionally conserved in evolution, and the most common pattern of telomeric sequences across eukaryotes is (TxAyGz)n maintained by telomerase. However, telomerase-added DNA repeats in some insect taxa frequently vary, show unusual features, and can even be absent. It has been speculated about factors that might allow frequent changes in telomere composition in Insecta. Coleoptera (beetles) is the largest of all insect orders and based on previously available data, it seemed that the telomeric sequence of beetles varies to a great extent. We performed an extensive mapping of the (TTAGG)n sequence, the ancestral telomeric sequence in Insects, across the main branches of Coleoptera. Our study indicates that the (TTAGG)n sequence has been repeatedly or completely lost in more than half of the tested beetle superfamilies. Although the exact telomeric motif in most of the (TTAGG)n-negative beetles is unknown, we found that the (TTAGG)n sequence has been replaced by two alternative telomeric motifs, the (TCAGG)n and (TTAGGG)n, in at least three superfamilies of Coleoptera. The diversity of the telomeric motifs was positively related to the species richness of taxa, regardless of the age of the taxa. The presence/absence of the (TTAGG)n sequence highly varied within the Curculionoidea, Chrysomeloidea, and Staphylinoidea, which are the three most diverse superfamilies within Metazoa. Our data supports the hypothesis that telomere dysfunctions can initiate rapid genomic changes that lead to reproductive isolation and speciation.

Long-read sequencing and de novo genome assemblies reveal complex chromosome end structures caused by telomere dysfunction at the single nucleotide level

Karyotype change and subsequent evolution is triggered by chromosome fusion and rearrangement events, which often occur when telomeres become dysfunctional. Telomeres protect linear chromosome ends from DNA damage responses (DDRs), and telomere dysfunction may result in genome instability. However, the complex chromosome end structures and the other possible consequences of telomere dysfunction have rarely been resolved at the nucleotide level due to the lack of the high-throughput methods needed to analyse these highly repetitive regions. Here we applied long-read sequencing technology to Caenorhabditis elegans survivor lines that emerged after telomere dysfunction. The survivors have preserved traces of DDRs in their genomes and our data revealed that variants generated by telomere dysfunction are accumulated along all chromosomes. The reconstruction of the chromosome end structures through de novo genome assemblies revealed diverse types of telomere damage processing at the nucleotide level. When telomeric repeats were totally eroded by telomere dysfunction, DDRs were mostly terminated by chromosome fusion events. We also partially reconstructed the most complex end structure and its DDR signatures, which would have been accumulated via multiple cell divisions. These finely resolved chromosome end structures suggest possible mechanisms regarding the repair processes after telomere dysfunction, providing insights into chromosome evolution in nature.

Long-lived termite kings and queens activate telomerase in somatic organs

Kings and queens of termites, like queens of other advanced eusocial insects, are endowed with admirable longevity, which dramatically exceeds the life expectancies of their non-reproducing nest-mates and related solitary insects. In the quest to find the mechanisms underlying the longevity of termite reproductives, we focused on somatic maintenance mediated by telomerase. This ribonucleoprotein is well established for pro-longevity functions in vertebrates, thanks primarily to its ability of telomere extension. However, its participation in lifespan regulation of insects, including the eusocial taxa, remains understudied. Here, we report a conspicuous increase of telomerase abundance and catalytic activity in the somatic organs of primary and secondary reproductives of the termite Prorhinotermes simplex and confirm a similar pattern in two other termite species. These observations stand in contrast with the telomerase downregulation characteristic for most adult somatic tissues in vertebrates and also in solitary insects and non-reproducing castes of termites. At the same time, we did not observe caste-specific differences in telomere lengths that might explain the differential longevity of termite castes. We conclude that although the telomerase activation in termite reproductives is in line with the broadly assumed association between telomerase and longevity, its direct phenotypic impact remains to be elucidated.

Omics Tools Applied to the Study of Chagas Disease Vectors: Cytogenomics and Genomics

Chagas disease is an illness caused by the protozoan Trypanosoma cruzi that is distributed in 21 countries of Latin America. The main way of transmission of T. cruzi is through the feces of triatomines (Hemiptera and Triatominae) infected with the parasite. With technological advances came new technologies called omics. In the pre-genomic era, the omics science was based on cytogenomic studies of triatomines. With the Rhodnius prolixus genome sequencing project, new omics tools were applied to understand the organism at a systemic level and not just from a genomic point of view. Thus, the present review aims to put together the cytogenomic and genomic information available in the literature for Chagas disease vectors. Here, we review all studies related to cytogenomics and genomics of Chagas disease vectors, contributing to the direction of further research with these insect vectors, because it was evident that most studies focus on cytogenomic knowledge of the species. Given the importance of genomic studies, which contributed to the knowledge of taxonomy, systematics, as well as the vector's biology, the need to apply these techniques in other genera and species of Triatominae subfamily is emphasized.

