Environmental DNA-based biodiversity profiling along the Houdong River in north-eastern Taiwan.

Background: This paper describes two datasets: species occurrences, which were determined by environmental DNA (eDNA) metabarcoding and their associated DNA sequences, originating from a research project which was carried out along the Houdong River (), Jiaoxi Township, Yilan, Taiwan. The Houdong River begins at an elevation of 860 m and flows for approximately 9 km before it empties into the Pacific Ocean. Meandering through mountains, hills, plains and alluvial valleys, this short river system is representative of the fluvial systems in Taiwan. The primary objective of this study was to determine eukaryotic species occurrences in the riverine ecosystem through the use of the eDNA analysis. The second goal was, based on the current dataset, to establish a metabarcoding eDNA data template that will be useful and replicable for all users, particularly the Taiwan community. The species occurrence data are accessible at the Global Biodiversity Information Facility (GBIF) portal and its associated DNA sequences have been deposited in the European Nucleotide Archive (ENA) at EMBL-EBI, respectively. A total of 12 water samples from the study yielded an average of 1.5 million reads. The subsequent species identification from the collected samples resulted in the classification of 432 Operational Taxonomic Units (OTUs) out of a total of 2,734. Furthermore, a total of 1,356 occurrences with taxon matches in GBIF were documented (excluding 4,941 incertae sedis, accessed 05-12-2023). These data will be of substantial importance for future species and habitat monitoring within the short river, such as assessment of biodiversity patterns across different elevations, zonations and time periods and its correlation to water quality, land uses and anthropogenic activities. Further, these datasets will be of importance for regional ecological studies, in particular the freshwater ecosystem and its status in the current global change scenarios. New information: The datasets are the first species diversity description of the Houdong River system using either eDNA or traditional monitoring processes.

Asymmetric segregation of maternal mRNAs and germline-related determinants in Cephalochordate embryos: implications for the evolution of early patterning events in chordates.

How animal embryos determine their early cell fates is an important question in developmental biology. In various model animals asymmetrically localized maternal transcripts play important roles in axial patterning and cell fate specification. Cephalochordates (amphioxus), which have three living genera (Asymmetron, Epigonichthys, Branchiostoma), are an early branching chordate lineage and thus occupy a key phylogenetic position for understanding the evolution of chordate developmental mechanisms. It has been shown that in the zygote of Brachiostoma amphioxus, which possess bilateral gonads flanking both sides of their trunk region, maternal transcripts of germline determinants form a compact granule. During early embryogenesis this granule is inherited by a single blastomere that subsequently gives rise to a cluster of cells displaying typical characteristics of primordial germ cells (PGC). These PGCs then come to lie in the tailbud region and proliferate during posterior elongation of the larva to join in the gonad anlagen at the ventral tip of the developing myomeres in amphioxus larvae. However, in Asymmetron and Epigonichthys amphioxus, whose gonads are present only on the right side of their body, nothing is known about their PGC development or the cellular/morphogenetic processes resulting in the asymmetric distribution of gonads. Using conserved germline determinants as markers, we show that similarly to Brachiostoma amphioxus, Asymmetron also employ a preformation mechanism to specify their PGCs, suggesting that this mechanism represents an ancient trait dating back to the common ancestor of Cephalochordates. Surprisingly, we found that Asymmetron PGCs are initially deposited on both sides of the body during early larval development; however, the left side PGCs cease to exist in young juveniles, suggesting that PGCs are eliminated from the left body side during larval development or following metamorphosis. This is reminiscent of the PGC development in the sea urchin embryo, and we discuss the implications of this observation for the evolution of developmental mechanisms.

Tracing the evolutionary origin of chordate somites in the hemichordate Ptychodera flava.

