Newly Identified Nematodes from Mono Lake Exhibit Extreme Arsenic Resistance

Newly identified nematodes from the Great Salt Lake are associated with microbialites and specially adapted to hypersaline conditions

Extreme environments enable the study of simplified food-webs and serve as models for evolutionary bottlenecks and early Earth ecology. We investigated the biodiversity of invertebrate meiofauna in the benthic zone of the Great Salt Lake (GSL), Utah, USA, one of the most hypersaline lake systems in the world. The hypersaline bays within the GSL are currently thought to support only two multicellular animals: brine fly larvae and brine shrimp. Here, we report the presence, habitat, and microbial interactions of novel free-living nematodes. Nematode diversity drops dramatically along a salinity gradient from a freshwater river into the south arm of the lake. In Gilbert Bay, nematodes primarily inhabit reef-like organosedimentary structures built by bacteria called microbialites. These structures likely provide a protective barrier to UV and aridity, and bacterial associations within them may support life in hypersaline environments. Notably, sampling from Owens Lake, another terminal lake in the Great Basin that lacks microbialites, did not recover nematodes from similar salinities. Phylogenetic divergence suggests that GSL nematodes represent previously undescribed members of the family Monhysteridae-one of the dominant fauna of the abyssal zone and deep-sea hydrothermal vents. These findings update our understanding of halophile ecosystems and the habitable limit of animals.

Geographical distribution and phoretic associations of the viviparous nematode Tokorhabditis atripennis with Onthophagus dung beetles in Japan

Viviparity is generally considered to be rare in animals. In nematodes, only six species of Rhabditida are viviparous. Five of these species have been identified in association with Onthophagus dung beetles, with Tokorhabditis atripennis being repeatedly isolated from the dung beetle Onthophagus atripennis in Japan. T. atripennis is easy to culture in a laboratory setting, and its host, O. atripennis, is distributed all over Japan. Therefore, T. atripennis is an ideal candidate for ecological and evolutionary studies on viviparity. However, the extent of their distribution and relationship with dung beetles, as well as habitats, remain unclear. In the present study, we conducted field surveys and successfully isolated 27 strains of viviparous nematodes associated with tunneler dung beetles from various regions of Japan, all of which were identified as T. atripennis. T. atripennis exhibited a strong association with Onthophagus dung beetles, especially O. apicetinctus and O. atripennis. And it was predominantly found in specific anatomical locations on the beetle bodies, such as the 'groove between pronotum and elytron' and the 'back of the wings'. Our findings suggest that Onthophagus species are the primary hosts for T. atripennis, and T. atripennis exhibits a close relationship with the living environments of tunneler beetles. This association may play a significant role in the evolution of viviparity in nematodes.

The developmental and structural uniqueness of the embryo of the extremophile viviparous nematode, Tokorhabditis tufae

Viviparity, a reproductive form that supplies nutrients to the embryo during gestation, has repeatedly and independently occurred in multiple lineages of animals. During the convergent evolution of viviparity, various modifications of development, structure, and physiology emerged. A new species of nematode, Tokorhabditis tufae, was discovered in the alkaline, hypersaline, and arsenic-rich environment of Mono lake. Its reproductive form is viviparity because it is obligately live-bearing and the embryo increases in size during development. However, the magnitude of the increase in size and nutrient provisioning are unclear. We measured egg and embryo sizes at three developmental stages in T. tufae. Eggs and embryos of T. tufae at the threefold stage were respectively 2.6- and 3.6-fold larger than at the single-cell stage. We then obtained T. tufae embryos at the single-cell, lima bean, and threefold developmental stages and investigated the egg hatching frequency at three different concentrations of egg salt buffer. Removal of embryos from the uterus halted embryonic development at the single-cell and lima bean stages in T. tufae irrespective of the solution used for incubation, indicating the provision of nutrients within the uterus. Ultrastructural and permeability evaluation showed that the permeability barrier did not form during embryonic development, resulting in increased molecular permeability. This high permeability caused by the absence of the permeability barrier likely enables supply of nutrients from the mother. The structural and physiological modifications in T. tufae are like those in other viviparous animals. We conclude that T. tufae is a viviparous rather than an ovoviviparous nematode. T. tufae will facilitate investigation of the evolution of viviparity in animals.

