A comparative ultrastructure study of storage cells in the eutardigrade Richtersius coronifer in the hydrated state and after desiccation and heating stress

Life on the dry side: a roadmap to understanding desiccation tolerance and accelerating translational applications

To thrive in extreme conditions, organisms have evolved a diverse arsenal of adaptations that confer resilience. These species, their traits, and the mechanisms underlying them comprise a valuable resource that can be mined for numerous conceptual insights and applied objectives. One of the most dramatic adaptations to water limitation is desiccation tolerance. Understanding the mechanisms underlying desiccation tolerance has important potential implications for medicine, biotechnology, agriculture, and conservation. However, progress has been hindered by a lack of standardization across sub-disciplines, complicating the integration of data and slowing the translation of basic discoveries into practical applications. Here, we synthesize current knowledge on desiccation tolerance across evolutionary, ecological, physiological, and cellular scales to provide a roadmap for advancing desiccation tolerance research. We also address critical gaps and technical roadblocks, highlighting the need for standardized experimental practices, improved taxonomic sampling, and the development of new tools for studying biology in a dry state. We hope that this perspective can serve as a roadmap to accelerating research breakthroughs and unlocking the potential of desiccation tolerance to address global challenges related to climate change, food security, and health.

Metabolite phosphatase from anhydrobiotic tardigrades

Terrestrial organisms have systems to escape from desiccation stresses. For example, tardigrades (also known as water bears) can survive severe dried and other extreme environments by anhydrobiosis. Although their extraordinary ability has enchanted people, little is known about the detailed molecular mechanisms of anhydrobiosis. Here, we focused on the tardigrade Ramazzottius varieornatus, one of the toughest animals on Earth. A transcriptome database of R. varieornatus shows that genes encoding a Ferritin-like protein are upregulated during desiccation or ultraviolet radiation. This protein shows sequence similarity to enigmatic proteins in desiccation-tolerant bacteria and plants, which are hypothesized to be desiccation-related. However, because these proteins lack detailed biological information, their functions are relatively unknown. We determined an atomic (1.05 Å) resolution crystal structure of a Ferritin-like protein from R. varieornatus. The structure revealed a dinuclear metal binding site, and we showed that this Ferritin-like protein has phosphatase activity toward several metabolite compounds including unusual nucleotide phosphates produced by oxidative or radiation damage. We also found that a homologous protein from a desiccation- and ultraviolet-tolerant bacterium Deinococcus radiodurans is a metabolite phosphatase. Our results indicate that through cleaning up damaged metabolites or regulation of metabolite levels, this phosphatase family can contribute to stress tolerances. This study provides a clue to one of the universal molecular bases of desiccation-stress tolerance.

A comparative ultrastructure study of the tardigrade Ramazzottius varieornatus in the hydrated state, after desiccation and during the process of rehydration

Tardigrades can survive hostile environments such as desiccation by adopting a state of anhydrobiosis. Numerous tardigrade species have been described thus far, and recent genome and transcriptome analyses revealed that several distinct strategies were employed to cope with harsh environments depending on the evolutionary lineages. Detailed analyses at the cellular and subcellular levels are essential to complete these data. In this work, we analyzed a tardigrade species that can withstand rapid dehydration, Ramazzottius varieornatus. Surprisingly, we noted an absence of the anhydrobiotic-specific extracellular structure previously described for the Hypsibius exemplaris species. Both Ramazzottius varieornatus and Hypsibius exemplaris belong to the same evolutionary class of Eutardigrada. Nevertheless, our observations reveal discrepancies in the anhydrobiotic structures correlated with the variation in the anhydrobiotic mechanisms.

A new, simplified, drying protocol to prepare tardigrades for scanning electron microscopy

A new protocol for preparation of tardigrades for scanning electron microscope (SEM) analysis is proposed. The more conventional protocols require various steps and a long time to obtain good drying of water bears, together with specific and uncommon instruments (i.e., critical point dryer) or highly volatile toxic compounds (i.e., hexametildisilazane). The new protocol can be performed using few and simple instruments and materials, all easily accessible, and produces a high yield in terms of dried animals in excellent condition for the observation of external morphological structures with SEM. The acquired data exhibit considerable promise, and the proposed methodology shows potential for application to other meiofaunal groups, including small arthropods, nematodes, and rotifers. RESEARCH HIGHLIGHTS: Cheap, safe, and fast new method for Tardigrada preparation for SEM. With the new protocol, the number of animals required for SEM studies is minimized. New protocol is potentially applicable to the study of other meiofaunal soft-bodied taxa.