Metameric somites are a novel character of chordates with unclear evolutionary origins. In the early branching chordate amphioxus, anterior somites are derived from the paraxial mesodermal cells that bud off the archenteron (i.e., enterocoely) at the end of gastrulation. Development of the anterior somites requires FGF signaling, and distinct somite compartments express orthologs of vertebrate non-axial mesodermal markers. Thus, it has been proposed that the amphioxus anterior somites are homologous to the vertebrate head mesoderm, paraxial mesoderm and lateral plate mesoderm. To trace the evolutionary origin of somites, it is essential to study the chordates' closest sister group, Ambulacraria, which includes hemichordates and echinoderms. The anterior coeloms of hemichordate and sea urchin embryos (respectively called protocoel and coelomic pouches) are also formed by enterocoely and require FGF signals for specification and/or differentiation. In this study, we applied RNA-seq to comprehensively screen for regulatory genes associated with the mesoderm-derived protocoel of the hemichordate Ptychodera flava. We also used a candidate gene approach to identify P. flava orthologs of chordate somite markers. In situ hybridization results showed that many of these candidate genes are expressed in distinct or overlapping regions of the protocoel, which indicates that molecular compartments exist in the hemichordate anterior coelom. Given that the hemichordate protocoel and amphioxus anterior somites share a similar ontogenic process (enterocoely), induction signal (FGF), and characteristic expression of orthologous genes, we propose that these two anterior coeloms are indeed homologous. In the lineage leading to the emergence of chordates, somites likely evolved from enterocoelic, FGF-dependent, and molecularly compartmentalized anterior coeloms of the deuterostome last common ancestor.

Embryonic and post-embryonic development in the parasitic copepod Ive ptychoderae (Copepoda: Iviidae): Insights into its phylogenetic position.

Parasitic copepods are frequently discovered in many marine animals, and they exhibit great species diversity with remarkable morphological adaptations to their parasitic lifestyle. Similar to their free-living relatives, parasitic copepods usually develop through complex life cycle, but they eventually transform into a modified adult form with reduced appendages. Although the life cycle and distinct larval stages have been described in a few species of parasitic copepods, particularly those infecting commercially valuable marine animals (such as fishes, oysters, and lobsters), very little is known about the developmental process of the species that transformed into extremely simplified adult body plan. This paucity also causes some difficulties when investigating the taxonomy and phylogeny of this kind of parasitic copepods. Here we describe the embryonic development and a series of sequential larval stages of a parasitic copepod, Ive ptychoderae, which is a vermiform endoparasite living inside the hemichordate acorn worms. We devised laboratory regimes that enable us raising large quantity of embryos and free living larvae, and obtaining post-infested I. ptychoderae samples from the host tissues. Using defined morphological features, the embryonic development of I. ptychoderae can be categorized into eight stages (1-, 2-, 4-, 8-, 16- cell stages, blastula, gastrula, and limb bud stages) and the post-embryonic development comprises six larval stages (2 naupliar and 4 copepodid stages). Based on the comparisons of morphological characters in the nauplius stage, our results provide evidence to support that the Ive-group is more closely related to the Cyclopoida, which represents one of the two major clades that contain many highly transformed parasitic copepods. Thus, our results help to resolve the problematic phylogenetic position of the Ive-group in previous study based on analysis using 18S rDNA sequences. Combining with more molecular data, future comparative analyses on the morphological features of copepodid stages will further refine our understanding of the phylogenetic relationships of parasitic copepods.

Three amphioxus reference genomes reveal gene and chromosome evolution of chordates.

The slow-evolving invertebrate amphioxus has an irreplaceable role in advancing our understanding of the vertebrate origin and innovations. Here we resolve the nearly complete chromosomal genomes of three amphioxus species, one of which best recapitulates the 17 chordate ancestor linkage groups. We reconstruct the fusions, retention, or rearrangements between descendants of whole-genome duplications, which gave rise to the extant microchromosomes likely existed in the vertebrate ancestor. Similar to vertebrates, the amphioxus genome gradually establishes its three-dimensional chromatin architecture at the onset of zygotic activation and forms two topologically associated domains at the Hox gene cluster. We find that all three amphioxus species have ZW sex chromosomes with little sequence differentiation, and their putative sex-determining regions are nonhomologous to each other. Our results illuminate the unappreciated interspecific diversity and developmental dynamics of amphioxus genomes and provide high-quality references for understanding the mechanisms of chordate functional genome evolution.

