Vasa, PL10, and Piwi gene expression during caudal regeneration of the polychaete annelid Alitta virens

Cloning, Characterization, and Expression Analysis of the DEAD-Box Family Genes, Vasa and PL10, in Pacific Abalone (Haliotis discus hannai)

BACKGROUND/OBJECTIVES

Vasa and PL10 belong to the DEAD-box protein family, which plays crucial roles in various cellular functions, such as DNA replication, DNA repair, and RNA processing. Additionally, DEAD-box family genes have also been identified as being related to gonadal development in many species. However, the function of vasa and PL10 in abalone is poorly understood on a molecular level.

METHODS

In the present study, we individually isolated and characterized the vasa and PL10 orthologs in Haliotis discus hannai (Hdh-vasa and Hdh-PL10). We also characterized the mRNA distributions of vasa and PL10 in various tissues from adult organisms and different embryonic developmental stages using real-time PCR (RT-qPCR) techniques. Furthermore, spatial and temporal expression of Hdh-vasa and Hdh-PL10 throughout embryonic and larval development was examined by whole-mount in situ hybridization (WMISH).

RESULTS

The two predicted amino acid sequences contained all of the conserved motifs characterized by the DEAD-box family. Homology and phylogenetic analyses indicate that they belong to the vasa and PL10 subfamilies. We found that vasa and PL10 mRNA were not solely restricted to gonads but were widely expressed in various tissues. WMISH showed that Hdh-vasa and Hdh-PL10 largely overlapped, with both being maternally expressed and specifically localized to the micromere lineage cells during early cleavage stages. By the gastrulation stage, Hdh-vasa were expressed strongly in two bilaterally symmetrical paraxial clusters, but Hdh-PL10 was dispersed in entire endodermal region. Our results suggest that Hdh-vasa-expressing cells are located as a subpopulation of undifferentiated multipotent cells that express Hdh-PL10. As such, we infer that primordial germ cells are specified from these vasa-expressing cells at some point during development, and inductive signals (epigenesis) play an important role in specifying primordial germ cells (PGCs) in H. discus hannai.

CONCLUSIONS

This study provides valuable insights into the molecular characteristics and expression patterns of Hdh-vasa and Hdh-PL10, contributing to a better understanding of their roles in germ cell specification and early embryonic development in H. discus hannai.

In vitro detection of marine invertebrate stem cells: utilizing molecular and cellular biology techniques and exploring markers

Marine invertebrate stem cells (MISCs) represent a distinct category of pluripotent and totipotent cells with remarkable abilities for self-renewal and differentiation into multiple germ layers, akin to their vertebrate counterparts. These unique cells persist throughout an organism's adult life and have been observed in various adult marine invertebrate phyla. MISCs play crucial roles in numerous biological processes, including developmental biology phenomena specific to marine invertebrates, such as senescence, delayed senescence, whole-body regeneration, and asexual reproduction. Furthermore, they serve as valuable models for studying stem cell biology. Despite their significance, information about MISCs remains scarce and scattered in the scientific literature. In this review, we have carefully collected and summarized valuable information about MISC detection by perusing the articles that study and detect MISCs in various marine invertebrate organisms. The review begins by defining MISCs and highlighting their unique features compared to vertebrates. It then discusses the common markers for MISC detection and in vitro techniques employed in invertebrate and vertebrates investigation. This comprehensive review provides researchers and scientists with a cohesive and succinct overview of MISC characteristics, detection methods, and associated biological phenomena in marine invertebrate organisms. We aim to offer a valuable resource to researchers and scientists interested in marine invertebrate stem cells, fostering a better understanding of their broader implications in biology. With ongoing advancements in scientific techniques and the continued exploration of marine invertebrate species, we anticipate that further discoveries will expand our knowledge of MISCs and their broader implications in biology.

