Postembryonic development and lifestyle shift in the commensal ribbon worm.

BACKGROUND: Various morphological adaptations are associated with symbiotic relationships between organisms. One such adaptation is seen in the nemertean genus Malacobdella. All species in the genus are commensals of molluscan hosts, attaching to the surface of host mantles with a terminal sucker. Malacobdella possesses several unique characteristics within the order Monostilifera, exhibiting the terminal sucker and the absence of eyes and apical/cerebral organs, which are related to their adaptation to a commensal lifestyle. Nevertheless, the developmental processes that give rise to these morphological characteristics during their transition from free-living larvae to commensal adults remain uncertain. RESULTS: In the present study, therefore, we visualized the developmental processes of the internal morphologies during postembryonic larval stages using fluorescent molecular markers. We demonstrated the developmental processes, including the formation of the sucker primordium and the functional sucker. Furthermore, our data revealed that sensory organs, including apical/cerebral organs, formed in embryonic and early postembryonic stages but degenerated in the late postembryonic stage prior to settlement within their host using a terminal sucker. CONCLUSIONS: This study reveals the formation of the terminal sucker through tissue invagination, shedding light on its adhesion mechanism. Sucker muscle development likely originates from body wall muscles. Notably, M. japonica exhibits negative phototaxis despite lacking larval ocelli. This observation suggests a potential role for other sensory mechanisms, such as the apical and cerebral organs identified in the larvae, in facilitating settlement and adhesive behaviors. The loss of sensory organs during larval development might reflect a transition from planktonic feeding to a stable, host-associated lifestyle. This study also emphasizes the need for further studies to explore the phylogenetic relationships within the infraorder Amphiporiina and investigate the postembryonic development of neuromuscular systems in closely related taxa to gain a more comprehensive understanding of ecological adaptations in Nemertea.

Secondary-tail formation during stolonization in the Japanese green syllid, Megasyllis nipponica.

Benthic annelids belonging to the family Syllidae show a distinctive sexual reproduction mode called "stolonization," in which posterior segments are transformed into a reproductive individual-like unit called a "stolon." Megasyllis nipponica forms a stolon head and a secondary tail in the middle of the trunk before a stolon detaches, while, in the case of posterior amputation, posterior regeneration initiates at the wound after amputation. To understand the difference between posterior regeneration and secondary-tail formation during stolonization, detailed comparisons between the developmental processes of these two tail-formation types were performed in this study. Morphological and inner structural observations (i.e., cell proliferation and muscular/nervous development) showed that some processes of posterior regeneration, such as blastema formation and muscular/nervous regeneration at the amputation site, are missing during secondary-tail formation. In contrast, the secondary tail showed some unique features, such as the formation of ventrolateral half-tail buds that later fused in the middle and muscle/nerve branches formed before the detachment of the stolon. These novel features in the process of stolonization are suggested to be adaptive since the animals need to recover a posterior end quickly to stolonize again.

[High Purity Solvents for qNMR Measurement].

Quantitative NMR (qNMR) has been adopted by documentary standards, including the Japanese Pharmacopoeia (JP), United States Pharmacopoeia (USP), and International Organization for Standardization (ISO), owing to its reliability and efficiency. Note that qNMR can be used for quantifying target components using the signal integration ratio of an analyte to a reference. In qNMR, a modern NMR instrument with high resolution and sensitivity is used to record reliable spectra. This instrument can detect small signals from impurities in a solvent, which may result in inaccurate signal integration in the spectrum. In this study, we investigated the influence of solvent quality on qNMR accuracy focusing on organic impurities, water content, and deuteration ratio. If signals from organic impurities and signals from the analyte overlap, the duplication of signal integration will directly affect the qNMR analytical result. To examine overlapping, we performed blank solvent tests. Additionally, a high water content and low deuteration ratio affect the detection sensitivity, thus reducing the signal-to-noise (S/N) ratio of the target. Thus, these factors must be considered to obtain accurate qNMR results.

