Embryonic Development Research Articles

Effects of Flurochloridone on the Developmental Toxicity in Zebrafish (Danio rerio) Embryo.

Flurochloridone (FLC) is a selective herbicide that can cause reproductive toxicity in male rats. However, limited information is available regarding the toxicity of FLC in the developmental stages of aquatic organisms. This study aimed to investigate the effects of FLC exposure during embryonic development and elucidate its potential mechanism of action. Zebrafish embryos were exposed to 6.25, 12.5, 25, and 50 μg/mL FLC for 4-144 hpf. The developmental status of embryos was recorded; the indicators of oxidative stress and embryonic apoptosis were determined. We found that FLC exposure caused severe embryonic malformations, such as pericardial edema, spinal curvature, and growth retardation, accompanied by a decreased hatching and survival rate. After exposure until 144 h postfertilization, the median lethal concentration (LC50) of FLC in zebrafish embryos was 36.9 μg/mL. Subsequently, FLC induced the accumulation of reactive oxygen species and malondialdehyde, enhanced the activity of superoxide dismutase, and activated the Keap1-Nrf2 signaling pathway. Further studies confirmed that FLC can induce apoptosis in zebrafish embryos through the activation of caspase. These results suggest that FLC induced developmental toxicity in zebrafish embryos, which provides new evidence regarding FLC toxicity in aquatic organisms and to assess human health risks.

Association Between Reduced Upstream Riparian Forest Cover and Impaired Development of Embryos From Eastern Hellbenders (Cryptobranchus alleganiensis)

ABSTRACT Populations of the Eastern Hellbender, (a large‐bodied, fully aquatic salamander) inhabiting stream reaches with low catchment‐wide riparian forest cover upstream, have experienced population declines and a shift toward a geriatric population age structure. These population declines and demographic shifts might be attributed to reduced embryo viability. Reduced egg quality/viability could negatively affect recruitment and has also been known to trigger filial cannibalism in other species. Therefore, we hypothesized that in comparison to high forest cover sites, hellbender eggs collected from low forest cover sites would have a greater incidence of developmental abnormalities and lower overall viability, and that this would predict whole‐clutch cannibalism by the attending male. We collected a subset of eggs (~20–35) from 99 clutches across sites with variable upstream riparian forest cover and reared these eggs through hatching in stream water under controlled laboratory conditions. At the same time, we monitored the fate of the remaining eggs from the same clutches in the field to document the frequency of whole‐clutch filial cannibalism. We found that eggs collected from sites with lower upstream forest cover had significantly shorter embryonic development times and produced a lower percentage of viable hatchlings (hatchlings with normal development times and morphology). The average modelled viability of hatchlings was 70% higher in sites with the highest forest cover compared to our sites with the lowest forest cover. In contrast to our predictions, we did not find evidence to suggest that egg viability in the lab predicted whole‐clutch cannibalism in the field. Although forest cover was a significant predictor of egg viability and underdevelopment, substantial variance in embryonic developmental traits was unaccounted for in our models suggesting that traits associated with adults (e.g., egg and/or sperm quality) may also play a role in determining developmental outcomes. Further experiments are needed to identify what factors (e.g., egg quality, water quality) disrupt the embryonic development of hellbenders as well as the proximate stimulus that causes adult male hellbenders to eat their young. Our results emphasise the importance of restoring and protecting riparian forest cover to conserve sensitive stream species.

An In-Depth Coho Salmon (Oncorhynchus kisutch) Ovarian Follicle Proteome Reveals Coordinated Changes Across Diverse Cellular Processes during the Transition From Primary to Secondary Growth.

Teleost fishes are a highly diverse, ecologically essential group of aquatic vertebrates that include coho salmon (Oncorhynchus kisutch). Coho are semelparous and all ovarian follicles develop synchronously. Owing to their ubiquitous distribution, teleosts provide critical sources of food worldwide through subsistence, commercial fisheries, and aquaculture. Enhancement of hatchery practices requires detailed knowledge of teleost reproductive physiology. Despite decades of research on teleost reproductive processes, an in-depth proteome of teleost ovarian development has yet to be generated. We have described a coho salmon ovarian proteome of over 5700 proteins, generated with data independent acquisition, revealing the proteins that change through the transition from primary to secondary ovarian follicle development. This transition is critical during the onset of puberty and for determining egg quality and embryonic development. Primary follicle development was marked by differential abundances of proteins in carbohydrate metabolism, protein turnover, and the complement pathway, suggesting elevated metabolism as the follicles develop through stages of oogenesis. The greatest proteomic shift occurred during the transition from primary to secondary follicle growth, with increased abundance of proteins underlying cortical alveoli formation, extracellular matrix reorganization, iron binding, and cell-cell signaling. This work provides a foundation for identifying biomarkers of salmon oocyte stage and quality.