SoxC and MmpReg promote blastema formation in whole-body regeneration of fragmenting potworms Enchytraeus japonensis

Regeneration in many animals involves the formation of a blastema, which differentiates and organizes into the appropriate missing body parts. Although the mechanisms underlying blastema formation are often fundamental to regeneration biology, information on the cellular and molecular basis of blastema formation remains limited. Here, we focus on a fragmenting potworm (Enchytraeus japonensis), which can regenerate its whole body from small fragments. We find soxC and mmpReg as upregulated genes in the blastema. RNAi of soxC and mmpReg reduce the number of blastema cells, indicating that soxC and mmpReg promote blastema formation. Expression analyses show that soxC-expressing cells appear to gradually accumulate in blastema and constitute a large part of the blastema. Additionally, similar expression dynamics of SoxC orthologue genes in frog (Xenopus laevis) are found in the regeneration blastema of tadpole tail. Our findings provide insights into the cellular and molecular mechanisms underlying blastema formation across species.

Annelid adult cell type diversity and their pluripotent cellular origins

Many annelids can regenerate missing body parts or reproduce asexually, generating all cell types in adult stages. However, the putative adult stem cell populations involved in these processes, and the diversity of cell types generated by them, are still unknown. To address this, we recover 75,218 single cell transcriptomes of the highly regenerative and asexually-reproducing annelid Pristina leidyi. Our results uncover a rich cell type diversity including annelid specific types as well as novel types. Moreover, we characterise transcription factors and gene networks that are expressed specifically in these populations. Finally, we uncover a broadly abundant cluster of putative stem cells with a pluripotent signature. This population expresses well-known stem cell markers such as vasa, piwi and nanos homologues, but also shows heterogeneous expression of differentiated cell markers and their transcription factors. We find conserved expression of pluripotency regulators, including multiple chromatin remodelling and epigenetic factors, in piwi+ cells. Finally, lineage reconstruction analyses reveal computational differentiation trajectories from piwi+ cells to diverse adult types. Our data reveal the cell type diversity of adult annelids by single cell transcriptomics and suggest that a piwi+ cell population with a pluripotent stem cell signature is associated with adult cell type differentiation.

Germ Line/Multipotency Genes Show Differential Expression during Embryonic Development of the Annelid Enchytraeus coronatus

Germ line development and the origin of the primordial germ cells (PGCs) are very variable and may occur across a range of developmental stages and in several developmental contexts. In establishing and maintaining germ line, a conserved set of genes is involved. On the other hand, these genes are expressed in multipotent/pluripotent cells that may give rise to both somatic and germline cells. To begin elucidating mechanisms by which the germ line is specified in Enchytraeus coronatus embryos, we identified twenty germline/multipotency genes, homologs of Vasa, PL10, Piwi, Nanos, Myc, Pumilio, Tudor, Boule, and Bruno, using transcriptome analysis and gene cloning, and characterized their expression by whole-mount in situ hybridization. To answer the question of the possible origin of PGCs in this annelid, we carried out an additional description of the early embryogenesis. Our results suggest that PGCs derive from small cells originating at the first two divisions of the mesoteloblasts. PGCs form two cell clusters, undergo limited proliferation, and migrate to the developing gonadal segments. In embryos and juvenile E. coronatus, homologs of the germline/multipotency genes are differentially expressed in both germline and somatic tissue including the presumptive germ cell precursors, posterior growth zone, developing foregut, and nervous system.

Vasa, Piwi, and Pl10 Expression during Sexual Maturation and Asexual Reproduction in the Annelid Pristina longiseta

Naidids are tiny, transparent freshwater oligochaetes, which are well known for their ability to propagate asexually. Despite the fact that sexually mature individuals and cocoons with embryos are sometimes found in nature, in long-period laboratory cultures, worms reproduce agametically only. In this paper, we showed, for the first time, the expression of Vasa, Piwi, and Pl10 homologs in mature Pristina longiseta worms with well-developed reproductive system structures and germ cells. Although the animals have been propagated asexually by paratomic fission for over 20 years in our lab, some individuals become sexualized under standard conditions for our laboratory culture and demonstrate various stages of maturation. The fully matured animals developed a complete set of sexual apparatus including spermatheca, atrium, seminal vesicles, and ovisac. They also had a clitellum and were able to form cocoons. The cues for the initiation of sexual maturation are still unknown for P. longiseta; nevertheless, our data suggest that the laboratory strain of P. longiseta maintains the ability to become fully sexually mature and to establish germline products even after a long period of agametic reproduction. On the other hand, many of the sexualized worms formed a fission zone and continued to reproduce asexually. Thus, in this species, the processes of asexual reproduction and sexual maturation do not preclude each other, and Vasa, Piwi, and Pl10 homologs are expressed in both somatic and germline tissue including the posterior growth zone, fission zone, nervous system, germline cells, and gametes.