Body part-specific development in termite caste differentiation: crosstalk between hormonal actions and developmental toolkit genes.

In social insects, interactions among colony members trigger caste differentiation with morphological modifications. During caste differentiation in termites, body parts and caste-specific morphologies are modified during postembryonic development under endocrine controls such as juvenile hormone (JH) and ecdysone. In addition to endocrine factors, developmental toolkit genes such as Hox- and appendage-patterning genes also contribute to the caste-specific body part modifications. These toolkits are thought to provide spatial information for specific morphogenesis. During social evolution, the complex crosstalks between physiological and developmental mechanisms should be established, leading to the sophisticated caste systems. This article reviews recent studies on these mechanisms underlying the termite caste differentiation and addresses implications for the evolution of caste systems in termites.

Reference transcriptome assembly of a protogynous sex change fish, harlequin sandsmelt (Parapercis pulchella).

The harlequin sandsmelt (Parapercis pulchella) is a female-to-male sex change fish in which functional females possess ovotestes that consist of both ovarian and testicular tissues. These features indicate that this species could be an excellent model for studying the flexibility of sex differentiation in vertebrates. However, genetic resources in this species have so far been limited. Therefore, in this study, the reference transcriptome of this fish was constructed through RNA-sequencing, de novo transcriptome assembly, superTranscripts construction, and functional annotations. To obtain as many genes as possible, RNA was extracted from various tissues (brains, gills, hearts, livers, guts, and gonads) and various sexual stages (females, individuals during sex change, and males) and then subjected to sequencing and downstream analyses. As a result, 91,884 representative transcripts with 32,627 protein-coding sequences were generated. 72.2% of protein-coding sequences (23,566 sequences) were functionally annotated. Also, our analysis shows that the superTranscripts method effectively removes redundant sequences from raw-assembled data compared with other strategies. The resultant dataset is a valuable resource for future molecular developmental studies on sex change in P. pulchella.

G protein-coupled receptor-based thermosensation determines temperature acclimatization of Caenorhabditis elegans.

Animals must sense and acclimatize to environmental temperatures for survival, yet their thermosensing mechanisms other than transient receptor potential (TRP) channels remain poorly understood. We identify a trimeric G protein-coupled receptor (GPCR), SRH-40, which confers thermosensitivity in sensory neurons regulating temperature acclimatization in Caenorhabditis elegans. Systematic knockdown of 1000 GPCRs by RNAi reveals GPCRs involved in temperature acclimatization, among which srh-40 is highly expressed in the ADL sensory neuron, a temperature-responsive chemosensory neuron, where TRP channels act as accessorial thermoreceptors. In vivo Ca2+ imaging demonstrates that an srh-40 mutation reduced the temperature sensitivity of ADL, resulting in supranormal temperature acclimatization. Ectopically expressing SRH-40 in a non-warmth-sensing gustatory neuron confers temperature responses. Moreover, temperature-dependent SRH-40 activation is reconstituted in Drosophila S2R+ cells. Overall, SRH-40 may be involved in thermosensory signaling underlying temperature acclimatization. We propose a dual thermosensing machinery through a GPCR and TRP channels in a single sensory neuron.

Quantitative 31P-NMR for the Purity Determination of the Organophosphorus Compound Brigatinib and Its Method Validation.

