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.

TeloBase: a community-curated database of telomere sequences across the tree of life

Discoveries over the recent decade have demonstrated the unexpected diversity of telomere DNA motifs in nature. However, currently available resources, 'Telomerase database' and 'Plant rDNA database', contain just fragments of all relevant literature published over decades of telomere research as they have a different primary focus and limited updates. To fill this gap, we gathered data about telomere DNA sequences from a thorough literature screen as well as by analysing publicly available NGS data, and we created TeloBase (http://cfb.ceitec.muni.cz/telobase/) as a comprehensive database of information about telomere motif diversity. TeloBase is supplemented by internal taxonomy utilizing popular on-line taxonomic resources that enables in-house data filtration and graphical visualisation of telomere DNA evolutionary dynamics in the form of heat tree plots. TeloBase avoids overreliance on administrators for future data updates by having a simple form and community-curation system for application and approval, respectively, of new telomere sequences by users, which should ensure timeliness of the database and topicality. To demonstrate TeloBase utility, we examined telomere motif diversity in species from the fungal genus Aspergillus, and discovered (TTTATTAGGG)n sequence as a putative telomere motif in the plant family Chrysobalanaceae. This was bioinformatically confirmed by analysing template regions of identified telomerase RNAs.

Seasonal changes in ultrastructure and gene expression in the fat body of worker honey bees

The anatomical, physiological, and behavioral characteristics of honey bees are affected by the season as well as division of labor. In this study, we examined the structure, ultrastructure, and gene expression of fat body cells in both long-lived winter and short-lived summer worker bees (the youngest stage of hive bees and forager bees). In contrast to hive bees, foragers and winter bees have a higher metabolism due to intensive muscle activity during their flight (foragers) or endothermic heat production (winter bees). These workers differ from hive bees in the biology of their mitochondria, peroxisomes, and lysosomes as well as in the expression of the genes involved in lipid, carbohydrate, amino acid metabolism, insulin, and TGF- β signaling. Additionally, the expression of genes related to phospholipid metabolism was higher in the hive bees. However, we found no differences between workers in the expression of genes controlling cell organelles, such as the Golgi apparatus, endoplasmic reticulum, ribosomes, nucleus, and vacuoles, as well as genes for DNA replication, cell cycle control, and autophagy. Furthermore, lysosomes, autophagic processes and lipofuscin particles were more frequently observed in winter bees using electron microscopy.

Identification of the Sequence and the Length of Telomere DNA

Telomeres are essential nucleoprotein structures at the very ends of linear eukaryote chromosomes. They shelter the terminal genome territories against degradation and prevent the natural chromosome ends from being recognized by repair mechanisms as double-strand DNA breaks.There are two basic characteristics of telomeric DNA, its sequence and its length. The telomere sequence is important as a "landing area" for specific telomere-binding proteins, which function as signals and moderate the interactions required for correct telomere function. While the sequence forms the proper "landing surface" of telomeric DNA, its length is similarly important. Too short or exceptionally long telomere DNA cannot perform its function properly. In this chapter, methods for the investigation of these two basic telomere DNA characteristics are described, namely, telomere motif identification and telomere length measurement.

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.

Extraordinary diversity of telomeres, telomerase RNAs and their template regions in Saccharomycetaceae

Telomerase RNA (TR) carries the template for synthesis of telomere DNA and provides a scaffold for telomerase assembly. Fungal TRs are long and have been compared to higher eukaryotes, where they show considerable diversity within phylogenetically close groups. TRs of several Saccharomycetaceae were recently identified, however, many of these remained uncharacterised in the template region. Here we show that this is mainly due to high variability in telomere sequence. We predicted the telomere sequences using Tandem Repeats Finder and then we identified corresponding putative template regions in TR candidates. Remarkably long telomere units and the corresponding putative TRs were found in Tetrapisispora species. Notably, variable lengths of the annealing sequence of the template region (1–10 nt) were found. Consequently, species with the same telomere sequence may not harbour identical TR templates. Thus, TR sequence alone can be used to predict a template region and telomere sequence, but not to determine these exactly. A conserved feature of telomere sequences, tracts of adjacent Gs, led us to test the propensity of individual telomere sequences to form G4. The results show highly diverse values of G4-propensity, indicating the lack of ubiquitous conservation of this feature across Saccharomycetaceae.