Spatial Colinear but Broken Temporal Expression of Duplicated ParaHox Genes in Asexually Reproducing Annelids, Nais communis and Pristina longiseta

ParaHox genes are key developmental regulators involved in the patterning of the digestive tract along the anteroposterior axis and the development of the nervous system. Most studies have focused on the function of these genes in embryogenesis, while their expression patterns in postembryonic development often remain unknown. In this study, we identified for the first time all ParaHox orthologs in two naidid oligochaetes, N. communis and P. longiseta, and described their expression patterns during normal growth and fission in these animals. We showed that Gsx and Cdx are presented by two paralogs, while Xlox is a single copy gene in both species. Using whole-mount in situ hybridization, we also found that orthologs, except for the Xlox gene, have similar activity patterns with minor differences in details, while the expression patterns of paralogs can differ significantly. However, all these genes are involved in axial patterning and/or in tissue remodeling during growth and asexual reproduction in naidids. Moreover, during paratomic fission, these genes are expressed with spatial colinearity but temporal colinearity is broken. The results of this study may be evidence of the functional diversification of duplicated genes and suggest involvement of the ParaHox genes in whole-body patterning during growth and asexual reproduction in annelids.

Cell Proliferation Indices in Regenerating Alitta virens (Annelida, Errantia)

In recent years, interest in the possible molecular regulators of cell proliferation and differentiation in a wide range of regeneration models has grown significantly, but the cell kinetics of this process remain largely a mystery. Here we try to elucidate the cellular aspects of regeneration by EdU incorporation in intact and posteriorly amputated annelid Alitta virens using quantitative analysis. We found that the main mechanism of blastema formation in A. virens is local dedifferentiation; mitotically active cells of intact segments do not significantly contribute to the blastemal cellular sources. Amputation-induced proliferation occurred predominantly within the epidermal and intestinal epithelium, as well as wound-adjacent muscle fibers, where clusters of cells at the same stage of the cell cycle were found. The resulting regenerative bud had zones of high proliferative activity and consisted of a heterogeneous population of cells that differed in their anterior-posterior positions and in their cell cycle parameters. The data presented allowed for the quantification of cell proliferation in the context of annelid regeneration for the first time. Regenerative cells showed an unprecedentedly high cycle rate and an exceptionally large growth fraction, making this regeneration model especially valuable for studying coordinated cell cycle entry in vivo in response to injury.

On the hormonal control of posterior regeneration in the annelid Platynereis dumerilii

Regeneration is the process by which many animals are able to restore lost or injured body parts. After amputation of the posterior part of its body, the annelid Platynereis dumerilii is able to regenerate the pygidium, the posteriormost part of its body that bears the anus, and a subterminal growth zone containing stem cells that allows the subsequent addition of new segments. The ability to regenerate their posterior part (posterior regeneration) is promoted, in juvenile worms, by a hormone produced by the brain and is lost when this hormonal activity becomes low at the time the worms undergo their sexual maturation. By characterizing posterior regeneration at the morphological and molecular levels in worms that have been decapitated, we show that the presence of the head is essential for multiple aspects of posterior regeneration, as well as for the subsequent production of new segments. We also show that methylfarnesoate, the molecule proposed to be the brain hormone, can partially rescue the posterior regeneration defects observed in decapitated worms. Our results are therefore consistent with a key role of brain hormonal activity in the control of regeneration and growth in P. dumerilii, and support the hypothesis of the involvement of methylfarnesoate in this control.

Transposon control as a checkpoint for tissue regeneration

Tissue regeneration requires precise temporal control of cellular processes such as inflammatory signaling, chromatin remodeling and proliferation. The combination of these processes forms a unique microenvironment permissive to the expression, and potential mobilization of, transposable elements (TEs). Here, we develop the hypothesis that TE activation creates a barrier to tissue repair that must be overcome to achieve successful regeneration. We discuss how uncontrolled TE activity may impede tissue restoration and review mechanisms by which TE activity may be controlled during regeneration. We posit that the diversification and co-evolution of TEs and host control mechanisms may contribute to the wide variation in regenerative competency across tissues and species.