Elevated external temperature affects cell ultrastructure and heat shock proteins (HSPs) in Paramacrobiotus experimentalis Kaczmarek, Mioduchowska, Poprawa, & Roszkowska, 2020

Increasing temperature influences the habitats of various organisms, including microscopic invertebrates. To gain insight into temperature-dependent changes in tardigrades, we isolated storage cells exposed to various temperatures and conducted biochemical and ultrastructural analysis in active and tun-state Paramacrobiotus experimentalis Kaczmarek, Mioduchowska, Poprawa, & Roszkowska, 2020. The abundance of heat shock proteins (HSPs) and ultrastructure of the storage cells were examined at different temperatures (20 °C, 30 °C, 35 °C, 37 °C, 40 °C, and 42 °C) in storage cells isolated from active specimens of Pam. experimentalis. In the active animals, upon increase in external temperature, we observed an increase in the levels of HSPs (HSP27, HSP60, and HSP70). Furthermore, the number of ultrastructural changes in storage cells increased with increasing temperature. Cellular organelles, such as mitochondria and the rough endoplasmic reticulum, gradually degenerated. At 42 °C, cell death occurred by necrosis. Apart from the higher electron density of the karyoplasm and the accumulation of electron-dense material in some mitochondria (at 42 °C), almost no changes were observed in the ultrastructure of tun storage cells exposed to different temperatures. We concluded that desiccated (tun-state) are resistant to high temperatures, but not active tardigrades (survival rates of tuns after 24 h of rehydration: 93.3% at 20 °C, 60.0% at 35 °C, 33.3% at 37 °C, 33.3% at 40 °C, and 20.0% at 42 °C).

The major inducible small heat shock protein HSP20-3 in the tardigrade Ramazzottius varieornatus forms filament-like structures and is an active chaperone

The tardigrade Ramazzottius varieornatus has remarkable resilience to a range of environmental stresses. In this study, we have characterised two members of the small heat shock protein (sHSP) family in R. varieornatus, HSP20-3 and HSP20-6. These are the most highly upregulated sHSPs in response to a 24 h heat shock at 35 0C of adult tardigrades with HSP20-3 being one of the most highly upregulated gene in the whole transcriptome. Both R. varieornatus sHSPs and the human sHSP, CRYAB (HSPB5), were produced recombinantly for comparative structure-function studies. HSP20-3 exhibited a superior chaperone activity than human CRYAB in a heat-induced protein aggregation assay. Both tardigrade sHSPs also formed larger oligomers than CRYAB as assessed by size exclusion chromatography and transmission electron microscopy of negatively stained samples. Whilst both HSP20-3 and HSP20-6 formed particles that were variable in size and larger than the particles formed by CRYAB, only HSP20-3 formed filament-like structures. The particles and filament-like structures formed by HSP20-3 appear inter-related as the filament-like structures often had particles located at their ends. Sequence analyses identified two unique features; an insertion in the middle region of the N-terminal domain (NTD) and preceding the critical-sequence identified in CRYAB, as well as a repeated QNTN-motif located in the C-terminal domain of HSP20-3. The NTD insertion is expected to affect protein-protein interactions and subunit oligomerisation. Removal of the repeated QNTN-motif abolished HSP20-3 chaperone activity and also affected the assembly of the filament-like structures. We discuss the potential contribution of HSP20-3 to protein condensate formation.

Effect of paracetamol on the storage cells of Hypsibius exemplaris—ultrastructural analysis

Tardigrades in their natural environment are exposed to various environmental toxicants, including non-steroidal anti-inflammatory drugs (NSAIDs) or antipyretics such as paracetamol. This drug can enter the animal’s body through the body wall or the digestive system with food and can affect the biology of organisms. In this paper, we report for the first time the effects of paracetamol on tardigrade storage cells. We analyzed the effects of short-term (7 days) and long-term (28 days) exposure of Hypsibius exemplaris storage cells to three paracetamol concentrations (0.2 µgxL−1, 230 µgxL−1, 1 mgxL−1). Our results showed that increasing paracetamol concentration and incubation time increases the number of damaged mitochondria in storage cells, and autophagy is activated and intensified. Moreover, the relocation of some organelles and cell deformation may indicate cytoskeleton damage.