On the origin and evolution of RNA editing in metazoans.

Extensive adenosine-to-inosine (A-to-I) editing of nuclear-transcribed mRNAs is the hallmark of metazoan transcriptional regulation. Here, by profiling the RNA editomes of 22 species that cover major groups of Holozoa, we provide substantial evidence supporting A-to-I mRNA editing as a regulatory innovation originating in the last common ancestor of extant metazoans. This ancient biochemistry process is preserved in most extant metazoan phyla and primarily targets endogenous double-stranded RNA (dsRNA) formed by evolutionarily young repeats. We also find intermolecular pairing of sense-antisense transcripts as an important mechanism for forming dsRNA substrates for A-to-I editing in some but not all lineages. Likewise, recoding editing is rarely shared across lineages but preferentially targets genes involved in neural and cytoskeleton systems in bilaterians. We conclude that metazoan A-to-I editing might first emerge as a safeguard mechanism against repeat-derived dsRNA and was later co-opted into diverse biological processes due to its mutagenic nature.

Gain of gene regulatory network interconnectivity at the origin of vertebrates.

SignificanceIn this manuscript, we address an essential question in developmental and evolutionary biology: How have changes in gene regulatory networks contributed to the invertebrate-to-vertebrate transition? To address this issue, we perturbed four signaling pathways critical for body plan formation in the cephalochordate amphioxus and in zebrafish and compared the effects of such perturbations on gene expression and gene regulation in both species. Our data reveal that many developmental genes have gained response to these signaling pathways in the vertebrate lineage. Moreover, we show that the interconnectivity between these pathways is much higher in zebrafish than in amphioxus. We conclude that this increased signaling pathway complexity likely contributed to vertebrate morphological novelties during evolution.

An Updated Staging System for Cephalochordate Development: One Table Suits Them All.

Chordates are divided into three subphyla: Vertebrata, Tunicata, and Cephalochordata. Phylogenetically, the Cephalochordata, more commonly known as lancelets or amphioxus, constitute the sister group of Vertebrata and Tunicata. Lancelets are small, benthic, marine filter feeders, and their roughly three dozen described species are divided into three genera: Branchiostoma, Epigonichthys, and Asymmetron. Due to their phylogenetic position and their stereotypical chordate morphology and genome architecture, lancelets are key models for understanding the evolutionary history of chordates. Lancelets have thus been studied by generations of scientists, with the first descriptions of adult anatomy and developmental morphology dating back to the 19th century. Today, several different lancelet species are used as laboratory models, predominantly for developmental, molecular and genomic studies. Surprisingly, however, a universal staging system and an unambiguous nomenclature for developing lancelets have not yet been adopted by the scientific community. In this work, we characterized the development of the European lancelet (Branchiostoma lanceolatum) using confocal microscopy and compiled a streamlined developmental staging system, from fertilization through larval life, including an unambiguous stage nomenclature. By tracing growth curves of the European lancelet reared at different temperatures, we were able to show that our staging system permitted an easy conversion of any developmental time into a specific stage name. Furthermore, comparisons of embryos and larvae from the European lancelet (B. lanceolatum), the Florida lancelet (Branchiostoma floridae), two Asian lancelets (Branchiostoma belcheri and Branchiostoma japonicum), and the Bahamas lancelet (Asymmetron lucayanum) demonstrated that our staging system could readily be applied to other lancelet species. Although the detailed staging description was carried out on developing B. lanceolatum, the comparisons with other lancelet species thus strongly suggested that both staging and nomenclature are applicable to all extant lancelets. We conclude that this description of embryonic and larval development will be of great use for the scientific community and that it should be adopted as the new standard for defining and naming developing lancelets. More generally, we anticipate that this work will facilitate future studies comparing representatives from different chordate lineages.