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.

Nanos Is Expressed in Somatic and Germline Tissue during Larval and Post-Larval Development of the Annelid Alitta virens

Nanos is a translational regulator that is involved in germline development in a number of diverse animals and is also involved in somatic patterning in several model organisms, including insects. Neither germline development nor somatic stem cell lines/undifferentiated multipotent cells have been characterized in the development of the annelid Alitta virens, nor is the mechanism of germ/stem-line specification generally well-understood in annelids. Here, I have cloned an Avi-nanos ortholog from A. virens and determined the spatial and temporal expression of Nanos. The results revealed that transcripts of nanos are expressed during differentiation of multiple tissues, including those that are derived from the 2d and 4d cells. In late embryonic stages and during larval development, these transcripts are expressed in the presumptive brain, ventral nerve cord, mesodermal bands, putative primordial germ cells (PGCs), and developing foregut and hindgut. During metamorphosis of the nectochaete larva into a juvenile worm, a posterior growth zone consisting of nanos-positive cells is established, and the PGCs begin to migrate. Later, the PGCs stop migrating and form a cluster of four nanos-expressing cells located immediately behind the jaws (segments 4–5). During posterior regeneration following caudal amputation, a robust Avi-nanos expression appears de novo at the site of injury and further accompanies all steps of regeneration. The obtained data suggest that blastemal cells are mostly derived from cells of the segment adjacent to the amputation site; this is consistent with the idea that the cluster of PGCs do not participate in regeneration.

It Cuts Both Ways: An Annelid Model System for the Study of Regeneration in the Laboratory and in the Classroom

The mechanisms supporting regeneration and successful recovery of function have fascinated scientists and the general public for quite some time, with the earliest description of regeneration occurring in the 8th century BC through the Greek mythological story of Prometheus. While most animals demonstrate the capacity for wound-healing, the ability to initiate a developmental process that leads to a partial or complete replacement of a lost structure varies widely among animal taxa. Variation also occurs within single species based on the nature and location of the wound and the developmental stage or age of the individual. Comparative studies of cellular and molecular changes that occur both during, and following, wound healing may point to conserved genomic pathways among animals of different regenerative capacity. Such insights could revolutionize studies within the field of regenerative medicine. In this review, we focus on several closely related species of Lumbriculus (Clitellata: Lumbriculidae), as we present a case for revisiting the use of an annelid model system for the study of regeneration. We hope that this review will provide a primer to Lumbriculus biology not only for regeneration researchers but also for STEM teachers and their students.

Regeneration in the Segmented Annelid Capitella teleta

The segmented worms, or annelids, are a clade within the Lophotrochozoa, one of the three bilaterian superclades. Annelids have long been models for regeneration studies due to their impressive regenerative abilities. Furthermore, the group exhibits variation in adult regeneration abilities with some species able to replace anterior segments, posterior segments, both or neither. Successful regeneration includes regrowth of complex organ systems, including the centralized nervous system, gut, musculature, nephridia and gonads. Here, regenerative capabilities of the annelid Capitella teleta are reviewed. C. teleta exhibits robust posterior regeneration and benefits from having an available sequenced genome and functional genomic tools available to study the molecular and cellular control of the regeneration response. The highly stereotypic developmental program of C. teleta provides opportunities to study adult regeneration and generate robust comparisons between development and regeneration.

Comparative genomic and transcriptomic analyses of transposable elements in polychaetous annelids highlight LTR retrotransposon diversity and evolution

Background

With the expansion of high throughput sequencing, we now have access to a larger number of genome-wide studies analyzing the Transposable elements (TEs) composition in a wide variety of organisms. However, genomic analyses often remain too limited in number and diversity of species investigated to study in depth the dynamics and evolutionary success of the different types of TEs among metazoans. Therefore, we chose to investigate the use of transcriptomes to describe the diversity of TEs in phylogenetically related species by conducting the first comparative analysis of TEs in two groups of polychaetes and evaluate the diversity of TEs that might impact genomic evolution as a result of their mobility.