A multiscale approach reveals elaborate circulatory system and intermittent heartbeat in velvet worms (Onychophora)

An antagonistic hemolymph-muscular system is essential for soft-bodied invertebrates. Many ecdysozoans (molting animals) possess neither a heart nor a vascular or circulatory system, whereas most arthropods exhibit a well-developed circulatory system. How did this system evolve and how was it subsequently modified in panarthropod lineages? As the closest relatives of arthropods and tardigrades, onychophorans (velvet worms) represent a key group for addressing this question. We therefore analyzed the entire circulatory system of the peripatopsid Euperipatoides rowelli and discovered a surprisingly elaborate organization. Our findings suggest that the last common ancestor of Onychophora and Arthropoda most likely possessed an open vascular system, a posteriorly closed heart with segmental ostia, a pericardial sinus filled with nephrocytes and an impermeable pericardial septum, whereas the evolutionary origin of plical and pericardial channels is unclear. Our study further revealed an intermittent heartbeat-regular breaks of rhythmic, peristaltic contractions of the heart-in velvet worms, which might stimulate similar investigations in arthropods.

How long can tardigrades survive in the anhydrobiotic state? A search for tardigrade anhydrobiosis patterns

Anhydrobiosis is a desiccation tolerance that denotes the ability to survive almost complete dehydration without sustaining damage. The knowledge on the survival capacity of various tardigrade species in anhydrobiosis is still very limited. Our research compares anhydrobiotic capacities of four tardigrade species from different genera, i.e. Echiniscus testudo, Paramacrobiotus experimentalis, Pseudohexapodibius degenerans and Macrobiotus pseudohufelandi, whose feeding behavior and occupied habitats are different. Additionally, in the case of Ech. testudo, we analyzed two populations: one urban and one from a natural habitat. The observed tardigrade species displayed clear differences in their anhydrobiotic capacity, which appear to be determined by the habitat rather than nutritional behavior of species sharing the same habitat type. The results also indicate that the longer the state of anhydrobiosis lasts, the more time the animals need to return to activity.

Verification of Hypsibius exemplaris Gąsiorek et al., 2018 (Eutardigrada; Hypsibiidae) application in anhydrobiosis research

Anhydrobiosis is considered to be an adaptation of important applicative implications because it enables resistance to the lack of water. The phenomenon is still not well understood at molecular level. Thus, a good model invertebrate species for the research is required. The best known anhydrobiotic invertebrates are tardigrades (Tardigrada), considered to be toughest animals in the world. Hypsibius. exemplaris is one of the best studied tardigrade species, with its name "exemplaris" referring to the widespread use of the species as a laboratory model for various types of research. However, available data suggest that anhydrobiotic capability of the species may be overestimated. Therefore, we determined anhydrobiosis survival by Hys. exemplaris specimens using three different anhydrobiosis protocols. We also checked ultrastructure of storage cells within formed dormant structures (tuns) that has not been studied yet for Hys. exemplaris. These cells are known to support energetic requirements of anhydrobiosis. The obtained results indicate that Hys. exemplaris appears not to be a good model species for anhydrobiosis research.

A pan-metazoan concept for adult stem cells: the wobbling Penrose landscape

Adult stem cells (ASCs) in vertebrates and model invertebrates (e.g. Drosophila melanogaster) are typically long-lived, lineage-restricted, clonogenic and quiescent cells with somatic descendants and tissue/organ-restricted activities. Such ASCs are mostly rare, morphologically undifferentiated, and undergo asymmetric cell division. Characterized by 'stemness' gene expression, they can regulate tissue/organ homeostasis, repair and regeneration. By contrast, analysis of other animal phyla shows that ASCs emerge at different life stages, present both differentiated and undifferentiated phenotypes, and may possess amoeboid movement. Usually pluri/totipotent, they may express germ-cell markers, but often lack germ-line sequestering, and typically do not reside in discrete niches. ASCs may constitute up to 40% of animal cells, and participate in a range of biological phenomena, from whole-body regeneration, dormancy, and agametic asexual reproduction, to indeterminate growth. They are considered legitimate units of selection. Conceptualizing this divergence, we present an alternative stemness metaphor to the Waddington landscape: the 'wobbling Penrose' landscape. Here, totipotent ASCs adopt ascending/descending courses of an 'Escherian stairwell', in a lifelong totipotency pathway. ASCs may also travel along lower stemness echelons to reach fully differentiated states. However, from any starting state, cells can change their stemness status, underscoring their dynamic cellular potencies. Thus, vertebrate ASCs may reflect just one metazoan ASC archetype.