Trioecy is maintained as a time-stable mating system in the pink sea urchin Toxopneustes roseus from the Mexican Pacific

Trioecy is a sexual system that consists of the co-occurrence of females, males and hermaphrodites in a population and is common in plants; however, in animals it is uncommon and poorly understood. In echinoderms, trioecy had never been recorded until now. Frequencies of females, males, and hermaphrodites were evaluated and gametogenic development was histologically characterized in a population of Toxopneustes roseus inhabiting the Mexican Pacific. Trioecy in this population is functional and temporally stable, since the three sexes coexisted in each sampling month. The hermaphrodites presented similar gametogenic development as the females and males and participated during the spawning season, contributing to the population's reproductive process. Trioecy is considered an evolutionarily transitory state, and it is extremely difficult to explain its presence in a species. We hypothesize that continuous ocean warming represents a threat to the survival of this population of T. roseus, since its early developmental stages, which represent a population bottleneck, are more vulnerable to high temperatures than other sea urchins inhabiting the area, while its population density is significantly lower. These conditions generate a strongly stressed environment, which is the determining factor that maintains the stability of trioecy in the species in which it has been studied.

Tokorhabditis tauri n. sp. and T. atripennis n. sp. (Rhabditida: Rhabditidae), isolated from Onthophagus dung beetles (Coleoptera: Scarabaeidae) from the Eastern USA and Japan

Two new species of Tokorhabditis, T. tauri n. sp. and T. atripennis n. sp., which were isolated from multiple Onthophagus species in North America and from O. atripennis in Japan, respectively, are described. The new species are each diagnosed by characters of the male tail and genitalia, in addition to molecular barcode differences that were previously reported. The description of T. tauri n. sp. expands the suite of known nematode associates of O. taurus, promoting ecological studies using a beetle that is an experimental model for insect–nematode–microbiota interactions in a semi-natural setting. Furthermore, our description of a third Tokorhabditis species, T. atripennis n. sp., sets up a comparative model for such ecological interactions, as well as other phenomena as previously described for T. tufae, including maternal care through obligate vivipary, the evolution of reproductive mode, and extremophilic living.

Reproductive plasticity in response to food source in the fungal-feeding nematode Bursaphelenchus okinawaensis

Organisms use various strategies to cope with fluctuating environments. Some organisms express different phenotypes in alternative conditions through a process known as ‘phenotypic plasticity’, which is presumably an evolutionary adaptation to environmental variation. Nematodes adapt to various environments; it has been suggested that phenotypic plasticity is a contributing factor in their high level of environmental adaptability. We investigated the reproductive plasticity in response to a food source in the fungal-feeding nematode, Bursaphelenchus okinawaensis. Bursaphelenchus okinawaensis is known to reproduce primarily as a self-fertilising hermaphrodite on a filamentous fungus and yeast; here, we showed that newly isolated SH3 strain hermaphrodites produced a small number of progenies (⩽9 progenies per hermaphrodite) on the yeast test plate, while they laid similar numbers of eggs to the SH1 strain on the fungus test plate. Subsequent sperm observation by 4′,6-diamidino-2-phenylindole (DAPI) revealed that SH3 hermaphrodites could produce only a small number of sperm on the yeast test plate. Some hermaphrodites did not produce any eggs, indicating that they had become females rather than hermaphrodites. These results showed that the hermaphrodite or female status of SH3 nematodes was a plastic character, dependent on the food stimulus. An intra-strain crossing test between SH1 and SH3 suggested that the reproductive plasticity was controlled by a single recessive gene. This study provided an insight into a novel type of phenotypic plasticity in nematodes.

OFF-switching property of quorum sensor LuxR via As(III)-induced insoluble form

Whole-cell sensors for arsenite detection have been developed exclusively based on the natural arsenite (As(III)) sensory protein ArsR for arsenic metabolism. This study reports that the quorum-sensing LuxR/Plux system from Vibrio fischeri, which is completely unrelated to arsenic metabolism, responds to As(III) in a dose-dependent manner. Due to as many as 9 cysteine residues, which has a high binding affinity with As(III), LuxR underwent As(III)-induced insoluble form, thereby reducing its effective cellular concentration. Accordingly, the expression level of green fluorescent protein under the control of Plux gradually decreased with increasing As(III) concentration in the medium. This is a novel As(III)-detection system that has never been proposed before, with a unique ON-to-OFF transfer function.