Results

We present a detailed analysis of TEs distribution in transcriptomes extracted from 15 polychaetes depending on the number of reads used during assembly, and also compare these results with additional TE scans on associated low-coverage genomes. We then characterized the clades defined by 1021 LTR-retrotransposon families identified in 26 species. Clade richness was highly dependent on the considered superfamily. Copia elements appear rare and are equally distributed in only three clades, GalEa, Hydra and CoMol. Among the eight BEL/Pao clades identified in annelids, two small clades within the Sailor lineage are new for science. We characterized 17 Gypsy clades of which only 4 are new; the C-clade largely dominates with a quarter of the families. Finally, all species also expressed for the majority two distinct transcripts encoding PIWI proteins, known to be involved in control of TEs mobilities.

Conclusions

This study shows that the use of transcriptomes assembled from 40 million reads was sufficient to access to the diversity and proportion of the transposable elements compared to those obtained by low coverage sequencing. Among LTR-retrotransposons Gypsy elements were unequivocally dominant but results suggest that the number of Gypsy clades, although high, may be more limited than previously thought in metazoans. For BEL/Pao elements, the organization of clades within the Sailor lineage appears more difficult to establish clearly. The Copia elements remain rare and result from the evolutionary consistent success of the same three clades.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13100-021-00252-0.

Comparative Aspects of Annelid Regeneration: Towards Understanding the Mechanisms of Regeneration

The question of why animals vary in their ability to regenerate remains one of the most intriguing questions in biology. Annelids are a large and diverse phylum, many members of which are capable of extensive regeneration such as regrowth of a complete head or tail and whole-body regeneration, even from few segments. On the other hand, some representatives of both of the two major annelid clades show very limited tissue regeneration and are completely incapable of segmental regeneration. Here we review experimental and descriptive data on annelid regeneration, obtained at different levels of organization, from data on organs and tissues to intracellular and transcriptomic data. Understanding the variety of the cellular and molecular basis of regeneration in annelids can help one to address important questions about the role of stem/dedifferentiated cells and "molecular morphallaxis" in annelid regeneration as well as the evolution of regeneration in general.

Structural and Functional Characterization of the FGF Signaling Pathway in Regeneration of the Polychaete Worm Alitta virens (Annelida, Errantia)

Epimorphic regeneration of lost body segments is a widespread phenomenon across annelids. However, the molecular inducers of the cell sources for this reparative morphogenesis have not been identified. In this study, we focused on the role of fibroblast growth factor (FGF) signaling in the posterior regeneration of Alitta virens. For the first time, we showed an early activation of FGF ligands and receptor expression in an annelid regenerating after amputation. The expression patterns indicate that the entire regenerative bud is competent to FGFs, whose activity precedes the initiation of cell proliferation. The critical requirement of FGF signaling, especially at early stages, is also supported by inhibitor treatments followed by proliferation assay, demonstrating that induction of blastemal cells depends on FGFs. Our results show that FGF signaling pathway is a key player in regenerative response, while the FGF-positive wound epithelium, ventral nerve cord and some mesodermal cells around the gut could be the inducing tissues. This mechanism resembles reparative regeneration of vertebrate appendages suggesting such a response to the injury may be ancestral for all bilaterians.

Injury-Induced Innate Immune Response During Segment Regeneration of the Earthworm, Eisenia andrei

Regeneration of body parts and their interaction with the immune response is a poorly understood aspect of earthworm biology. Consequently, we aimed to study the mechanisms of innate immunity during regeneration in Eisenia andrei earthworms. In the course of anterior and posterior regeneration, we documented the kinetical aspects of segment restoration by histochemistry. Cell proliferation peaked at two weeks and remitted by four weeks in regenerating earthworms. Apoptotic cells were present throughout the cell renewal period. Distinct immune cell (e.g., coelomocyte) subsets were accumulated in the newly-formed blastema in the close proximity of the apoptotic area. Regenerating earthworms have decreased pattern recognition receptors (PRRs) (e.g., TLR, except for scavenger receptor) and antimicrobial peptides (AMPs) (e.g., lysenin) mRNA patterns compared to intact earthworms. In contrast, at the protein level, mirroring regulation of lysenins became evident. Experimental coelomocyte depletion caused significantly impaired cell divisions and blastema formation during anterior and posterior regeneration. These obtained novel data allow us to gain insight into the intricate interactions of regeneration and invertebrate innate immunity.