Tolerance to Anhydrobiotic Conditions Among Two Coexisting Tardigrade Species Differing in Life Strategies

Water availability is one of the most important factors for terrestrial life. Terrestrial habitats may periodically become dry, which can be overcome by an organism's capability to undergo anhydrobiosis. In animals, this phenomenon has been reported for invertebrates, with tardigrades being the best-known. However, different tardigrade species appear to significantly differ in their anhydrobiotic abilities. While several studies have addressed this issue, established experimental protocols for tardigrade dehydration differ both within and among species, leading to ambiguous results. Therefore, we apply unified conditions to estimate intra-and interspecies differences in anhydrobiosis ability reflected by the return to active life. We analysed Milnesium inceptum and Ramazzottius subanomalus representing predatory and herbivorous species, respectively, and often co-occur in the same habitat. The results indicated that the carnivorous Mil. inceptum displays better anhydrobiosis survivability than the herbivorous Ram. subanomalus. This tendency to some degree coincides with the time of "waking up" since Mil. inceptum showed first movements and full activity of any first individual later than Ram. subanomalus. The movements of all individuals were however observed to be faster for Mil. inceptum. Differences between the experimental groups varying in anhydrobiosis length were also observed: the longer tun state duration, the more time was necessary to return to activity.

Tardigrada: An Emerging Animal Model to Study the Endoplasmic Reticulum Stress Response to Environmental Extremes

Tardigrada (also known as "water bears") are hydrophilous microinvertebrates with a bilaterally symmetrical body and four pairs of legs usually terminating with claws. Water bears are quite complex animals and range from 50 to 1200 μm in length. Their body is divided into a head segment and four trunk segments, each bearing a pair of legs. They inhabit almost all terrestrial and aquatic environments, from the ocean depths to highest mountains ranges. However, one of their best known and unusual features is their capability for cryptobiosis. In this state tardigrades are able to survive extremely low and high temperatures and atmospheric pressures, complete lack of water, high doses of radiation, high concentrations of toxins and even a cosmic vacuum. The cellular mechanisms enabling cryptobiosis are poorly understood, although it appears the synthesis of certain types of molecules (sugars and proteins) enable the prevention of cellular damage at different levels. The endoplasmic reticulum (ER) is a morphologically and functionally diverse organelle able to integrate multiple extracellular and internal signals and generate adaptive cellular responses. However, the ER morphology and activity in the case of tardigrades has been studied rarely and in the context of oogenesis, functioning of the digestive system, and in the role and function of storage cells. Thus, there are no direct studies on the contribution of the ER in the ability of this organism to cope with environmental stress during cryptobiosis. Nevertheless, it is highly probable that the ER has a crucial role in this uncommon process. Since water bears are easy to handle laboratory animals, they may represent an ideal model organism to uncover the important role of the ER in the cell response to extreme environmental stress conditions.

Ultrastructural analysis of the dehydrated tardigrade Hypsibius exemplaris unveils an anhydrobiotic-specific architecture

Tardigrades can cope with adverse environmental conditions by turning into anhydrobiotes with a characteristic tun shape. Tun formation is an essential morphological adaptation for tardigrade entry into the anhydrobiotic state. The tun cell structure and ultrastructure have rarely been explored in tardigrades in general and never in Hypsibius exemplaris. We used transmission electron microscopy to compare cellular organization and ultrastructures between hydrated and anhydrobiotic H. exemplaris. Despite a globally similar cell organelle structure and a number of cells not significantly different between hydrated and desiccated tardigrades, reductions in the sizes of both cells and mitochondria were detected in dehydrated animals. Moreover, in anhydrobiotes, secretory active cells with a dense endoplasmic reticulum network were observed. Interestingly, these anhydrobiote-specific cells are in a close relationship with a specific extracellular structure surrounding each cell. It is possible that this rampart-like extracellular structure resulted from the accumulation of anhydrobiotic-specific material to protect the cells. Interestingly, after five hours of rehydration, the number of secretory cells decreased, and the specific extracellular structure began to disappear. Twenty-four hours after the beginning of rehydration, the cellular structure and ultrastructure were comparable to those observed in hydrated tardigrades.