The spectrum of 31P-NMR is fundamentally simpler than that of 1H-NMR; consequently identifying the target signal(s) for quantitation is simpler using quantitative 31P-NMR (31P-qNMR) than using quantitative 1H-NMR (1H-qNMR), which has been already established as an absolute determination method. We have previously reported a 31P-qNMR method for the absolute determination of cyclophosphamide hydrate and sofosbuvir as water-soluble and water-insoluble organophosphorus compounds, respectively. This study introduces the purity determination of brigatinib (BR), an organophosphorus compound with limited water solubility, using 31P-qNMR at multiple laboratories. Phosphonoacetic acid (PAA) and 1,4-BTMSB-d4 were selected as the reference standards (RSs) for 31P-qNMR and 1H-qNMR, respectively. The qNMR solvents were chosen based on the solubilities of BR and the RSs for qNMR. CD3OH was selected as the solvent for 31P-qNMR measurements to prevent the influence of deuterium exchange caused by the presence of exchangeable intramolecular protons of BR and PAA on the quantitative values, while CD3OD was the solvent of choice for the 1H-qNMR measurements to prevent the influence of water signals and the exchangeable intramolecular protons of BR and PAA. The mean purity of BR determined by 31P-qNMR was 97.94 ± 0.69%, which was in agreement with that determined by 1H-qNMR (97.26 ± 0.71%), thus indicating the feasibility of purity determination of BR by 31P-qNMR. Therefore, the findings of this study may provide an effective method that is simpler than conventional 1H-qNMR for the determination of organophosphorus compounds.

Morphological, histological and gene-expression analyses on stolonization in the Japanese Green Syllid, Megasyllis nipponica (Annelida, Syllidae).

Benthic annelids belonging to the family Syllidae (Annelida, Errantia, Phyllodocida) exhibit a unique reproduction mode called "schizogamy" or "stolonization", in which the posterior body part filled with gametes detaches from the original body, as a reproductive unit (stolon) that autonomously swims and spawns. In this study, morphological and histological observations on the developmental processes during stolonization were carried out in Megasyllis nipponica. Results suggest that the stolon formation started with maturation of gonads, followed by the formation of a head ganglion in the anteriormost segment of the developing stolon. Then, the detailed stolon-specific structures such as stolon eyes and notochaetae were formed. Furthermore, expression profiles of genes involved in the anterior-posterior identity (Hox genes), head determination, germ-line, and hormone regulation were compared between anterior and posterior body parts during the stolonization process. The results reveal that, in the posterior body part, genes for gonadal development were up-regulated, followed by hormone-related genes and head-determination genes. Unexpectedly, Hox genes known to identify body parts along the anterior-posterior axis showed no significant temporal expression changes. These findings suggest that during stolonization, gonad development induces the head formation of a stolon, without up-regulation of anterior Hox genes.

Homeotic transformation in a terrestrial isopod: insights into the appendage identity in crustaceans.

In many crustacean species, an individual possesses both uniramous and biramous appendages that enable us to compare the two types on the same genetic background. Therefore, among the diverse morphologies of arthropod appendages, crustacean biramous appendages provide interesting subjects for studying the developmental mechanisms underlying appendage modifications. In this study, we report a malformed specimen of the terrestrial isopod Porcellio scaber, in which one of the pleopods was transformed into a different structure. Morphological observations of exoskeletons and musculatures by confocal scanning laser microscopy revealed that the transformed appendage was three-segmented, with at least the apical two segments having pereopod-like musculoskeletal structures. The apical segment of the transformed appendage lacked muscles, and the following segment had a pair of muscle bundles. These findings together with those of some previous studies of gene expression patterns in this species suggest that this anomaly could be caused by homeotic transformation of a flap-like pleopod into a three-segmented pereopod tip, which may be a homologous structure of the pleopod.

Differences of Sucker Formation Processes Depending on Benthic or Pelagic Posthatching Lifestyles in Two Octopus Species.

AbstractMorphologies of animal appendages are highly diversified depending on animal lifestyles. In cephalopods (Mollusca, Cephalopoda), an individual possesses multiple arms that contribute to elaborate behaviors, and suckers on them enable various arm functions. In octopus hatchlings, arm and sucker morphologies can be divided into two different types due to alternative posthatching lifestyles, that is, pelagic or benthic lifestyles, although the underlying developmental differences have yet to be elucidated. In this study, therefore, detailed developmental processes of arms and suckers were observed during embryogenesis in two different octopus species, Octopus parvus and Amphioctopus fangsiao, showing pelagic and benthic posthatching lifestyles, respectively. In O. parvus, sucker formation stopped at a relatively early stage in which three suckers on an arm were produced. In addition, at late embryonic stages, cell proliferation was hardly detected in whole arms, while in A. fangsiao, sucker production continued throughout embryogenesis and cell proliferation also remained active in whole arms even in the late stages. Therefore, although further investigations in other octopus species are required, it is suggested that in octopus evolution, the developmental program of suckers has been modified in accordance with the acquisition of a novel lifestyle.