Expression of Piwi Genes during the Regeneration of Lineus sanguineus (Nemertea, Pilidiophora, Heteronemertea)

The transposon silencer piwi genes play important roles in germline determination and maintenance, gametogenesis, and stem-cell self-renewal, and the expression of certain piwi genes is indispensable for regeneration. Knowledge about piwi genes is needed for phylum Nemertea, which contains members (e.g., Lineus sanguineus) with formidable regeneration capacity. By searching the L. sanguineus genome, we identified six Argonaute genes including three ago (Ls-Ago2, Ls-Ago2a, and Ls-Ago2b) and three piwi (Ls-piwi1, Ls-piwi2, and Ls-piwi3) genes. In situ hybridization revealed that, in intact females, Ls-piwi2 and Ls-piwi3 were not expressed, while Ls-piwi1 was expressed in ovaries. During regeneration, Ls-piwi1 and Ls-pcna (proliferating cell nuclear antigen) had strong and similar expressions. The expression of Ls-piwi1 became indetectable while Ls-pcna continued to be expressed when the differentiation of new organs was finished. During anterior regeneration, expression signals of Ls-piwi2 and Ls-piwi3 were weak and only detected in the blastema stage. During posterior regeneration, no expression was observed for Ls-piwi2. To date, no direct evidence has been found for the existence of congenital stem cells in adult L. sanguineus. The "pluripotent cells" in regenerating tissues are likely to be dedifferentiated from other type(s) of cells.

Comparative transcriptomics in Syllidae (Annelida) indicates that posterior regeneration and regular growth are comparable, while anterior regeneration is a distinct process

Background

Annelids exhibit remarkable postembryonic developmental abilities. Most annelids grow during their whole life by adding segments through the action of a segment addition zone (SAZ) located in front of the pygidium. In addition, they show an outstanding ability to regenerate their bodies. Experimental evidence and field observations show that many annelids are able to regenerate their posterior bodies, while anterior regeneration is often limited or absent. Syllidae, for instance, usually show high abilities of posterior regeneration, although anterior regeneration varies across species. Some syllids are able to partially restore the anterior end, while others regenerate all lost anterior body after bisection. Here, we used comparative transcriptomics to detect changes in the gene expression profiles during anterior regeneration, posterior regeneration and regular growth of two syllid species: Sphaerosyllis hystrix and Syllis gracilis; which exhibit limited and complete anterior regeneration, respectively.

Results

We detected a high number of genes with differential expression: 4771 genes in S. hystrix (limited anterior regeneration) and 1997 genes in S. gracilis (complete anterior regeneration). For both species, the comparative transcriptomic analysis showed that gene expression during posterior regeneration and regular growth was very similar, whereas anterior regeneration was characterized by up-regulation of several genes. Among the up-regulated genes, we identified putative homologs of regeneration-related genes associated to cellular proliferation, nervous system development, establishment of body axis, and stem-cellness; such as rup and JNK (in S. hystrix); and glutamine synthetase, elav, slit, Hox genes, β-catenin and PL10 (in S. gracilis).

Conclusions

Posterior regeneration and regular growth show no significant differences in gene expression in the herein investigated syllids. However, anterior regeneration is associated with a clear change in terms of gene expression in both species. Our comparative transcriptomic analysis was able to detect differential expression of some regeneration-related genes, suggesting that syllids share some features of the regenerative mechanisms already known for other annelids and invertebrates.

FoxA expression pattern in two polychaete species, Alitta virens and Platynereis dumerilii: Examination of the conserved key regulator of the gut development from cleavage through larval life, postlarval growth, and regeneration

BACKGROUND

foxA orthologs are involved in various processes from embryo patterning to regulation of metabolism. Since foxA conserved role in the development of the gut of errant annelids has never been thoroughly studied, we used a candidate gene approach to unravel the molecular profile of the alimentary canal in two closely related nereid worms with a trochophore-type lecithotrophic larva.

RESULTS

The character of foxA expression in the two polychaetes was similar but not identical. The genes were successively activated first in blastoporal cells, then in the stomodeum, the midgut, and hindgut primordia, and in the cells of central and peripheral nervous system. Before the start of active feeding of nectochaetes, we observed a short phase of foxA expression in the entire digestive tract. After amputation of posterior segments, foxA expression was established de novo in the new terminal part of the intestine, and then in the developing hindgut and the anus.