Rapid evolution at the Drosophila telomere: transposable element dynamics at an intrinsically unstable locus

Drosophila telomeres have been maintained by three families of active transposable elements (TEs), HeT-A, TAHRE, and TART, collectively referred to as HTTs, for tens of millions of years, which contrasts with an unusually high degree of HTT interspecific variation. While the impacts of conflict and domestication are often invoked to explain HTT variation, the telomeres are unstable structures such that neutral mutational processes and evolutionary tradeoffs may also drive HTT evolution. We leveraged population genomic data to analyze nearly 10,000 HTT insertions in 85  Drosophila melanogaster genomes and compared their variation to other more typical TE families. We observe that occasional large-scale copy number expansions of both HTTs and other TE families occur, highlighting that the HTTs are, like their feral cousins, typically repressed but primed to take over given the opportunity. However, large expansions of HTTs are not caused by the runaway activity of any particular HTT subfamilies or even associated with telomere-specific TE activity, as might be expected if HTTs are in strong genetic conflict with their hosts. Rather than conflict, we instead suggest that distinctive aspects of HTT copy number variation and sequence diversity largely reflect telomere instability, with HTT insertions being lost at much higher rates than other TEs elsewhere in the genome. We extend previous observations that telomere deletions occur at a high rate, and surprisingly discover that more than one-third do not appear to have been healed with an HTT insertion. We also report that some HTT families may be preferentially activated by the erosion of whole telomeres, implying the existence of HTT-specific host control mechanisms. We further suggest that the persistent telomere localization of HTTs may reflect a highly successful evolutionary strategy that trades away a stable insertion site in order to have reduced impact on the host genome. We propose that HTT evolution is driven by multiple processes, with niche specialization and telomere instability being previously underappreciated and likely predominant.

Seasonality in telomerase activity in relation to cell size, DNA replication, and nutrients in the fat body of Apis mellifera

In honeybees (Apis mellifera), the rate of aging is modulated through social interactions and according to caste differentiation and the seasonal (winter/summer) generation of workers. Winter generation workers, which hatch at the end of summer, have remarkably extended lifespans as an adaptation to the cold season when the resources required for the growth and reproduction of colonies are limited and the bees need to maintain the colony until the next spring. In contrast, the summer bees only live for several weeks. To better understand the lifespan differences between summer and winter bees, we studied the fat bodies of honeybee workers and identified several parameters that fluctuate in a season-dependent manner. In agreement with the assumption that winter workers possess greater fat body mass, our data showed gradual increases in fat body mass, the size of the fat body cells, and Vg production as the winter season proceeded, as well as contrasting gradual decreases in these parameters in the summer season. The differences in the fat bodies between winter and summer bees are accompanied by respective increases and decreases in telomerase activity and DNA replication in the fat bodies. These data show that although the fat bodies of winter bees differ significantly from those of summer bees, these differences are not a priori set when bees hatch at the end of summer or in early autumn but instead gradually evolve over the course of the season, depending on environmental factors.

Chromosome End Diversification in Sciarid Flies

Background: Dipterans exhibit a remarkable diversity of chromosome end structures in contrast to the conserved system defined by telomerase and short repeats. Within dipteran families, structure of chromosome termini is usually conserved within genera. With the aim to assess whether or not the evolutionary distance between genera implies chromosome end diversification, this report exploits two representatives of Sciaridae, Rhynchosciara americana, and Trichomegalosphyspubescens. Methods: Probes and plasmid microlibraries obtained by chromosome end microdissection, in situ hybridization, cloning, and sequencing are among the methodological approaches employed in this work. Results: The data argue for the existence of either specific terminal DNA sequences for each chromosome tip in T. pubescens, or sequences common to all chromosome ends but their extension does not allow detection by in situ hybridization. Both sciarid species share terminal sequences that are significantly underrepresented in chromosome ends of T. pubescens. Conclusions: The data suggest an unusual terminal structure in T. pubescens chromosomes compared to other dipterans investigated. A putative, evolutionary process of repetitive DNA expansion that acted differentially to shape chromosome ends of the two flies is also discussed.

Composition and Function of Telomerase-A Polymerase Associated with the Origin of Eukaryotes

The canonical DNA polymerases involved in the replication of the genome are unable to fully replicate the physical ends of linear chromosomes, called telomeres. Chromosomal termini thus become shortened in each cell cycle. The maintenance of telomeres requires telomerase—a specific RNA-dependent DNA polymerase enzyme complex that carries its own RNA template and adds telomeric repeats to the ends of chromosomes using a reverse transcription mechanism. Both core subunits of telomerase—its catalytic telomerase reverse transcriptase (TERT) subunit and telomerase RNA (TR) component—were identified in quick succession in Tetrahymena more than 30 years ago. Since then, both telomerase subunits have been described in various organisms including yeasts, mammals, birds, reptiles and fish. Despite the fact that telomerase activity in plants was described 25 years ago and the TERT subunit four years later, a genuine plant TR has only recently been identified by our group. In this review, we focus on the structure, composition and function of telomerases. In addition, we discuss the origin and phylogenetic divergence of this unique RNA-dependent DNA polymerase as a witness of early eukaryotic evolution. Specifically, we discuss the latest information regarding the recently discovered TR component in plants, its conservation and its structural features.