A new redescription of Richtersius coronifer, supported by transcriptome, provides resources for describing concealed species diversity within the monotypic genus Richtersius (Eutardigrada)

Richtersius coronifer, the nominal species for the family Richtersiidae and a popular laboratory model, exemplifies a common problem in modern tardigrade taxonomy. Despite undeniable progress in the field, many old and incomplete descriptions of taxa hinder both species delimitation and the estimation of species diversity and distribution. Although for over a century this species has been recorded throughout the world, recent research indicates that records to date are likely to represent a species complex rather than a single cosmopolitan species. However, in order to recognise and name species diversity within the complex, an integrative redescription of the nominal species is first needed. Here, we describe an R. coronifer population collected from Spitsbergen, i.e., one of the two localities mentioned in the original description, with detailed morphological and morphometric data associated with standard DNA sequences of four standard genetic markers (18S rRNA, 28S rRNA, ITS-2, and COI) and supported by transcriptome sequencing. We propose replacement of the neotype designated in 1981 by Maucci and Ramazzotti, as it is impossible to verify whether the existing neotype is conspecific with specimens studied by Richters in 1903 and 1904. Finally, using newly obtained cytochrome c oxidase subunit I (COI) sequences of populations from Spitsbergen, Italy, Poland, and Greece together with sequences deposited in GenBank (China, Greenland, Italy, Mongolia), we performed genetic species delimitation, which indicated seven distinct potential species within the genus Richtersius, in addition to the nominal taxon. This study marks a starting point for further research on the taxonomy of and species diversity within the genus. Moreover, this work has the potential to be the first tardigrade redescription to provide both genetic barcodes and a transcriptome of the species in question.

Fine structure of the midgut epithelium of Thulinius ruffoi (Tardigrada, Eutardigrada, Parachela) in relation to oogenesis and simplex stage

Thulinius ruffoi is a small freshwater tardigrade that lives in both non-polluted and polluted freshwater environments. As a result of tardigradan body miniaturization, the digestive system is reduced and simplified. It consists of a short fore- and hindgut, and the midgut in the shape of a short tube is lined with a simple epithelium. The midgut epithelium is formed by the digestive cells and two rings of crescent-shaped cells were also detected. The anterior ring is located at the border between the fore- and midgut, while the posterior ring is situated between the mid- and hindgut. The precise ultrastructure of the digestive and crescent-shaped cells was examined using transmission electron microscopy, serial block face scanning electron microscopy and histochemical methods. We analyzed the changes that occurred in the midgut epithelial cells according to oogenesis (the species is parthenogenetic and there were only females in the laboratory culture). We focused on the accumulation of reserve material and the relationship between this and the intensity of autophagy. We concluded that autophagy supplies energy during a natural period of starvation (the simplex stage) and delivers the energy and probably the substances that are required during oogenesis. Apoptosis was not detected in the midgut epithelium of T. ruffoi.

Miniaturization of tardigrades (water bears): Morphological and genomic perspectives

Tardigrades form a monophyletic group of microscopic ecdysozoans best known for surviving extreme environmental conditions. Due to their key phylogenetic position as a subgroup of the Panarthropoda, understanding tardigrade biology is important for comparative studies with related groups like Arthropoda. Panarthropods - and Ecdysozoa as a whole - likely evolved from macroscopic ancestors, with several taxa becoming secondarily miniaturized. Morphological and genomic evidence likewise points to a miniaturized tardigrade ancestor. The five-segmented tardigrade body typically measures less than 1 mm in length and consists of only about 1000 cells. Most organs comprise a relatively small number of cells, with the highest proportion belonging to the central nervous system, while muscles are reduced to a single cell each. Similarly, fully sequenced genomes of three tardigrade species - together with Hox gene expression data - point to extensive modifications, rearrangements, and major losses of genes and even a large body region. Parallels are evident with related ecdysozoans that may have also undergone genomic reductions, such as the nematode Caenorhabditis elegans. We interpret these data together as evidence of miniaturization in the tardigrade lineage, while cautioning that the effects of miniaturization may manifest in different ways depending on the organ or organ system under examination.