CONCLUSIONS

We discovered an early marker of endoderm formation previously unknown in errant annelids. Its expression dynamics provided valuable insights into the gut development. Comparative analysis of foxA activity suggests its primary role in gastrulation morphogenesis independently of its type and in midgut and foregut specification. Developmental Dynamics 248:728-743, 2019. © 2018 Wiley Periodicals, Inc.

Morphological, cellular and molecular characterization of posterior regeneration in the marine annelid Platynereis dumerilii

Regeneration, the ability to restore body parts after an injury or an amputation, is a widespread but highly variable and complex phenomenon in animals. While having fascinated scientists for centuries, fundamental questions about the cellular basis of animal regeneration as well as its evolutionary history remain largely unanswered. Here, we present a study of regeneration of the marine annelid Platynereis dumerilii, an emerging comparative developmental biology model, which, like many other annelids, displays important regenerative abilities. When P. dumerilii worms are amputated, they are able to regenerate the posteriormost differentiated part of their body and a stem cell-rich growth zone that allows the production of new segments replacing the amputated ones. We show that posterior regeneration is a rapid process that follows a well reproducible path and timeline, going through specific stages that we thoroughly defined. Wound healing is achieved one day after amputation and a regeneration blastema forms one day later. At this time point, some tissue specification already occurs, and a functional posterior growth zone is re-established as early as three days after amputation. Regeneration timing is only influenced, in a minor manner, by worm size. Comparable regenerative abilities are found for amputations performed at different positions along the antero-posterior axis of the worm, except when amputation planes are very close to the pharynx. Regenerative abilities persist upon repeated amputations without important alterations of the process. We also show that intense cell proliferation occurs during regeneration and that cell divisions are required for regeneration to proceed normally. Finally, 5-ethynyl-2'-deoxyuridine (EdU) pulse and chase experiments suggest that blastemal cells mostly derive from the segment immediately abutting the amputation plane. The detailed characterization of P. dumerilii posterior body regeneration presented in this article provides the foundation for future mechanistic and comparative studies of regeneration in this species.

Characterization of the piRNA pathway during development of the sea anemone Nematostella vectensis

PIWI-interacting RNAs (piRNAs) and associated proteins comprise a conserved pathway for silencing transposons in metazoan germlines. piRNA pathway components are also expressed in multipotent somatic stem cells in various organisms. piRNA functions have been extensively explored in bilaterian model systems, however, comprehensive studies in non-bilaterian phyla remain limited. Here we investigate the piRNA pathway during the development of Nematostella vectensis, a well-established model system belonging to Cnidaria, the sister group to Bilateria. To date, no population of somatic stem cells has been identified in this organism, despite its long life-span and regenerative capacities that require a constant cell-renewal. We show that Nematostella piRNA pathway components are broadly expressed in early developmental stages, while piRNAs themselves show differential expression, suggesting specific developmental roles of distinct piRNA families. In adults, piRNA associated proteins are enriched in the germline but also expressed in somatic cells, indicating putative stem cell properties. Furthermore, we provide experimental evidence that Nematostella piRNAs cleave transposable elements as well as protein-coding genes. Our results demonstrate that somatic expression of piRNA associated proteins as well as the roles of piRNAs in transposon repression and gene regulation are likely ancestral features that evolved before the split between Cnidaria and Bilateria.

The Conservation of the Germline Multipotency Program, from Sponges to Vertebrates: A Stepping Stone to Understanding the Somatic and Germline Origins

The germline definition in metazoans was first based on few bilaterian models. As a result, gene function interpretations were often based on phenotypes observed in those models and led to the definition of a set of genes, considered as specific of the germline, named the “germline core”. However, some of these genes were shown to also be involved in somatic stem cells, thus leading to the notion of germline multipotency program (GMP). Because Porifera and Ctenophora are currently the best candidates to be the sister-group to all other animals, the comparative analysis of gene contents and functions between these phyla, Cnidaria and Bilateria is expected to provide clues on early animal evolution and on the links between somatic and germ lineages. Our present bioinformatic analyses at the metazoan scale show that a set of 18 GMP genes was already present in the last common ancestor of metazoans and indicate more precisely the evolution of some of them in the animal lineage. The expression patterns and levels of 11 of these genes in the homoscleromorph sponge Oscarella lobularis show that they are expressed throughout their life cycle, in pluri/multipotent progenitors, during gametogenesis, embryogenesis and during wound healing. This new study in a nonbilaterian species reinforces the hypothesis of an ancestral multipotency program.