Gonadal and cellular dynamics during protogynous sex change in the harlequin sandsmelt Parapercis pulchella.

Some teleost fishes change their sex, and some of these fishes have specific gonads known as "ovotestes," that is, gonads containing both ovarian and testicular tissues. In this study, we revealed the gonadal transformation process and cell dynamics during the female-to-male sex change in the harlequin sandsmelt, Parapercis pulchella (Pinguipetidae), in which females possess ovotestes. Histological observations revealed that although female ovotestes were composed of oocytes, a few cysts of male germ cells were observed among them. At the initial phase of sex change, male germ cells increased, and spermatogenesis proceeded. After that, oocytes decreased and finally disappeared, and the gonads became functional testes. Immunohistochemistry using antibodies against Pcna (proliferating cell nuclear antigen) as a cell proliferation marker revealed that spermatogonia were Pcna positive, whereas spermatocytes were negative, in female ovotestes. This suggests that spermatogenesis is arrested at the spermatocyte stage. In addition, some somatic cells surrounding oocytes, which were thought to be the female follicle cells, were Pcna positive during sex change, indicating that these cells proliferate during sex change and are reused in male testes after sex change. Also, immunostaining using antibodies against active cleaved-Caspase3a as an apoptosis marker demonstrated that oocytes degenerated through apoptotic cell death at the late transition stage. Together with previous findings in other fishes, these findings suggested that the histological processes in gonads during sex change, such as the order of developmental events, developmental fates of ovarian cavities, and ovotestis structures, are diversified among fish species. In contrast, cellular dynamics of female germ and somatic cells during sex change are common among protogynous species.

[Quantitative Determination Method of Aconitum Monoester Alkaloids Using Relative Molar Sensitivity (RMS) for the Assay in the Japanese Pharmacopoeia].

Recently, a novel quantitative method using relative molar sensitivity (RMS) was applied to quantify the ingredients of drugs and foods. An important development in this regard can be observed in the Japanese Pharmacopoeia (JP) 18, where the quantification of perillaldehyde, an unstable compound, in crude drug "Perilla Herb," was revised to incorporate the RMS method. In this study, the primary objective was to improve the tester safety and reduce the amount of reagents used in the JP test. To achieve this, the quantification of three toxic Aconitum monoester alkaloids (AMAs) was explored using the RMS method, employing a single reference compound for all three targets. These AMAs, namely benzoylmesaconine hydrochloride, benzoylhypaconine hydrochloride, and 14-anisoylaconine hydrochloride, which are the quantitative compounds of Kampo extracts containing Aconite Root (AR), were quantified using the reference compound benzoic acid (BA). Reliable RMS values were obtained using both 1H-quantitative NMR and HPLC/UV. Using the RMS of three AMAs relative to the BA, the AMA content (%) in commercial AMAs quantitative reagents were determined without analytical standards. Moreover, the quantitative values of AMAs using the RMS method and the calibration curve method using the three analytical standards were similar. Additionally, similar values were achieved for the three AMAs in the Kampo extracts containing AR using the RMS and the modified JP18 calibration curve methods. These results suggest that the RMS method is suitable for quantitative assays of the Kampo extracts containing AR and can serve as an alternative to the current method specified in the JP18.

Upregulation of Hox genes leading to caste-specific morphogenesis in a termite.