Silence at the End: How Drosophila Regulates Expression and Transposition of Telomeric Retroelements

The maintenance of chromosome ends in Drosophila is an exceptional phenomenon because it relies on the transposition of specialized retrotransposons rather than on the activity of the enzyme telomerase that maintains telomeres in almost every other eukaryotic species. Sequential transpositions of Het-A, TART, and TAHRE (HTT) onto chromosome ends produce long head-to-tail arrays that are reminiscent to the long arrays of short repeats produced by telomerase in other organisms. Coordinating the activation and silencing of the HTT array with the recruitment of telomere capping proteins favors proper telomere function. However, how this coordination is achieved is not well understood. Like other Drosophila retrotransposons, telomeric elements are regulated by the piRNA pathway. Remarkably, HTT arrays are both source of piRNA and targets of gene silencing thus making the regulation of Drosophila telomeric transposons a unique event among eukaryotes. Herein we will review the genetic and molecular mechanisms underlying the regulation of HTT transcription and transposition and will discuss the possibility of a crosstalk between piRNA-mediated regulation, telomeric chromatin establishment, and telomere protection.

Essential factors involved in the precise targeting and insertion of telomere-specific non-LTR retrotransposon, SART1Bm

Telomere length maintenance is essential for most eukaryotes to ensure genome stability and integrity. A non-long terminal repeat (LTR) retrotransposon, SART1Bm, targets telomeric repeats (TTAGG)n of the silkworm Bombyx mori and is presumably involved in telomere length maintenance. However, how many telomeric repeats are required for its retrotransposition and how reverse transcription is initiated at the target site are not well understood. Here, using an ex vivo and trans-in vivo recombinant baculovirus retrotransposition system, we demonstrated that SART1Bm requires at least three (TTAGG) telomeric repeats and a longer poly(A) tail for its accurate retrotransposition. We found that SART1Bm retrotransposed only in the third (TTAGG) tract of three repeats and that the A residue of the (TTAGG) unit was essential for its retrotransposition. Interestingly, SART1Bm also retrotransposed into telomeric repeats of other species, such as human (TTAGGG)n repeats, albeit with low retrotransposition efficiency. We further showed that the reverse transcription of SART1Bm occurred inaccurately at the internal site of the 3' untranslated region (UTR) when using a short poly(A) tail but at the accurate site when using a longer poly(A) tail. These findings promote our understanding of the general mechanisms of site-specific retrotransposition and aid the development of a site-specific gene knock-in tool.

Origin, Diversity, and Evolution of Telomere Sequences in Plants

Telomeres are basic structures of eukaryote genomes. They distinguish natural chromosome ends from double-stranded breaks in DNA and protect chromosome ends from degradation or end-to-end fusion with other chromosomes. Telomere sequences are usually tandemly arranged minisatellites, typically following the formula (TxAyGz)n. Although they are well conserved across large groups of organisms, recent findings in plants imply that their diversity has been underestimated. Changes in telomeres are of enormous evolutionary importance as they can affect whole-genome stability. Even a small change in the telomere motif of each repeat unit represents an important interference in the system of sequence-specific telomere binding proteins. Here, we provide an overview of telomere sequences, considering the latest phylogenomic evolutionary framework of plants in the broad sense (Archaeplastida), in which new telomeric sequences have recently been found in diverse and economically important families such as Solanaceae and Amaryllidaceae. In the family Lentibulariaceae and in many groups of green algae, deviations from the typical plant telomeric sequence have also been detected recently. Ancestry and possible homoplasy in telomeric motifs, as well as extant gaps in knowledge are discussed. With the increasing availability of genomic approaches, it is likely that more telomeric diversity will be uncovered in the future. We also discuss basic methods used for telomere identification and we explain the implications of the recent discovery of plant telomerase RNA on further research about the role of telomerase in eukaryogenesis or on the molecular causes and consequences of telomere variability.

The Telomere Paradox: Stable Genome Preservation with Rapidly Evolving Proteins

Telomeres ensure chromosome length homeostasis and protection from catastrophic end-to-end chromosome fusions. All eukaryotes require this essential, strictly conserved telomere-dependent genome preservation. However, recent evolutionary analyses of mammals, plants, and flies report pervasive rapid evolution of telomere proteins. The causes of this paradoxical observation - that unconserved machinery underlies an essential, conserved function - remain enigmatic. Indeed, these fast-evolving telomere proteins bind, extend, and protect telomeric DNA, which itself evolves slowly in most systems. We hypothesize that the universally fast-evolving subtelomere - the telomere-adjacent, repetitive sequence - is a primary driver of the 'telomere paradox'. Under this model, radical sequence changes in the subtelomere perturb subtelomere-dependent, telomere functions. Compromised telomere function then spurs adaptation of telomere proteins to maintain telomere length homeostasis and protection. We propose an experimental framework that leverages both protein divergence and subtelomeric sequence divergence to test the hypothesis that subtelomere sequence evolution shapes recurrent innovation of telomere machinery.