Somite Compartments in Amphioxus and Its Implications on the Evolution of the Vertebrate Skeletal Tissues.

Mineralized skeletal tissues of vertebrates are an evolutionary novelty within the chordate lineage. While the progenitor cells that contribute to vertebrate skeletal tissues are known to have two embryonic origins, the mesoderm and neural crest, the evolutionary origin of their developmental process remains unclear. Using cephalochordate amphioxus as our model, we found that cells at the lateral wall of the amphioxus somite express SPARC (a crucial gene for tissue mineralization) and various collagen genes. During development, some of these cells expand medially to surround the axial structures, including the neural tube, notochord and gut, while others expand laterally and ventrally to underlie the epidermis. Eventually these cell populations are found closely associated with the collagenous matrix around the neural tube, notochord, and dorsal aorta, and also with the dense collagen sheets underneath the epidermis. Using known genetic markers for distinct vertebrate somite compartments, we showed that the lateral wall of amphioxus somite likely corresponds to the vertebrate dermomyotome and lateral plate mesoderm. Furthermore, we demonstrated a conserved role for BMP signaling pathway in somite patterning of both amphioxus and vertebrates. These results suggest that compartmentalized somites and their contribution to primitive skeletal tissues are ancient traits that date back to the chordate common ancestor. The finding of SPARC-expressing skeletal scaffold in amphioxus further supports previous hypothesis regarding SPARC gene family expansion in the elaboration of the vertebrate mineralized skeleton.

The Ontology of the Amphioxus Anatomy and Life Cycle (AMPHX).

An ontology is a computable representation of the different parts of an organism and its different developmental stages as well as the relationships between them. The ontology of model organisms is therefore a fundamental tool for a multitude of bioinformatics and comparative analyses. The cephalochordate amphioxus is a marine animal representing the earliest diverging evolutionary lineage of chordates. Furthermore, its morphology, its anatomy and its genome can be considered as prototypes of the chordate phylum. For these reasons, amphioxus is a very important animal model for evolutionary developmental biology studies aimed at understanding the origin and diversification of vertebrates. Here, we have constructed an amphioxus ontology (AMPHX) which combines anatomical and developmental terms and includes the relationships between these terms. AMPHX will be used to annotate amphioxus gene expression patterns as well as phenotypes. We encourage the scientific community to adopt this amphioxus ontology and send recommendations for future updates and improvements.

Evidence for BMP-mediated specification of primordial germ cells in an indirect-developing hemichordate.

Primordial germ cells (PGCs) are specified during development by either one of two major mechanisms, the preformation mode or the inductive mode. Because the inductive mode is widely employed by many bilaterians and early branching metazoan lineages, it has been postulated as an ancestral mechanism. However, among the deuterostome species that have been studied, invertebrate chordates use the preformation mode, while many vertebrate and echinoderm species are known to utilize an inductive mechanism, thus leaving the evolutionary history of PGC specification in the deuterostome lineage unclear. Hemichordates are the sister phylum of echinoderms, and together they form a clade called Ambulacraria that represents the closest group to the chordates. Thus, research in hemichordates is highly informative for resolving this issue. In this study, we investigate the developmental process of PGCs in an indirect-developing hemichordate, Ptychodera flava. We show that maternal transcripts of the conserved germline markers vasa, nanos, and piwi1 are ubiquitously distributed in early P. flava embryos, and these genes are coexpressed specifically in the dorsal hindgut starting from the gastrula stage. Immunostaining revealed that Vasa protein is concentrated toward the vegetal pole in early P. flava embryos, and it is restricted to cells in the dorsal hindgut of gastrulae and newly hatched larvae. The Vasa-positive cells later contribute to the developing trunk coeloms of the larvae and eventually reside in the adult gonads. We further show that bone morphogenetic protein (BMP) signaling is required to activate expression of the germline determinants in the gastrula hindgut, suggesting that PGC specification is induced by BMP signaling in P. flava. Our data support the hypothesis that the inductive mode is a conserved mechanism in Ambulacraria, which might even trace back to the common ancestor of Deuterostomes.