Cryopreservation of C. elegans and Other Nematodes with Dimethyl Sulfoxide and Trehalose

One of the key attributes that has contributed to the popularity of Caenorhabditis elegans as a model system is its ability to survive freezing. By preserving stocks at ultralow temperature, researchers have been able to generate an unlimited number of strains without the burden of constantly maintaining them. This has facilitated the use of large-scale forward genetic screens and CRISPR-mediated genome editing where large numbers of novel and informative mutants can be generated. Traditionally, C. elegans and other nematodes were frozen using glycerol as a cryoprotectant. While effective, a large majority of animals do not survive a typical freeze-thaw cycle. Here I describe an alternative method based on the popular combination of DMSO and trehalose as a cryoprotectant. This method allows the survival of large numbers of worms and effectively protects most developmental stages including adults.

Tokorhabditis n. gen. (Rhabditida, Rhabditidae), a comparative nematode model for extremophilic living

Life in extreme environments is typically studied as a physiological problem, although the existence of extremophilic animals suggests that developmental and behavioral traits might also be adaptive in such environments. Here, we describe a new species of nematode, Tokorhabditis tufae, n. gen., n. sp., which was discovered from the alkaline, hypersaline, and arsenic-rich locale of Mono Lake, California. The new species, which offers a tractable model for studying animal-specific adaptations to extremophilic life, shows a combination of unusual reproductive and developmental traits. Like the recently described sister group Auanema, the species has a trioecious mating system comprising males, females, and self-fertilizing hermaphrodites. Our description of the new genus thus reveals that the origin of this uncommon reproductive mode is even more ancient than previously assumed, and it presents a new comparator for the study of mating-system transitions. However, unlike Auanema and almost all other known rhabditid nematodes, the new species is obligately live-bearing, with embryos that grow in utero, suggesting maternal provisioning during development. Finally, our isolation of two additional, molecularly distinct strains of the new genus-specifically from non-extreme locales-establishes a comparative system for the study of extremophilic traits in this model.

Fisher vs. the Worms: Extraordinary Sex Ratios in Nematodes and the Mechanisms that Produce Them

Parker, Baker, and Smith provided the first robust theory explaining why anisogamy evolves in parallel in multicellular organisms. Anisogamy sets the stage for the emergence of separate sexes, and for another phenomenon with which Parker is associated: sperm competition. In outcrossing taxa with separate sexes, Fisher proposed that the sex ratio will tend towards unity in large, randomly mating populations due to a fitness advantage that accrues in individuals of the rarer sex. This creates a vast excess of sperm over that required to fertilize all available eggs, and intense competition as a result. However, small, inbred populations can experience selection for skewed sex ratios. This is widely appreciated in haplodiploid organisms, in which females can control the sex ratio behaviorally. In this review, we discuss recent research in nematodes that has characterized the mechanisms underlying highly skewed sex ratios in fully diploid systems. These include self-fertile hermaphroditism and the adaptive elimination of sperm competition factors, facultative parthenogenesis, non-Mendelian meiotic oddities involving the sex chromosomes, and environmental sex determination. By connecting sex ratio evolution and sperm biology in surprising ways, these phenomena link two "seminal" contributions of G. A. Parker.

Genome-Wide Transcriptional Responses of Marine Nematode Litoditis marina to Hyposaline and Hypersaline Stresses

Maintenance of osmotic homeostasis is essential for all organisms, especially for marine animals in the ocean with 3% salinity or higher. However, the underlying molecular mechanisms that how marine animals adapt to high salinity environment compared to their terrestrial relatives, remain elusive. Here, we investigated marine animal’s genome-wide transcriptional responses to salinity stresses using an emerging marine nematode model Litoditis marina. We found that the transthyretin-like family genes were significantly increased in both hyposaline and hypersaline conditions, while multiple neurotransmitter receptor and ion transporter genes were down-regulated in both conditions, suggesting the existence of conserved strategies for response to stressful salinity environments in L. marina. Unsaturated fatty acids biosynthesis related genes, neuronal related tubulins and intraflagellar transport genes were specifically up-regulated in hyposaline treated worms. By contrast, cuticle related collagen genes were enriched and up-regulated for hypersaline response. Given a wide range of salinity tolerance of the marine nematodes, this study and further genetic analysis of key gene(s) of osmoregulation in L. marina will likely provide important insights into biological evolution and environmental adaptation mechanisms in nematodes and other invertebrate animals in general.

Why are nematodes so successful extremophiles?

Extreme environments constitute the largest habitat on earth, but our understanding of life in such environments is rudimentary. The hostility of extreme environments such as the deep sea, earth’s crust, and toxic lakes limits the sampling, culturing, and studying of extremophiles, the organisms that live in these habitats. Thus, in terms of ecological research, extreme environments are the earth’s final frontier. A growing body of data suggests that nematodes are the most common animal taxon in different types of extreme settings such as the deep-subsurface and sediments in the deep sea. Notably, the reasons for the abundance of nematodes in extreme habitats remain mostly unknown. I propose that a unique combination of several characteristics of nematodes may explain, additively or synergistically, their successful adaptation to extreme habitats. Novel functional genetic and genomic approaches are expected to reveal molecular mechanisms of adaptation of nematodes to the many fascinating extreme environments on earth.