Telomere-Specialized Retroelements in Drosophila: Adaptive Symbionts of the Genome, Neutral, or in Conflict?

Linear chromosomes shorten in every round of replication. In Drosophila, telomere-specialized long interspersed retrotransposable elements (LINEs) belonging to the jockey clade offset this shortening by forming head-to-tail arrays at Drosophila telomere ends. As such, these telomeric LINEs have been considered adaptive symbionts of the genome, protecting it from premature decay, particularly as Drosophila lacks a conventional telomerase holoenzyme. However, as reviewed here, recent work reveals a high degree of variation and turnover in the telomere-specialized LINE lineages across Drosophila. There appears to be no absolute requirement for LINE activity to maintain telomeres in flies, hence the suggestion that the telomere-specialized LINEs may instead be neutral or in conflict with the host, rather than adaptive.

Telomerase activity is upregulated in the fat bodies of pre-diapause bumblebee queens (Bombus terrestris)

The attrition of telomeres, the ends of eukaryote chromosomes, and activity of telomerase, the enzyme that restores telomere length, play a role in the ageing process and act as indicators of biological age. A notable feature of advanced eusocial insects is the longevity of reproductive individuals (queens and kings) compared to those from non-reproductive castes (workers and soldiers) within a given species, with a proposed link towards upregulation of telomerase activity in the somatic tissues of reproductive individuals. Given this, eusocial insects provide excellent model systems for research into ageing. We tested telomerase activity and measured telomere length in Bombus terrestris, which is a primitively eusocial insect species with several distinct features compared to advanced social insects. In somatic tissues, telomerase activity was upregulated only in the fat bodies of pre-diapause queens, and this upregulation was linked to heightened DNA synthesis. Telomere length was shorter in old queens compared to that in younger queens or workers. We speculate that (1) the upregulation of telomerase activity, together with DNA synthesis, is the essential step for intensifying metabolic activity in the fat body to build up a sufficient energy reserve prior to diapause, and that (2) the lifespan differences between B. terrestris workers and queens are related to the long diapause period of the queen. A possible relationship between telomere length regulation and TOR, FOXO, and InR as cell signaling components, was tested.

New evidence for the presence of the telomere motif (TTAGG) n in the family Reduviidae and its absence in the families Nabidae and Miridae (Hemiptera, Cimicomorpha)

Male karyotype and meiosis in four true bug species belonging to the families Reduviidae, Nabidae, and Miridae (Cimicomorpha) were studied for the first time using Giemsa staining and FISH with 18S ribosomal DNA and telomeric (TTAGG)_n probes. We found that Rhynocoris punctiventris (Herrich-Schäffer, 1846) and R. iracundus (Poda, 1761) (Reduviidae: Harpactorinae) had 2n = 28 (24 + X₁X₂X₃Y), whereas Nabis sareptanus Dohrn, 1862 (Nabidae) and Horistus orientalis (Gmelin, 1790) (Miridae) had 2n = 34 (32 + XY) and 2n = 32 (30 + XY), respectively. FISH for 18S rDNA revealed hybridization signals on a sex chromosome, the X or the Y, in H. orientalis, on both X and Y chromosomes in N. sareptanus, and on two of the four sex chromosomes, Y and one of the Xs, in both species of Rhynocoris Hahn, 1834. The results of FISH with telomeric probes support with confidence the absence of the "insect" telomere motif (TTAGG)_n in the families Nabidae and Miridae and its presence in both species of genus Rhynocoris of the Reduviidae, considered as a basal family of Cimicomorpha. Increasing evidence reinforces the hypothesis of the loss of the canonical "insect" telomere motif (TTAGG)_n by at least four cimicomorphan families, Nabidae, Miridae, Tingidae, and Cimicidae, for which data are currently available.

Long-read sequencing reveals intra-species tolerance of substantial structural variations and new subtelomere formation in C. elegans

Long-read sequencing technologies have contributed greatly to comparative genomics among species and can also be applied to study genomics within a species. In this study, to determine how substantial genomic changes are generated and tolerated within a species, we sequenced a C. elegans strain, CB4856, which is one of the most genetically divergent strains compared to the N2 reference strain. For this comparison, we used the Pacific Biosciences (PacBio) RSII platform (80×, N50 read length 11.8 kb) and generated de novo genome assembly to the level of pseudochromosomes containing 76 contigs (N50 contig = 2.8 Mb). We identified structural variations that affected as many as 2694 genes, most of which are at chromosome arms. Subtelomeric regions contained the most extensive genomic rearrangements, which even created new subtelomeres in some cases. The subtelomere structure of Chromosome VR implies that ancestral telomere damage was repaired by alternative lengthening of telomeres even in the presence of a functional telomerase gene and that a new subtelomere was formed by break-induced replication. Our study demonstrates that substantial genomic changes including structural variations and new subtelomeres can be tolerated within a species, and that these changes may accumulate genetic diversity within a species.