BACKGROUND: In social insects, interactions among colony members trigger caste differentiation with morphological modifications. In termite caste differentiation, caste-specific morphologies (such as mandibles in soldiers, genital organs in reproductives or wings in alates) are well developed during post-embryonic development under endocrine controls (e.g., juvenile hormone and ecdysone). Since body part-specific morphogenesis in caste differentiation is hormonally regulated by global factors circulated throughout the body, positional information should be required for the caste-specific and also body part-specific morphogenesis. To identify factors providing the positional information, expression and functional analyses of eight Hox genes were carried out during the three types of caste differentiation (i.e., soldier, neotenic and alate differentiation) in a termite, Hodotermopsis sjostedti. RESULTS: Spatio-temporal patterns of Hox gene expression during caste differentiation were elucidated by real-time qPCR, showing the caste-specific upregulations of Hox genes during the differentiation processes. Among eight Hox genes, Deformed (Dfd) was upregulated specifically in mandibles in soldier differentiation, abdominal-A (abd-A) and Abdominal-B (Abd-B) were upregulated in the abdomen in neotenic differentiation, while Sex-comb reduced (Scr) and Antennapedia (Antp) were upregulated during alate differentiation. Furthermore, RNAi knockdown of Dfd in soldier differentiation and of abd-A and Abd-B in neotenic differentiation distorted the modifications of caste-specific morphologies. CONCLUSIONS: Gene expression and functional analyses in this study revealed that, in the caste differentiation in termites, upregulation of Hox genes provide positional identities of body segments, resulting in the caste-specific morphogenesis. The acquisition of such developmental modifications would have enabled the evolution of sophisticated caste systems in termites.

Appearance of a transparent protrusion containing two pairs of legs on the apodous ring preceding the anamorphic molt in a millipede, Niponia nodulosa.

BACKGROUND: Arthropods gradually change their forms through repeated molting events during postembryonic development. Anamorphosis, i.e., segment addition during postembryonic development, is seen in some arthropod lineages. In all millipede species (Myriapoda, Diplopoda), for example, postembryonic processes go through anamorphosis. Jean-Henri Fabre proposed 168 years ago the "law of anamorphosis", that is, "new rings appear between the penultimate ring and the telson" and "all apodous rings in a given stadium become podous rings in the next stadium", but the developmental process at the anamorphic molt remains largely unknown. In this study, therefore, by observing the morphological and histological changes at the time of molting, the detailed processes of leg- and ring-addition during anamorphosis were characterized in a millipede, Niponia nodulosa (Polydesmida, Cryptodesmidae). RESULTS: In the preparatory period, a few days before molting, scanning electron microscopy, confocal laser scanning microscopy, and histological observations revealed that two pairs of wrinkled leg primordia were present under the cuticle of each apodous ring. In the rigidation period, just prior to molt, observations of external morphology showed that a transparent protrusion was observed on the median line of the ventral surface on each apodous ring. Confocal laser scanning microscopy and histological observations revealed that the transparent protrusion covered by an arthrodial membrane contained a leg bundle consisting of two pairs of legs. On the other hand, ring primordia were observed anterior to the telson just before molts. CONCLUSIONS: Preceding the anamorphic molt in which two pairs of legs are added on an apodous ring, a transparent protrusion containing the leg pairs (a leg bundle) appears on each apodous ring. The morphogenetic process of the rapid protrusion of leg bundles, that is enabled by thin and elastic cuticle, suggested that millipedes have acquired a resting period and unique morphogenesis to efficiently add new legs and rings.

Quantitative 31P-NMR for Purity Determination of Sofosbuvir and Method Validation.