Molecular asymmetry in the cephalochordate embryo revealed by single-blastomere transcriptome profiling.

Studies in various animals have shown that asymmetrically localized maternal transcripts play important roles in axial patterning and cell fate specification in early embryos. However, comprehensive analyses of the maternal transcriptomes with spatial information are scarce and limited to a handful of model organisms. In cephalochordates (amphioxus), an early branching chordate group, maternal transcripts of germline determinants form a compact granule that is inherited by a single blastomere during cleavage stages. Further blastomere separation experiments suggest that other transcripts associated with the granule are likely responsible for organizing the posterior structure in amphioxus; however, the identities of these determinants remain unknown. In this study, we used high-throughput RNA sequencing of separated blastomeres to examine asymmetrically localized transcripts in two-cell and eight-cell stage embryos of the amphioxus Branchiostoma floridae. We identified 111 and 391 differentially enriched transcripts at the 2-cell stage and the 8-cell stage, respectively, and used in situ hybridization to validate the spatial distribution patterns for a subset of these transcripts. The identified transcripts could be categorized into two major groups: (1) vegetal tier/germ granule-enriched and (2) animal tier/anterior-enriched transcripts. Using zebrafish as a surrogate model system, we showed that overexpression of one animal tier/anterior-localized amphioxus transcript, zfp665, causes a dorsalization/anteriorization phenotype in zebrafish embryos by downregulating the expression of the ventral gene, eve1, suggesting a potential function of zfp665 in early axial patterning. Our results provide a global transcriptomic blueprint for early-stage amphioxus embryos. This dataset represents a rich platform to guide future characterization of molecular players in early amphioxus development and to elucidate conservation and divergence of developmental programs during chordate evolution.

Deeply conserved synteny resolves early events in vertebrate evolution.

Although it is widely believed that early vertebrate evolution was shaped by ancient whole-genome duplications, the number, timing and mechanism of these events remain elusive. Here, we infer the history of vertebrates through genomic comparisons with a new chromosome-scale sequence of the invertebrate chordate amphioxus. We show how the karyotypes of amphioxus and diverse vertebrates are derived from 17 ancestral chordate linkage groups (and 19 ancestral bilaterian groups) by fusion, rearrangement and duplication. We resolve two distinct ancient duplications based on patterns of chromosomal conserved synteny. All extant vertebrates share the first duplication, which occurred in the mid/late Cambrian by autotetraploidization (that is, direct genome doubling). In contrast, the second duplication is found only in jawed vertebrates and occurred in the mid-late Ordovician by allotetraploidization (that is, genome duplication following interspecific hybridization) from two now-extinct progenitors. This complex genomic history parallels the diversification of vertebrate lineages in the fossil record.

Genetic Reprogramming of Positional Memory in a Regenerating Appendage.

Certain vertebrates such as salamanders and zebrafish are able to regenerate complex tissues (e.g., limbs and fins) with remarkable fidelity. However, how positional information of the missing structure is recalled by appendage stump cells has puzzled researchers for centuries. Here, we report that sizing information for adult zebrafish tailfins is encoded within proliferating blastema cells during a critical period of regeneration. Using a chemical mutagenesis screen, we identified a temperature-sensitive allele of the gene encoding DNA polymerase alpha subunit 2 (pola2) that disrupts fin regeneration in zebrafish. Temperature shift assays revealed a 48-h window of regeneration, during which positional identities could be disrupted in pola2 mutants, leading to regeneration of miniaturized appendages. These fins retained memory of the new size in subsequent rounds of amputation and regeneration. Similar effects were observed upon transient genetic or pharmacological disruption of progenitor cell proliferation after plucking of zebrafish scales or head or tail amputation in amphioxus and annelids. Our results provide evidence that positional information in regenerating tissues is not hardwired but malleable, based on regulatory mechanisms that appear to be evolutionarily conserved across distantly related phyla.