Diversification and collapse of a telomere elongation mechanism

In most eukaryotes, telomerase counteracts chromosome erosion by adding repetitive sequence to terminal ends. Drosophila melanogaster instead relies on specialized retrotransposons that insert exclusively at telomeres. This exchange of goods between host and mobile element-wherein the mobile element provides an essential genome service and the host provides a hospitable niche for mobile element propagation-has been called a "genomic symbiosis." However, these telomere-specialized, jockey family retrotransposons may actually evolve to "selfishly" overreplicate in the genomes that they ostensibly serve. Under this model, we expect rapid diversification of telomere-specialized retrotransposon lineages and, possibly, the breakdown of this ostensibly symbiotic relationship. Here we report data consistent with both predictions. Searching the raw reads of the 15-Myr-old melanogaster species group, we generated de novo jockey retrotransposon consensus sequences and used phylogenetic tree-building to delineate four distinct telomere-associated lineages. Recurrent gains, losses, and replacements account for this retrotransposon lineage diversity. In Drosophila biarmipes, telomere-specialized elements have disappeared completely. De novo assembly of long reads and cytogenetics confirmed this species-specific collapse of retrotransposon-dependent telomere elongation. Instead, telomere-restricted satellite DNA and DNA transposon fragments occupy its terminal ends. We infer that D. biarmipes relies instead on a recombination-based mechanism conserved from yeast to flies to humans. Telomeric retrotransposon diversification and disappearance suggest that persistently "selfish" machinery shapes telomere elongation across Drosophila rather than completely domesticated, symbiotic mobile elements.

How RNAi machinery enters the world of telomerase

Human telomerase holoenzyme consists of the catalytic component TERT and the template RNA TERC. However, a network of accessory proteins plays key roles in its assembly, localization and stability. Defects in genes involved in telomerase biology affect the renewal of critical stem cell populations and cause disorders such as telomeropathies. Moreover, activation of telomerase in somatic cells allows neoplastic cells to proliferate indefinitely, thus contributing to tumorigenesis. For these reasons, identification of new players involved in telomerase regulation is crucial for the determination of novel therapeutic targets and biomarkers. In the very last years, increasing evidence describes components of the RNAi machinery as a new layer of complexity in human telomerase activity. In this review, we will discuss how AGO2 and other proteins which collaborate with AGO2 in RNAi pathway play a pivotal role in TERC stability and function.

Presence of the canonical TTAGG insect telomeric repeat in the Tenthredinidae (Symphyta) suggests its ancestral nature in the order Hymenoptera

Telomeric repeats in two members of the sawfly family Tenthredinidae (Hymenoptera), namely, Tenthredo omissa (Förster, 1844) and Taxonus agrorum (Fallén, 1808) (both have n = 10), were studied using fluorescence in situ hybridization (FISH). Chromosomes of both species were demonstrated to contain the canonical TTAGG insect telomeric repeat, which constitutes the first report of the (TTAGG)_n telomeric motif for the Tenthredinidae as well as for the clade Eusymphyta and the suborder Symphyta in general. Taken together with the presence of this repeat in many other Holometabola as well as in the hymenopteran families Formicidae and Apidae from the suborder Apocrita, these results collectively suggest the ancestral nature of the (TTAGG)_n telomeric motif in the Hymenoptera as well as its subsequent loss within the clade Unicalcarida and independent reappearance in ants and bees. If this is true, the loss of the TTAGG repeat can be considered as a synapomorphy of the corresponding clade.

Chronic low-dose pro-oxidant treatment stimulates transcriptional activity of telomeric retroelements and increases telomere length in Drosophila

It has been proposed that oxidative stress, elicited by high levels of reactive oxygen species, accelerates telomere shortening by erosion of telomeric DNA repeats. While most eukaryotes counteract telomere shortening by telomerase-driven addition of these repeats, telomeric loss in Drosophila is compensated by retrotransposition of the telomeric retroelements HeT-A, TART and TAHRE to chromosome ends. In this study we tested the effect of chronic exposure of flies to non-/sub-lethal doses of paraquat, which is a redox cycling compound widely used to induce oxidative stress in various experimental paradigms including telomere length analyses. Indeed, chronic paraquat exposure for five generations resulted in elevated transcriptional activity of both telomeric and non-telomeric transposable elements, and extended telomeric length in the tested fly lines. We propose that low oxidative stress leads to increased telomere length within Drosophila populations. For a mechanistic understanding of the observed phenomenon we discuss two scenarios: adaption, acting through a direct stimulation of telomere extension, or positive selection favoring individuals with longer telomeres within the population.

Are the TTAGG and TTAGGG telomeric repeats phylogenetically conserved in aculeate Hymenoptera?