Quantitative 1H-NMR (1H-qNMR) is useful for determining the absolute purity of organic molecules; however, it is sometimes difficult to identify the target signal(s) for quantitation because of their overlap and complexity. Therefore, we focused on the 31P nucleus because of the simplicity of its signals and previously reported 31P-qNMR in D2O. Here we report 31P-qNMR of an organophosphorus compound, sofosbuvir (SOF), which is soluble in organic solvents. Phosphonoacetic acid (PAA) and 1,4-bis(trimethylsilyl)benzene-d4 (1,4-BTMSB-d4) were used as reference standards for 31P-qNMR and 1H-qNMR, respectively, in methanol-d4. The purity of SOF determined by 31P-qNMR was 100.63 ± 0.95%, whereas that determined by 1H-qNMR was 99.07 ± 0.50%. The average half bandwidths of the 31P signal of PAA and SOF were 3.38 ± 2.39 and 2.22 ± 0.19 Hz, respectively, suggesting that the T2 relaxation time of the PAA signal was shorter than that of SOF and varied among test laboratories. This difference most likely arose from the instability in the chemical shift due to the deuterium exchange of the acidic protons of PAA, which decreased the integrated intensity of the PAA signal. Next, an aprotic solvent, dimethyl sulfoxide-d6 (DMSO-d6), was used as the dissolving solvent with PAA and sodium 4,4-dimethyl-4-silapentanesulfonate-d6 (DSS-d6) as reference standards for 31P-qNMR and 1H-qNMR, respectively. SOF purities determined by 31P-qNMR and 1H-qNMR were 99.10 ± 0.30 and 99.44 ± 0.29%, respectively. SOF purities determined by 31P-qNMR agreed with the established 1H-qNMR values, suggesting that an aprotic solvent is preferable for 31P-qNMR because it is unnecessary to consider the effect of deuterium exchange.

Ontogenetic Expressions of Sexually Dimorphic Traits in the Skeleton Shrimp Caprella scaura (Crustacea: Amphipoda).

In sexual dimorphism, males often exhibit exaggerated characters as weapons or ornaments. Among the numerous amphipod species (Amphipoda, Crustacea) showing sexual dimorphism, caprellids (Caprellidae) are characterized by considerably larger males that possess weapons, although the developmental processes underlying these sex-related differences are largely unknown. Therefore, to clarify the process of sexual differentiation during postembryonic development in caprellids, morphometric analyses of Caprella scaura were conducted. Principal component analysis using 31 morphometric traits showed drastic allometric changes occurring at two ontogenetic body length (BL) points (i.e., 3.8 and 8.8 mm). In individuals larger than 3 mm, head spines appeared in both sexes, and penises did only in males, allowing the discrimination of juveniles from larvae. Moreover, in larger males (BL > 8.8 mm), traits used in reproductive behavior, i.e., the first antenna, second gnathopod, and first to fifth pereonites, were extremely exaggerated. Observations of pre-copulatory behavior along with morphological assays revealed that sexually mature males could be identified by the size ratio between the triangular projection and palmar spine on the propodus of the second gnathopod. In contrast, female maturation could be determined by the marginal setae of oostegites forming a brood pouch. The body size distribution of sexually mature females was concentrated within a narrow range of BLs (6-9 mm), whereas that of sexually mature males showed a broader range (BL 9-18 mm), suggesting that, in C. scaura, males continue to molt and grow even after sexual maturation via indeterminate growth, to increase their lifetime reproductive success.

Pleopodal lung development in a terrestrial isopod, Porcellio scaber (Oniscidea).

During evolution, various lineages of arthropods colonized land and independently acquired air-breathing organs. Some taxa of oniscidean isopods (Crustacea, Isopoda, Oniscidea) are the most successful crustacean lineages on land and possess organs called "lungs" or "pseudotrachea" for air-breathing in their abdominal appendages, i.e., in pleopods. Although these lungs are important for adapting to the terrestrial environment, their developmental process has not yet been elucidated. In the present study, we investigated the process of lung development in Porcellio scaber, the common rough woodlouse with pleopodal lungs in the first two pairs of pleopods. The lungs in the second pleopods developed at the manca 1 stage (immediately after hatching) and became functional at the manca 2 stage. In the first pleopods, which appear at the manca 3 stage, the lungs were gradually developed during the manca 3 stage and became functional in post-manca juveniles. In the second pleopods, epithelial invaginations led to lung development. These results suggest that some novel developmental mechanisms with epithelial invaginations and cuticle formation were acquired during terrestrialization, resulting in the development of functional lungs in the terrestrial isopod lineages.