Clinicopathological significance of SOX2 and FOXG1 expression patterns in ovarian immature teratomas

Objective: To investigate the relationship between the expression patterns of SOX2 and FOXG1 and the differentiation/development level of neural components in immature teratoma and to determine the clinical significance and potential application of this correlation in a clinical setting. Methods: We conducted a comprehensive whole transcriptome sequencing analysis to identify differentially expressed genes (DEGs) across various subtypes of ovarian germ cell tumors. Additionally, immunohistochemical staining of paraffin-embedded tissue sections was employed to assess the nuclear staining pattern of SOX2 and FOXG1 proteins within the tumor tissues. Results: The transcriptome sequencing data showed that transcription factors SOX2 and FOXG1 exhibited high levels of expression typically in immature teratoma and occupied a pivotal position within the protein-protein interaction network. Immunohistochemical staining revealed the absence of both SOX2 and FOXG1 protein expression in dysgerminoma and yolk sac tumor samples. In immature teratoma, immunohistochemical staining demonstrated diffuse expression of SOX2 and FOXG1 proteins within the inner layer of densely-arranged primitive neuroepithelial tubules. This pattern of expression suggested the presence of stem cell-like properties within these tumor cells. In the sparsely peripheral neurogliocytes, FOXG1 maintained a diffuse nuclear staining pattern resembling that of neuroepithelial cells, while SOX2 exhibited a scattered pattern of positive staining, hinting at a neural lineage differentiation potential. This spatial differential expression pattern of SOX2 and FOXG1 proteins in immature teratoma suggested that primitive neural components within these tumors often recapitulated the trajectory of neural formation and cortical development that was typically observed during embryogenesis. The primitive neural tube acted as the center that constantly moved from inside to outside, with a dynamic shift from the interior to the exterior, paralleled by the sequential differentiation of cell lineages from primitive neuroepithelial stem cells to radial glia, intermediate progenitor cells, and ultimately to precursor glia. Conclusions: This spatial expression pattern of SOX2 and FOXG1 proteins observed in immature teratoma mirrors the lineage differentiation and migration trajectories of primitive neuroepithelial components typically seen in embryonic neurogenesis and cortical development. In daily practice, the combined application of SOX2 and FOXG1 SOX2 and FOXG1 helps identify the primitive neuroepithelial components in immature teratoma, avoid misjudgment of similar morphologies, and thereby assist in the histological grading and clinical decision-making.

3-Chloro-1,2-Propanediol Exposure Induces Cardiotoxicity and Behavioural Abnormalities in Zebrafish Embryos.

Numerous contemporary diseases are linked to food contamination. Pathogenic agents might stem from certain food ingredients or result from pollution stemming from food processing or packaging. One such contaminant is 3-Chloro-1,2-propanediol (3-MCPD), it has been previously reported to be produced during the preparation of chemical sauces, as well as during the heating of baked goods. Yet, uncertainty surrounds its potential to induce embryonic developmental toxicity. In this study, zebrafish were employed as the focal point to assess the impact of 3-MCPD on initial embryonic development, heart functionality, and behavior. The research unveiled that exposure of zebrafish embryos to 18, 36, and 54 mM 3-MCPD led to cardiac anomalies, including pericardial edema, reduced heart rate, and elongated SV-BA distance. Additionally, 3-MCPD exposure triggered aberrations in cardiac-related gene expression and an elevation in oxidative stress. Notably, behavioral changes were observed in 3-MCPD-exposed zebrafish embryos, while vascular development appeared unaffected. This study introduces a novel basis for comprehensive exploration of 3-MCPD toxicity.

Novel variants in PADI6 genes cause female infertility due to early embryo arrest.