Despite the (TTAGG)_n telomeric repeat supposed being the ancestral DNA motif of telomeres in insects, it was repeatedly lost within some insect orders. Notably, parasitoid hymenopterans and the social wasp Metapolybia decorata (Gribodo) lack the (TTAGG)_n sequence, but in other representatives of Hymenoptera, this motif was noticed, such as different ant species and the honeybee. These findings raise the question of whether the insect telomeric repeat is or not phylogenetically predominant in Hymenoptera. Thus, we evaluated the occurrence of both the (TTAGG)_n sequence and the vertebrate telomere sequence (TTAGGG)_n using dot-blotting hybridization in 25 aculeate species of Hymenoptera. Our results revealed the absence of (TTAGG)_n sequence in all tested species, elevating the number of hymenopteran families lacking this telomeric sequence to 13 out of the 15 tested families so far. The (TTAGGG)_n was not observed in any tested species. Based on our data and compiled information, we suggest that the (TTAGG)_n sequence was putatively lost in the ancestor of Apocrita with at least two subsequent independent regains (in Formicidae and Apidae).

A chromosomal analysis of Nepa cinerea Linnaeus, 1758 and Ranatra linearis (Linnaeus, 1758) (Heteroptera, Nepidae)

An account is given of the karyotypes and male meiosis of the Water Scorpion Nepa cinerea Linnaeus, 1758 and the Water Stick Insect Ranatra linearis (Linnaeus, 1758) (Heteroptera, Nepomorpha, Nepidae). A number of different approaches and techniques were tried: the employment of both male and female gonads and mid-guts as the sources of chromosomes, squash and air-drying methods for chromosome preparations, C-banding and fluorescence in situ hybridization (FISH) for chromosome study. We found that N. cinerea had a karyotype comprising 14 pairs of autosomes and a multiple sex chromosome system, which is X₁X₂X₃X₄Y (♂) / X₁X₁X₂X₂X₃X₃X₄X₄ (♀), whereas R. linearis had a karyotype comprising 19 pairs of autosomes and a multiple sex chromosome system X₁X₂X₃X₄Y (♂) / X₁X₁X₂X₂X₃X₃X₄X₄ (♀). In both N. cinerea and R. linearis, the autosomes formed chiasmate bivalents in spermatogenesis, and the sex chromosome univalents divided during the first meiotic division and segregated during the second one suggesting thus a post-reductional type of behaviour. These results confirm and amplify those of Steopoe (1925, 1927, 1931, 1932) but are inconsistent with those of other researchers. C-banding appeared helpful in pairing up the autosomes for karyotype assembly; however in R. linearis the chromosomes were much more uniform in size and general appearance than in N. cinerea. FISH for 18S ribosomal DNA (major rDNA) revealed hybridization signals on two of the five sex chromosomes in N. cinerea. In R. linearis, rDNA location was less obvious than in N. cinerea; however it is suggested to be similar. We have detected the presence of the canonical "insect" (TTAGG) _n telomeric repeat in chromosomes of these species. This is the first application of C-banding and FISH in the family Nepidae.

The Drosophila telomere-capping protein Verrocchio binds single-stranded DNA and protects telomeres from DNA damage response

Drosophila telomeres are sequence-independent structures maintained by transposition to chromosome ends of three specialized retroelements rather than by telomerase activity. Fly telomeres are protected by the terminin complex that includes the HOAP, HipHop, Moi and Ver proteins. These are fast evolving, non-conserved proteins that localize and function exclusively at telomeres, protecting them from fusion events. We have previously suggested that terminin is the functional analogue of shelterin, the multi-protein complex that protects human telomeres. Here, we use electrophoretic mobility shift assay (EMSA) and atomic force microscopy (AFM) to show that Ver preferentially binds single-stranded DNA (ssDNA) with no sequence specificity. We also show that Moi and Ver form a complex in vivo. Although these two proteins are mutually dependent for their localization at telomeres, Moi neither binds ssDNA nor facilitates Ver binding to ssDNA. Consistent with these results, we found that Ver-depleted telomeres form RPA and γH2AX foci, like the human telomeres lacking the ssDNA-binding POT1 protein. Collectively, our findings suggest that Drosophila telomeres possess a ssDNA overhang like the other eukaryotes, and that the terminin complex is architecturally and functionally similar to shelterin.