WALTER: an easy way to online evaluate telomere lengths from terminal restriction fragment analysis

BACKGROUND

Telomeres, nucleoprotein structures comprising short tandem repeats and delimiting the ends of linear eukaryotic chromosomes, play an important role in the maintenance of genome stability. Therefore, the determination of the length of telomeres is of high importance for many studies. Over the last years, new methods for the analysis of the length of telomeres have been developed, including those based on PCR or analysis of NGS data. Despite that, terminal restriction fragment (TRF) method remains the gold standard to this day. However, this method lacks universally accepted and precise tool capable to analyse and statistically evaluate TRF results.

RESULTS

To standardize the processing of TRF results, we have developed WALTER, an online toolset allowing rapid, reproducible, and user-friendly analysis including statistical evaluation of the data. Given its web-based nature, it provides an easily accessible way to analyse TRF data without any need to install additional software.

CONCLUSIONS

WALTER represents a major upgrade from currently available tools for the image processing of TRF scans. This toolset enables a rapid, highly reproducible, and user-friendly evaluation of almost any TRF scan including in-house statistical evaluation of the data. WALTER platform together with user manual describing the evaluation of TRF scans in detail and presenting tips and troubleshooting, as well as test data to demo the software are available at https://www.ceitec.eu/chromatin-molecular-complexes-jiri-fajkus/rg51/tab?tabId=125#WALTER and the source code at https://github.com/mlyc93/WALTER .

Human-like telomeres in Zostera marina reveal a mode of transition from the plant to the human telomeric sequences

A previous study describing the genome of Zostera marina, the most widespread seagrass in the Northern hemisphere, revealed some genomic signatures of adaptation to the aquatic environment such as the loss of stomatal genes, while other functions such as an algal-like cell wall composition were acquired. Beyond these, the genome structure and organization were comparable with those of the majority of plant genomes sequenced, except for one striking feature that went unnoticed at that time: the presence of human-like instead of the expected plant-type telomeric sequences. By using different experimental approaches including fluorescence in situ hybridization (FISH), genome skimming by next-generation sequencing (NGS), and analysis of non-coding transcriptome, we have confirmed its telomeric location in the chromosomes of Z. marina. We have also identified its telomerase RNA (TR) subunit, confirming the presence of the human-type telomeric sequence in the template region. Remarkably, this region was found to be very variable even in clades with a highly conserved telomeric sequence across their species. Based on this observation, we propose that alternative annealing preferences in the template borders can explain the transition between the plant and human telomeric sequences. The further identification of paralogues of TR in several plant genomes led us to the hypothesis that plants may retain an increased ability to change their telomeric sequence. We discuss the implications of this occurrence in the evolution of telomeres while introducing a mechanistic model for the transition from the plant to the human telomeric sequences.

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.

Two faces of Solanaceae telomeres: a comparison between Nicotiana and Cestrum telomeres and telomere-binding proteins

While most Solanaceae genera (e.g.Solanum, Nicotiana) possess Arabidopsis-type telomeres of (TTTAGGG)n maintained by telomerase, the genera Cestrum, Vestia and Sessea (Cestrum group) lack these telomeres. Here we show that in the Cestrum-group the activity of telomerase has been lost. Nevertheless, proteins binding the single-stranded G-rich strand of the Arabidopsis-type and related human-type (TTAGGG)n telomeric sequences are present in nuclear extracts of both Nicotiana and Cestrum species. These proteins may have a role in telomere function or other cellular activities. In addition to characterizing DNA binding specificity and molecular weights of these proteins, we searched in both N. tabacum (tobacco) and C. parqui for the presence of POT1-like proteins, involved in telomere capping and telomerase regulation. Analysis of POT1-like proteins available on public databases and cloned by us from C. parqui, revealed the N-terminal OB folds typical for this protein family and a novel, plant-specific conserved C-terminal OB-fold domain (CTOB). We propose that CTOB is involved in protein-protein interactions.