Nematode epibionts on skin of the Florida manatee, Trichechus manatus latirostris

A survey for the presence of nematodes on the skin of the native Florida manatee, Trichechus manatus latirostris from Crystal River, Florida was conducted during annual manatee health assessments. A putative isolate of Cutidiplogaster manati (Diplogastridae) and two other nematodes belonging to the same family were recovered from mid-dorsal tail skin-scrapings from all sampled winter-collected healthy wild adult manatees during two successive years (2018–2019). Qualitative abundance estimates of these three species of diplogastrid nematodes suggest that an average wild Florida manatee adult might possess between 30,000 and 120,000 nematodes on its tail dorsum and that the entire body dorsum including the tail might possess 160,000–640,000 nematodes in roughly equal ratios. Attempts to culture these nematodes on a variety of different culture media were unsuccessful but examination of the mouth (stomatal) morphology suggests specialized feeding on microbes such as diatoms or predation on other nematodes. No skin lesions were observed during the 2018–2019 samplings suggesting that under normal conditions these nematodes are highly specialized free-living epibionts of the skin that are tightly bound to this niche and horizontally transferred between individual manatees in an analogous fashion to human skin mites (Demodex folliculorum and D. brevis). Molecular phylogenetic inferences using sequences of near full length SSU and D2–D3 expansion segments of LSU rRNA genes revealed a putative new morphospecies in Cutidiplogaster sister to C. manati that was monophyletic with several named Mononchoides species, and another putative new morphospecies that formed a clade with several undescribed species similar in appearance to Mononchoides as well as Tylopharyx, Eudiplogasterium, Paroigolaimella and Sachsia.

The O-GlcNAc transferase OGT is a conserved and essential regulator of the cellular and organismal response to hypertonic stress

The conserved O-GlcNAc transferase OGT O-GlcNAcylates serine and threonine residues of intracellular proteins to regulate their function. OGT is required for viability in mammalian cells, but its specific roles in cellular physiology are poorly understood. Here we describe a conserved requirement for OGT in an essential aspect of cell physiology: the hypertonic stress response. Through a forward genetic screen in Caenorhabditis elegans, we discovered OGT is acutely required for osmoprotective protein expression and adaptation to hypertonic stress. Gene expression analysis shows that ogt-1 functions through a post-transcriptional mechanism. Human OGT partially rescues the C. elegans phenotypes, suggesting that the osmoregulatory functions of OGT are ancient. Intriguingly, expression of O-GlcNAcylation-deficient forms of human or worm OGT rescue the hypertonic stress response phenotype. However, expression of an OGT protein lacking the tetracopeptide repeat (TPR) domain does not rescue. Our findings are among the first to demonstrate a specific physiological role for OGT at the organismal level and demonstrate that OGT engages in important molecular functions outside of its well described roles in post-translational O-GlcNAcylation of intracellular proteins.

Evolution and Developmental System Drift in the Endoderm Gene Regulatory Network of Caenorhabditis and Other Nematodes

Developmental gene regulatory networks (GRNs) underpin metazoan embryogenesis and have undergone substantial modification to generate the tremendous variety of animal forms present on Earth today. The nematode Caenorhabditis elegans has been a central model for advancing many important discoveries in fundamental mechanistic biology and, more recently, has provided a strong base from which to explore the evolutionary diversification of GRN architecture and developmental processes in other species. In this short review, we will focus on evolutionary diversification of the GRN for the most ancient of the embryonic germ layers, the endoderm. Early embryogenesis diverges considerably across the phylum Nematoda. Notably, while some species deploy regulative development, more derived species, such as C. elegans, exhibit largely mosaic modes of embryogenesis. Despite the relatively similar morphology of the nematode gut across species, widespread variation has been observed in the signaling inputs that initiate the endoderm GRN, an exemplar of developmental system drift (DSD). We will explore how genetic variation in the endoderm GRN helps to drive DSD at both inter- and intraspecies levels, thereby resulting in a robust developmental system. Comparative studies using divergent nematodes promise to unveil the genetic mechanisms controlling developmental plasticity and provide a paradigm for the principles governing evolutionary modification of an embryonic GRN.