Chromosomal distribution of interstitial telomeric sequences as signs of evolution through chromosome fusion in six species of the giant water bugs (Hemiptera, Belostoma)

Tandem arrays of TTAGG repeats show a highly conserved location at the telomeres across the phylogenetic tree of arthropods. In giant water bugs Belostoma, the chromosome number changed during speciation by fragmentation of the single ancestral X chromosome, resulting in a multiple sex chromosome system. Several autosome-autosome fusions and a fusion between the sex chromosome pair and an autosome pair resulted in the reduced number in several species. We mapped the distribution of telomeric sequences and interstitial telomeric sequences (ITSs) in Belostoma candidulum (2n = 12 + XY/XX; male/female), B. dentatum (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. elegans (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. elongatum (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. micantulum (2n = 14 + XY/XX), and B. oxyurum (2n = 6 + XY/XX) by FISH with the (TTAGG) _n probes. Hybridization signals confirmed the presence of TTAGG repeats in the telomeres of all species examined. The three species with reduced chromosome numbers showed additional hybridization signals in interstitial positions, indicating the occurrence of ITS. From the comparison of all species here analyzed, we observed inverse relationships between chromosome number and chromosome size, and between presence/absence of ITS and chromosome number. The ITS distribution between these closely related species supports the hypothesis that several telomere-telomere fusions of the chromosomes from an ancestral diploid chromosome number 2n = 26 + XY/XX played a major role in the karyotype evolution of Belostoma. Consequently, our study provide valuable features that can be used to understand the karyotype evolution, may contribute to a better understanding of taxonomic relationships, and also elucidate the high plasticity of nuclear genomes at the chromosomal level during the speciation processes.

Chromosomal distribution of interstitial telomeric sequences as signs of evolution through chromosome fusion in six species of the giant water bugs (Hemiptera, Belostoma)

Tandem arrays of TTAGG repeats show a highly conserved location at the telomeres across the phylogenetic tree of arthropods. In giant water bugs Belostoma, the chromosome number changed during speciation by fragmentation of the single ancestral X chromosome, resulting in a multiple sex chromosome system. Several autosome–autosome fusions and a fusion between the sex chromosome pair and an autosome pair resulted in the reduced number in several species. We mapped the distribution of telomeric sequences and interstitial telomeric sequences (ITSs) in Belostoma candidulum (2n = 12 + XY/XX; male/female), B. dentatum (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. elegans (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. elongatum (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. micantulum (2n = 14 + XY/XX), and B. oxyurum (2n = 6 + XY/XX) by FISH with the (TTAGG)_n probes. Hybridization signals confirmed the presence of TTAGG repeats in the telomeres of all species examined. The three species with reduced chromosome numbers showed additional hybridization signals in interstitial positions, indicating the occurrence of ITS. From the comparison of all species here analyzed, we observed inverse relationships between chromosome number and chromosome size, and between presence/absence of ITS and chromosome number. The ITS distribution between these closely related species supports the hypothesis that several telomere–telomere fusions of the chromosomes from an ancestral diploid chromosome number 2n = 26 + XY/XX played a major role in the karyotype evolution of Belostoma. Consequently, our study provide valuable features that can be used to understand the karyotype evolution, may contribute to a better understanding of taxonomic relationships, and also elucidate the high plasticity of nuclear genomes at the chromosomal level during the speciation processes.

The protein subunit of telomerase displays patterns of dynamic evolution and conservation across different metazoan taxa

BACKGROUND

Most animals employ telomerase, which consists of a catalytic subunit known as the telomerase reverse transcriptase (TERT) and an RNA template, to maintain telomere ends. Given the importance of TERT and telomere biology in core metazoan life history traits, like ageing and the control of somatic cell proliferation, we hypothesised that TERT would have patterns of sequence and regulatory evolution reflecting the diverse life histories across the Animal Kingdom.

RESULTS

We performed a complete investigation of the evolutionary history of TERT across animals. We show that although TERT is almost ubiquitous across Metazoa, it has undergone substantial sequence evolution within canonical motifs. Beyond the known canonical motifs, we also identify and compare regions that are highly variable between lineages, but show conservation within phyla. Recent data have highlighted the importance of alternative splice forms of TERT in non-canonical functions and although animals may share some conserved introns, we find that the selection of exons for alternative splicing appears to be highly variable, and regulation by alternative splicing appears to be a very dynamic feature of TERT evolution. We show that even within a closely related group of triclad flatworms, where alternative splicing of TERT was previously correlated with reproductive strategy, we observe highly diverse splicing patterns.

CONCLUSIONS

Our work establishes that the evolutionary history and structural evolution of TERT involves previously unappreciated levels of change and the emergence of lineage specific motifs. The sequence conservation we describe within phyla suggests that these new motifs likely serve essential biological functions of TERT, which along with changes in splicing, underpin diverse functions of TERT important for animal life histories.

The presence of the ancestral insect telomeric motif in kissing bugs (Triatominae) rules out the hypothesis of its loss in evolutionarily advanced Heteroptera (Cimicomorpha)

Next-generation sequencing data analysis on Triatoma infestans Klug, 1834 (Heteroptera, Cimicomorpha, Reduviidae) revealed the presence of the ancestral insect (TTAGG)_n telomeric motif in its genome. Fluorescence in situ hybridization confirms that chromosomes bear this telomeric sequence in their chromosomal ends. Furthermore, motif amount estimation was about 0.03% of the total genome, so that the average telomere length in each chromosomal end is almost 18 kb long. We also detected the presence of (TTAGG)_n telomeric repeat in mitotic and meiotic chromosomes in other three species of Triatominae: Triatoma dimidiata Latreille, 1811, Dipetalogaster maxima Uhler, 1894, and Rhodnius prolixus Ståhl, 1859. This is the first report of the (TTAGG)_n telomeric repeat in the infraorder Cimicomorpha, contradicting the currently accepted hypothesis that evolutionarily recent heteropterans lack this ancestral insect telomeric sequence.