Early embryo arrest is characterized by premature termination of development in preimplantation embryos. Human subcortical maternal complex (SCMC) is a protein complex that is specifically expressed in mammalian oocytes and early embryos and is essential for embryonic cell division. Peptidyl arginine deiminase 6 (PADI6) is proven to be a member of SCMC. Variants in the PADI6 gene have been shown to induce early embryo arrest. In this study, we performed genetic analysis in patients with female infertility due to early embryo arrest to identify the disease-causing gene variants. Whole-exome sequencing and Sanger sequencing were used to identify the variants in the patients and their families. Western blotting and immunofluorescence staining were used to check the effects of the variants on expression and function of PADI6. We identified a novel homozygous variant (c.358A > C [p.Thr120Pro]) and novel compound-heterozygous variants (c.2044C > T [p.Arg682Trp] and c.707dupT [p.Leu237Alafs*24]) in PADI6 in two infertile individuals with early embryo arrest. We found that these variants resulted in a decrease in the expression level of PADI6, which may lead to abnormal protein function. Immunofluorescence staining also suggested that these variants affected the expression of PADI6. Our study expands the spectrum of genetic defects in female early embryo arrest and further supports the causality between PADI6 variants and female infertility.

Effects of Caffeic Acid Phenethyl Ester on Embryonic Development Through Regulation of Mitochondria and Endoplasmic Reticulum

Caffeic acid phenethyl ester (CAPE) is one of the main active components of the natural medicine propolis, which has antioxidant, anti-tumor, and immunomodulatory activities. This study aimed to analyze the effects and underlying mechanisms of CAPE added to the medium of in vitro cultures on the developmental competence, mitochondria, and endoplasmic reticulum of porcine embryos. The results demonstrated that 1 nM of CAPE significantly improved the quality of porcine embryos, increased the rate of blastocyst formation, and enhanced the proliferation ability. It also enhanced mitochondrial function by increasing the level of mitochondrial membrane potential and expression of the mitochondrial biogenesis-related protein PPARgamma coactivator 1 alpha and beta (PGC1 alpha and beta), regulating mitochondrial biogenesis, and increasing adenosine triphosphate (ATP) content. In addition, CAPE alleviated oxidative and endoplasmic reticulum (ER) stress in embryos by decreasing ROS accumulation and increasing glutathione content, as well as elevating Nrf2 and reducing GRP78 (ER stress marker) expression levels. Moreover, CAPE reduced the levels of apoptosis and autophagy in the cultivated embryos. These results indicate that CAPE improves the quality and enhances the mitochondrial function of in vitro-produced porcine embryos by alleviating oxidative and ER stress.

Investigating the Expression and Function of HIF-1α in Neocaridina davidi During Embryo Cleavage Stage

Neocaridina davidi, a member of Decapoda within Crustacea, stands out due to its petite stature, robust reproductive capabilities and short molting cycle. Consequently, it has emerged as a valuable experimental model for investigations spanning ecology, development, physiology, and toxicology. Despite the pivotal role of Hypoxia Inducible Factor-1α (HIF-1α) in diverse physiological processes like biological hypoxia stress, cellular growth, and stress resistance, its functionality in crustaceans remains underexplored. Moreover, the role and function of HIF-1α in crustaceans, especially in embryonic stages, have been insufficiently addressed. This study delves into the function of HIF-1α during embryonic development. Initially, the complete sequence of the HIF-1α gene was acquired. Notably, the expression of HIF-1α during the cleavage stage surpassed that of subsequent phases. Subsequently, dsRNAHIF-1α was introduced into sexually mature shrimp, revealing that the inhibitory impact of dsRNA on gene expression in the parental generation could be inherited by the offspring, exerting a specific gene silencing effect in the embryo. The consequences of silencing HIF-1α in embryos manifested as varying degrees of premature division termination, besides, the glycolysis in the embryos was also affected by HIF-1α suppression. These results provide data support for understanding the early embryonic development of crustaceans and HIF-1α functions within it.

Not just a garbage truck: No-go decay plays a role during embryo development in zebrafish.

In eukaryotes, defective mRNAs that impede the movement of the ribosome are subject to rapid decay via no-go decay (NGD). In this issue of PLOS Biology, Ishibashi and colleagues expand on the role of NGD and reveal new endogenous targets for the process in zebrafish.

Membrane progesterone receptor e (paqr9) is necessary for chorion elevation in zebrafish.