PL10 DEAD-Box Protein is Expressed during Germ Cell Differentiation in the Reptile Podarcis sicula (Family Lacertidae)

Among genes involved in the regulation of germ cell differentiation, those of DDX4/Vasa and the Ded1/DDX3 subfamilies encode for DEAD-box ATP-dependent RNA helicases, proteins involved in many mechanisms related to RNA processing. For the first time in reptiles, using specific antibodies at confocal microscopy, we analysed the localization pattern of a Ded1/DDX3 subfamily member in testis and ovary of Podarcis sicula (Ps-PL10) during the reproductive cycle. In testis, Ps-PL10 is expressed in the cytoplasm of spermatocytes and it is not detected in spermatogonia. Differently from Ps-VASA, in round spermatids, Ps-PL10 is not segregated in the chromatoid body but it accumulates in the cytoplasm of residual bodies, and mature spermatozoa are unstained. These observations suggest that in males, Ps-PL10 (1) is involved in spermatogenesis and (2) is then eliminated with residual bodies. In the ovary, Ps-PL10 is present with granules in the cytoplasm of early meiotic cells of the germinal bed (GB), while it is not present in oogonia and somatic cells of the GB stroma. In follicular cells of ovarian follicles, Ps-PL10 expression starts after their fusion with the oocyte. Numerous Ps-PL10 spots are visible in pyriform (nurse-like) cells concomitantly with the protein accumulation in the cytoplasm of differentiating oocyte. In pyriform cells, Ps-PL10 spots are present in the cytoplasm and nuclei, as observed for Ps-VASA, and in the nucleoli, suggesting for Ps-PL10 a role in rRNA processing and in the transport of molecules from the nucleus to cytoplasm and from nurse cells to the oocyte.

Developmental and molecular biology of annelid regeneration: a comparative review of recent studies

Studies of annelid regeneration have greatly increased in frequency in recent years, providing new insights into the developmental basis and evolution of regeneration. In this review, we summarize recent findings related to regeneration in annelids, focusing on molecular and developmental studies of epimorphic (blastema-based) regeneration, morphallactic (tissue-remodeling based) regeneration, and development and regeneration of putative stem cells of the posterior growth zone and germline. Regeneration is being investigated in a broad range of annelids spanning the phylum, and comparing findings among species reveals both widely conserved features that may be ancestral for the phylum as well as features that are variable across the group.

Mesoderm patterning and morphogenesis in the polychaete Alitta virens (Spiralia, Annelida): Expression of mesodermal markers Twist, Mox, Evx and functional role for MAP kinase signaling

Mesoderm represents the evolutionary youngest germ layer and forms numerous novel tissues in bilaterian animals. Despite the established conservation of the gene regulatory networks that drive mesoderm differentiation (e.g. myogenesis), mechanisms of mesoderm specification are highly variable in distant model species. Thus, broader phylogenetic sampling is required to reveal common features of mesoderm formation across bilaterians. Here we focus on a representative of Spiralia, the marine annelid Alitta virens, whose mesoderm development is still poorly investigated on the molecular level. We characterize three novel early mesodermal markers for A. virens - Twist, Mox, and Evx - which are differentially expressed within the mesodermal lineages. The Twist mRNA is ubiquitously distributed in the fertilized egg and exhibits specific expression in endomesodermal- and ectomesodermal-founder cells at gastrulation. Twist is expressed around the blastopore and later in a segmental metameric pattern. We consider this expression to be ancestral, and in support of the enterocoelic hypothesis of mesoderm evolution. We also revealed an early pattern of the MAPK activation in A. virens that is different from the previously reported pattern in spiralians. Inhibition of the MAPK pathway by U0126 disrupts the metameric Twist and Mox expression, indicating an early requirement of the MAPK cascade for proper morphogenesis of endomesodermal tissues.