BAL31-NGS approach for identification of telomeres de novo in large genomes

This article describes a novel method to identify as yet undiscovered telomere sequences, which combines next generation sequencing (NGS) with BAL31 digestion of high molecular weight DNA. The method was applied to two groups of plants: i) dicots, genus Cestrum, and ii) monocots, Allium species (e.g. A. ursinum and A. cepa). Both groups consist of species with large genomes (tens of Gb) and a low number of chromosomes (2n=14-16), full of repeat elements. Both genera lack typical telomeric repeats and multiple studies have attempted to characterize alternative telomeric sequences. However, despite interesting hypotheses and suggestions of alternative candidate telomeres (retrotransposons, rDNA, satellite repeats) these studies have not resolved the question. In a novel approach based on the two most general features of eukaryotic telomeres, their repetitive character and sensitivity to BAL31 nuclease digestion, we have taken advantage of the capacity and current affordability of NGS in combination with the robustness of classical BAL31 nuclease digestion of chromosomal termini. While representative samples of most repeat elements were ensured by low-coverage (less than 5%) genomic shot-gun NGS, candidate telomeres were identified as under-represented sequences in BAL31-treated samples.

Evolutionary perspectives of telomerase RNA structure and function

Telomerase is the eukaryotic solution to the ‘end-replication problem’ of linear chromosomes by synthesising the highly repetitive DNA constituent of telomeres, the nucleoprotein cap that protects chromosome termini. Functioning as a ribonucleoprotein (RNP) enzyme, telomerase is minimally composed of the highly conserved catalytic telomerase reverse transcriptase (TERT) and essential telomerase RNA (TR) component. Beyond merely providing the template for telomeric DNA synthesis, TR is an innate telomerase component and directly facilitates enzymatic function. TR accomplishes this by having evolved structural elements for stable assembly with the TERT protein and the regulation of the telomerase catalytic cycle. Despite its prominence and prevalence, TR has profoundly diverged in length, sequence, and biogenesis pathway among distinct evolutionary lineages. This diversity has generated numerous structural and mechanistic solutions for ensuring proper RNP formation and high fidelity telomeric DNA synthesis. Telomerase provides unique insights into RNA and protein coevolution within RNP enzymes.

Beyond DNA puffs: What can we learn from studying sciarids?

Members of the Sciaridae family attracted the interest of researchers because of the demonstration that the DNA puff regions, which are formed in the salivary gland polytene chromosomes at the end of the fourth larval instar, constitute sites of developmentally regulated gene amplification. Besides contributing to a deeper understanding of the process of gene amplification, the study of sciarids has also provided important insights on other biological processes such as sex determination, programmed cell death, insect immunity, telomere maintenance, and nucleolar organizing regions (NOR) formation. Open questions in sciarids include among others, early development, the role of noncoding RNAs in gene amplification and the relationship between gene amplification and transcription in DNA puff forming regions. These and other questions can now be pursued with next generation sequencing techniques and experiments using RNAi experiments, since this latter technique has been shown to be feasible in sciarids. These new perspectives in the field of sciarid biology open the opportunity to consolidate sciarid species as important emerging models. genesis 54:361-378, 2016. © 2016 Wiley Periodicals, Inc.

The Deadbeat Paternal Effect of Uncapped Sperm Telomeres on Cell Cycle Progression and Chromosome Behavior in Drosophila melanogaster

Telomere-capping complexes (TCCs) protect the ends of linear chromosomes from illegitimate repair and end-to-end fusions and are required for genome stability. The identity and assembly of TCC components have been extensively studied, but whether TCCs require active maintenance in nondividing cells remains an open question. Here we show that Drosophila melanogaster requires Deadbeat (Ddbt), a sperm nuclear basic protein (SNBP) that is recruited to the telomere by the TCC and is required for TCC maintenance during genome-wide chromatin remodeling, which transforms spermatids to mature sperm. Ddbt-deficient males produce sperm lacking TCCs. Their offspring delay the initiation of anaphase as early as cycle 1 but progress through the first two cycles. Persistence of uncapped paternal chromosomes induces arrest at or around cycle 3. This early arrest can be rescued by selective elimination of paternal chromosomes and production of gynogenetic haploid or haploid mosaics. Progression past cycle 3 can also occur if embryos have reduced levels of the maternally provided checkpoint kinase Chk2. The findings provide insights into how telomere integrity affects the regulation of the earliest embryonic cell cycles. They also suggest that other SNBPs, including those in humans, may have analogous roles and manifest as paternal effects on embryo quality.