Paqr9 is a gene encoding membrane progestin receptor e (mPRe), the fifth subtype of the five mPR subtypes, and is currently classified a member of the progestin and adipoQ receptor (PAQR) family, which consists of 11 genes. To elucidate the physiological functions of the mPR subtypes, we established gene knockout (KO) fish via genome editing of seven paqr genes in zebrafish and analyzed their phenotypes. The null-mutant strain of paqr9 (paqr9-/-) that we established in this study presented reduced chorion elevation and a high percentage of abnormal embryos. Embryos exhibit various kinds of abnormal morphology, which are thought to be caused by insufficient elevation of the chorion. Immunohistochemical staining of ovaries with an anti-Paqr9 antibody revealed that Paqr9 was expressed in the periplasm of oocytes and the surface of chorion in the wild type, whereas signals were absent in paqr9-/- zebrafish. In histological sections, the periplasmic connection between the oocyte plasma membrane and chorion was absent in paqr9-/- oocytes. The number of cortical alveoli (CA) that are responsible for chorion elevation was significantly reduced in paqr9-/- zebrafish. SEM revealed that fiber-supported knob-like structures (KSs) on the chorion were absent in paqr9-/- zebrafish. These results indicate that Paqr9 is required for the preparation of CA during oogenesis. Insufficient formation of the chorion resulted in the abnormal development of embryos.

Decoding complex transport patterns in flow-induced autologous chemotaxis of multicellular systems.

Cell migration via autologous chemotaxis in the presence of interstitial fluid flow is important in cancer metastasis and embryonic development. Despite significant recent progress, our understanding of flow-induced autologous chemotaxis of multicellular systems remains poor. The literature presents inconsistent findings regarding the effectiveness of collective autologous chemotaxis of densely packed cells under interstitial fluid flow. Here, we present a high-fidelity computational model to analyze the migration of multicellular systems performing autologous chemotaxis in the presence of interstitial fluid flow. Our simulations show that the details of the complex transport dynamics of the chemoattractant and fluid flow patterns that occur in the extracellular space, previously overlooked, are essential to understand this cell migration mechanism. We find that, although flow-induced autologous chemotaxis is a robust migration mechanism for individual cells, the cell-cell interactions that occur in multicellular systems render autologous chemotaxis an inefficient mechanism of collective cell migration. Our results offer new perspectives on the potential role of autologous chemotaxis in the tumor microenvironment, where fluid flow is an important modulator of transport.

Spinal neural tube formation and tail development in human embryos

Spinal neural tube formation and tail development in human embryos

Spinal neural tube formation and tail development in human embryos.

Primary and secondary neurulation - processes that form the spinal cord - are incompletely understood in humans, largely due to the challenge of accessing neurulation-stage embryos (3-7 weeks post-conception). Here, we describe findings from 108 human embryos, spanning Carnegie stages (CS) 10-18. Primary neurulation is completed at the posterior neuropore with neural plate bending that is similar, but not identical, to the mouse. Secondary neurulation proceeds from CS13 with formation of a single lumen as in mouse, not coalescence of multiple lumens as in chick. There is no evidence of a 'transition zone' from primary to secondary neurulation. Secondary neural tube 'splitting' occurs in 60% of proximal human tail regions. A somite is formed every 7 hr in human, compared with 2 hr in mice and a 5 hr 'segmentation clock' in human organoids. Termination of axial elongation occurs after down-regulation of WNT3A and FGF8 in the CS15 embryonic tailbud, with a 'burst' of apoptosis that may remove neuro-mesodermal progenitors. Hence, the main differences between human and mouse/rat spinal neurulation relate to timing. Investigators are now attempting to recapitulate neurulation events in stem cell-derived organoids, and our results provide 'normative data' for interpretation of such research findings.

The intestine-specific homeobox (ISX) modulates β-carotene-dependent regulation of microsomal triglyceride transfer protein (MTP) in a tissue-specific manner

Vitamin A is an essential nutrient crucial to ensuring proper mammalian embryonic development. β-Carotene is the most prevalent form of vitamin A in food that, when transferred in its intact form from mother to the developing tissues, can serve as an in situ source of retinoic acid, the active form of vitamin A. We have previously provided evidence that the maternal-fetal transfer of β-carotene across the placenta is mediated by lipoproteins and that β-carotene itself regulates placenta lipoprotein biogenesis by means of its derivatives β-apo-10′-carotenoids and retinoic acid. These metabolites exert antagonistic transcriptional activity on placental microsomal triglyceride transfer protein (MTP) and apolipoprotein B (APOB), two key players of lipoprotein biosynthesis. Here, we analyzed the time-dependency of this regulation over the course of 24 h upon a single maternal administration of β-carotene. We also tested the hypothesis that the transcriptional repressor intestine-specific homeobox (ISX) plays a role in the regulation of Mttp in placenta. We observed that ISX is expressed in placenta of mouse dams and is regulated by β-carotene availability. Furthermore, we demonstrated that the absence of Isx disrupts the β-carotene-mediated regulation of placental MTP. We also showed that this mechanism is organ-specific, as it was not observed in enterocytes of the intestine, a major place of Isx expression. Therefore, we identified ISX as a “master” regulator of a placental β-carotene-dependent transcriptional regulatory cascade that fine-tunes the flux of provitamin A carotenoid towards the developing fetus.