BMP controls dorsoventral and neural patterning in indirect-developing hemichordates providing insight into a possible origin of chordates.

A defining feature of chordates is the unique presence of a dorsal hollow neural tube that forms by internalization of the ectodermal neural plate specified via inhibition of BMP signaling during gastrulation. While BMP controls dorsoventral (DV) patterning across diverse bilaterians, the BMP-active side is ventral in chordates and dorsal in many other bilaterians. How this phylum-specific DV inversion occurs and whether it is coupled to the emergence of the dorsal neural plate are unknown. Here we explore these questions by investigating an indirect-developing enteropneust from the hemichordate phylum, which together with echinoderms form a sister group of the chordates. We found that in the hemichordate larva, BMP signaling is required for DV patterning and is sufficient to repress neurogenesis. We also found that transient overactivation of BMP signaling during gastrulation concomitantly blocked mouth formation and centralized the nervous system to the ventral ectoderm in both hemichordate and sea urchin larvae. Moreover, this mouthless, neurogenic ventral ectoderm displayed a medial-to-lateral organization similar to that of the chordate neural plate. Thus, indirect-developing deuterostomes use BMP signaling in DV and neural patterning, and an elevated BMP level during gastrulation drives pronounced morphological changes reminiscent of a DV inversion. These findings provide a mechanistic basis to support the hypothesis that an inverse chordate body plan emerged from an indirect-developing ancestor by tinkering with BMP signaling.

Co-option of the PRDM14-CBFA2T complex from motor neurons to pluripotent cells during vertebrate evolution.

Gene regulatory networks underlying cellular pluripotency are controlled by a core circuitry of transcription factors in mammals, including POU5F1. However, the evolutionary origin and transformation of pluripotency-related transcriptional networks have not been elucidated in deuterostomes. PR domain-containing protein 14 (PRDM14) is specifically expressed in pluripotent cells and germ cells, and is required for establishing embryonic stem cells (ESCs) and primordial germ cells in mice. Here, we compared the functions and expression patterns of PRDM14 orthologues within deuterostomes. Amphioxus PRDM14 and zebrafish PRDM14, but not sea urchin PRDM14, compensated for mouse PRDM14 function in maintaining mouse ESC pluripotency. Interestingly, sea urchin PRDM14 together with sea urchin CBFA2T, an essential partner of PRDM14 in mouse ESCs, complemented the self-renewal defect in mouse Prdm14 KO ESCs. Contrary to the Prdm14 expression pattern in mouse embryos, Prdm14 was expressed in motor neurons of amphioxus embryos, as observed in zebrafish embryos. Thus, Prdm14 expression in motor neurons was conserved in non-tetrapod deuterostomes and the co-option of the PRDM14-CBFA2T complex from motor neurons into pluripotent cells may have maintained the transcriptional network for pluripotency during vertebrate evolution.This article has an associated 'The people behind the papers' interview.

Using Amphioxus as a Basal Chordate Model to Study BMP Signaling Pathway.

The BMP signaling pathway has been shown to be involved in different aspects of embryonic development across diverse metazoan phyla. Comparative studies on the roles of the BMP signaling pathway provide crucial insights into the evolution of the animal body plans. In this chapter, we present the general workflow on how to investigate the roles of BMP signaling pathway during amphioxus embryonic development. As amphioxus are basal invertebrate chordates, studies on the BMP signaling pathway in amphioxus could elucidate the functional evolution of BMP pathway in the chordate group. Here, we describe methods for animal husbandry, spawning induction, and manipulation of the BMP signaling pathway during embryonic development through drug inhibitors and recombinant proteins. We also introduce an efficient method of using mesh baskets to handle amphioxus embryos for fluorescence immunostaining and multicolor fluorescence in situ hybridization and to assay the effects of manipulating BMP signaling pathway during amphioxus embryogenesis.

Amphioxus functional genomics and the origins of vertebrate gene regulation.