Construction of a massive genetic resource by transcriptome sequencing and genetic characterization of Megasyllis nipponica (Annelida: Syllidae).

Understanding the processes and consequences of the morphological diversity of organisms is one of the major goals of evolutionary biology. Studies on the evolution of developmental mechanisms of morphologies, or evo-devo, have been extensively conducted in many taxa and have revealed many interesting phenomena at the molecular level. However, many other taxa exhibiting intriguing morphological diversity remain unexplored in the field of evo-devo. Although the annelid family Syllidae shows spectacular diversity in morphological development associated with reproduction, its evo-devo study, especially on molecular development, has progressed slowly. In this study, we focused on Megasyllis nipponica as a new model species for evo-devo in syllids and performed transcriptome sequencing to develop a massive genetic resource, which will be useful for future molecular studies. From the transcriptome data, we identified candidate genes that are likely involved in morphogenesis, including genes involved in hormone regulation, sex determination and appendage development. Furthermore, a computational analysis of the transcriptome sequence data indicated the occurrence of DNA methylation in coding regions of the M. nipponica genome. In addition, flow cytometry analysis showed that the genome size of M. nipponica was approximately 524 megabases. These results facilitate the study of morphogenesis in molecular terms and contribute to our understanding of the morphological diversity in syllids.

The pentameric hydrocoel lobes organize adult pentameral structures in a sea cucumber, Apostichopus japonicus.

Despite being one of the bilaterians, the body plan of echinoderms shifts from bilateral symmetry to five-fold radial, or pentaradial symmetry during embryogenesis or their metamorphosis. While the clarification of the developmental mechanism behind this transition will be a basis for understanding their unique body plan evolution, it is still poorly understood. With this regard, the hydrocoel, a mesodermal coelom formed on the left side of bilateral larva, would be a clue for understanding the mechanism as it is the first pentaradial structure that appears before metamorphosis and develops into the water vascular system of adults. By analyzing the development of a sea cucumber, Apostichopus japonicus, we found that the hydrocoel expresses genes related in muscle and neural formation such as myosin heavy chain, tropomyosin, soxC, and elav, implying that cells of the hydrocoel contributes to muscle and neural structures in the adult. Furthermore, ablation of one of the hydrocoel lobes led to incomplete development of adult pentameral structures. The ablation of primary hydrocoel lobes resulted in the reduction in tentacles and the ablation of secondary hydrocoel lobes resulted in the reduction in water vascular canals and nerve cords. Our findings suggest that the hydrocoel lobes may serve as a potential organizing center for establishing the pentaradial body plan in echinoderms.

Hydrocoel morphogenesis forming the pentaradial body plan in a sea cucumber, Apostichopus japonicus.

Echinoderms constitute an animal phylum characterized by the pentaradial body plan. During the development from bilateral larvae to pentaradial adults, the formation of the multiple of five hydrocoel lobes, i.e., the buddings from the mesodermal coelom, is the firstly emerging pentameral character. The developmental mechanism underlying the hydrocoel-lobe formation should be revealed to understand the evolutionary process of this unique and highly derived body plan of echinoderms, although the morphogenetic mechanisms of hydrocoel lobes are largely uninvestigated. In this study, using the sea cucumber Apostichopus japonicus, in which hydrocoel is easily observable, the developmental process of hydrocoel lobes was described in detail, focusing on cell proliferation and rearrangement. Cell proliferation was not specifically distributed in the growing tips of the hydrocoel lobes, and inhibition of cell proliferation did not affect lobe formation. During lobe formation, the epithelium of the hydrocoel lobes was firstly thickened and then transformed into a simple epithelium, suggesting that tissue expansion via tissue remodeling contributes to the hydrocoel-lobe formation.