Cell Biology of the Tardigrades: Current Knowledge and Perspectives

The invertebrate phylum Tardigrada has received much attention for containing species adapted to the most challenging environmental conditions where an ability to survive complete desiccation or freezing in a cryptobiotic state is necessary for persistence. Although research on tardigrades has a long history, the last decade has seen a dramatic increase in molecular biological ("omics") studies, most of them with the aim to reveal the biochemical mechanisms behind desiccation tolerance of tardigrades. Several other aspects of tardigrade cell biology have been studied, and we review some of them, including karyology, embryology, the role of storage cells, and the question of whether tardigrades are eutelic animals. We also review some of the theories about how anhydrobiotic organisms are able to maintain cell integrity under dry conditions, and our current knowledge on the role of vitrification and DNA protection and repair. Many aspects of tardigrade stress tolerance have relevance for human medicine, and the first transfers of tardigrade stress genes to human cells have now appeared. We expect this field to develop rapidly in the coming years, as more genomic information becomes available. However, many basic cell biological aspects remain to be investigated, such as immunology, cell cycle kinetics, cell metabolism, and culturing of tardigrade cells. Such development will be necessary to allow tardigrades to move from a nonmodel organism position to a true model organism with interesting associations with the current models C. elegans and D. melanogaster.

X-ray imaging of a water bear offers a new look at tardigrade internal anatomy

BACKGROUND

Tardigrades (water bears) are microscopic invertebrates of which the anatomy has been well studied using traditional techniques, but a comprehensive three-dimensional reconstruction has never been performed. In order to close this gap, we employed X-ray computed tomography (CT), a technique that is becoming increasingly popular in zoology for producing high-resolution, three-dimensional (3D) scans of whole specimens. While CT has long been used to scan larger samples, its use in some microscopic animals can be problematic, as they are often too small for conventional CT yet too large for high-resolution, optics-based soft X-ray microscopy. This size gap continues to be narrowed with advancements in technology, with high-resolution imaging now being possible using both large synchrotron devices and, more recently, laboratory-based instruments.

RESULTS

Here we use a recently developed prototype lab-based nano-computed tomography device to image a 152 μm-long tardigrade at high resolution (200-270 nm pixel size). The resulting dataset allowed us to visualize the anatomy of the tardigrade in 3D and analyze the spatial relationships of the internal structures. Segmentation of the major structures of the body enabled the direct measurement of their respective volumes. Furthermore, we segmented every storage cell individually and quantified their volume distribution. We compare our measurements to those from published studies in which other techniques were used.

CONCLUSIONS

The data presented herein demonstrate the utility of CT imaging as a powerful supplementary tool for studies of tardigrade anatomy, especially for quantitative volume measurements. This nanoCT study represents the smallest complete animal ever imaged using CT, and offers new 3D insights into the spatial relationships of the internal organs of water bears.

Reproduction, Gonad Structure, and Oogenesis in Tardigrades

Even though tardigrades have been known since 1772, their phylogenetic position is still controversial. Tardigrades are regarded as either the sister group of arthropods, onychophorans, or onychophorans plus arthropods. Furthermore, the knowledge about their gametogenesis, especially oogenesis, is still poor and needs further analysis. The process of oogenesis has been studied solely for several eutardigradan species. Moreover, the spatial organization of the female germ-line clusters has been described for three species only. Meroistic ovaries characterize all analyzed species. In species of the Parachela, one cell per germ-cell cluster differentiates into the oocyte, while the remaining cells become the trophocytes. In Apochela several cells in the cluster differentiate into oocytes. Vitellogenesis is of a mixed type. The eggs are covered with the egg capsule that is composed of two shells: the thin vitelline envelope that adheres to the oolemma and the thick three-layered chorion. Chorion is formed as a first followed by vitelline envelope. Several features related to the oogenesis and structure of the ovary confirm the hypothesis that tardigrades are the sister group rather for arthropods than for onychophorans.