Vertebrates have greatly elaborated the basic chordate body plan and evolved highly distinctive genomes that have been sculpted by two whole-genome duplications. Here we sequence the genome of the Mediterranean amphioxus (Branchiostoma lanceolatum) and characterize DNA methylation, chromatin accessibility, histone modifications and transcriptomes across multiple developmental stages and adult tissues to investigate the evolution of the regulation of the chordate genome. Comparisons with vertebrates identify an intermediate stage in the evolution of differentially methylated enhancers, and a high conservation of gene expression and its cis-regulatory logic between amphioxus and vertebrates that occurs maximally at an earlier mid-embryonic phylotypic period. We analyse regulatory evolution after whole-genome duplications, and find that-in vertebrates-over 80% of broadly expressed gene families with multiple paralogues derived from whole-genome duplications have members that restricted their ancestral expression, and underwent specialization rather than subfunctionalization. Counter-intuitively, paralogues that restricted their expression increased the complexity of their regulatory landscapes. These data pave the way for a better understanding of the regulatory principles that underlie key vertebrate innovations.

Chitin-based barrier immunity and its loss predated mucus-colonization by indigenous gut microbiota.

Mammalian gut microbiota are integral to host health. However, how this association began remains unclear. We show that in basal chordates the gut space is radially compartmentalized into a luminal part where food microbes pass and an almost axenic peripheral part, defined by membranous delamination of the gut epithelium. While this membrane, framed with chitin nanofibers, structurally resembles invertebrate peritrophic membranes, proteome supports its affinity to mammalian mucus layers, where gut microbiota colonize. In ray-finned fish, intestines harbor indigenous microbes, but chitinous membranes segregate these luminal microbes from the surrounding mucus layer. These data suggest that chitin-based barrier immunity is an ancient system, the loss of which, at least in mammals, provided mucus layers as a novel niche for microbial colonization. These findings provide a missing link for intestinal immune systems in animals, revealing disparate mucosal environment in model organisms and highlighting the loss of a proven system as innovation.

Reiterative use of FGF signaling in mesoderm development during embryogenesis and metamorphosis in the hemichordate Ptychodera flava.

BACKGROUND: Mesoderm is generally considered to be a germ layer that is unique to Bilateria, and it develops into diverse tissues, including muscle, and in the case of vertebrates, the skeleton and notochord. Studies on various deuterostome animals have demonstrated that fibroblast growth factor (FGF) signaling is required for the formation of many mesodermal structures, such as vertebrate somites, from which muscles are differentiated, and muscles in sea urchin embryos, suggesting an ancient role of FGF signaling in muscle development. However, the formation of trunk muscles in invertebrate chordates is FGF-independent, leading to ambiguity about this ancient role in deuterostomes. To further understand the role of FGF signaling during deuterostome evolution, we investigated the development of mesodermal structures during embryogenesis and metamorphosis in Ptychodera flava, an indirect-developing hemichordate that has larval morphology similar to echinoderms and adult body features that are similar to chordates. RESULTS: Here we show that genes encoding FGF ligands, FGF receptors and transcription factors that are known to be involved in mesoderm formation and myogenesis are expressed dynamically during embryogenesis and metamorphosis. FGF signaling at the early gastrula stage is required for the specification of the mesodermal cell fate in P. flava. The mesoderm cells are then differentiated stepwise into the hydroporic canal, the pharyngeal muscle and the muscle string; formation of the last two muscular structures are controlled by FGF signaling. Moreover, augmentation of FGF signaling during metamorphosis accelerated the process, facilitating the transformation from cilia-driven swimming larvae into muscle-driven worm-like juveniles. CONCLUSIONS: Our data show that FGF signaling is required for mesoderm induction and myogenesis in the P. flava embryo, and it is reiteratively used for the morphological transition during metamorphosis. The dependence of muscle development on FGF signaling in both planktonic larvae and sand-burrowing worms supports its ancestral role in deuterostomes.