Bmlark is essential for embryonic development

BackgroundTranscription factor lark has been demonstrated to play multiple functions in Drosophila, but the function of this gene in embryonic development remains to be elucidated.ResultsIn this study, the CRISPR/Cas9 gene-editing method was used to construct a Bmlark mutant strain of Bombyx mori to investigate the roles of this gene. The results showed that the homozygous mutant Bmlark−/− was lethal. The Bmlark−/− embryos showed obvious developmental defects, such as defective sclerotization and melanization of the exoskeleton. A transcriptomic comparison of Bmlark−/− and wild-type embryos showed that the differentially expressed genes were mainly enriched in the structure and metabolic processes of chitin and cuticles. While the expression levels of chitin metabolism-related enzyme genes did not significantly change, in the mutant embryos compared to the wild-type embryos, the expression levels of 63 putative cuticle protein genes showed significant differences. Among which, 35 genes were downregulated and 28 genes were upregulated. The expression levels of the transcription factor BmPOUM2 and eight wing disc cuticle protein genes (WCP) also changed. BmPOUM2, WCP5, WCP9, WCP10, WCP11 were downregulated and WCP1, WCP2, WCP3, WCP6 were upregulated in Bmlark−/− embryos. While the expression level of TH in the tyrosine-mediated pigmentation pathway was upregulated in the mutant embryos, the expression levels of the four key pigment synthesis genes DDC, aaNAT, Laccase2A, and yellow-f2 were significantly downregulated.ConclusionsThe expression levels of 63 putative cuticle protein genes, eight WCP genes, and five pigment synthesis genes significantly changed in Bmlark mutant B. mori compared to those of the wildtype. These results suggest that Bmlark is essential for normal development of cuticle and tyrosine-mediated melanization in silkworm embryos.

Kinesin-like motor protein KIF23 maintains neural stemand progenitor cell pools in the developing cortex.

Accurate mitotic division of neural stem and progenitor cells (NSPCs) is crucial for the coordinated generation of progenitors and mature neurons, which determines cortical size and structure. While mutations in the kinesin-like motor protein KIF23 gene have been recently linked to microcephaly in humans, the underlying mechanisms remain elusive. Here, we explore the pivotal role of KIF23 in embryonic cortical development. We characterize the dynamic expression of KIF23 in the cortical NSPCs of mice, ferrets, and humans during embryonic neurogenesis. Knockdown of Kif23 in mice results in precocious neurogenesis and neuronal apoptosis, attributed to an accelerated cell cycle exit, likely resulting from disrupted mitotic spindle orientation and impaired cytokinesis. Additionally, KIF23 depletion perturbs the apical surface structure of NSPCs by affecting the localization of apical junction proteins. We further demonstrate that the phenotypes induced by Kif23 knockdown are rescued by introducing wild-type human KIF23, but not by a microcephaly-associated variant. Our findings unveil a previously unexplored role of KIF23 in neural stem and progenitor cell maintenance via regulating spindle orientation and apical structure in addition to cytokinesis, shedding light on microcephaly pathogenesis.

Low-input CUT&Tag for efficient epigenomic profiling of zebrafish stage I oocytes

Histone modification signatures mark sites of transcriptional regulatory elements and regions of gene activation and repression. These sites vary among cell types and undergo dynamic changes during development and in diseases. Oocytes produce numerous maternal factors essential for early embryonic development, which are significantly influenced by epigenetic modifications. The profiling of epigenetic modifications during oogenesis remains uniquely challenging due to the presence of numerous tightly wrapped granulosa cells. Here, we successfully established a low-input CUT&Tag (Cleavage Under Targets and Tagmentation) method tailored for zebrafish stage I oocytes. This advanced technique enables high-resolution profiling of histone modifications and DNA-binding proteins, critical for understanding chromatin dynamics in developing oocytes. In this study, we detailed the workflow for this technique, including the isolation of pure stage I oocytes without somatic cells, library construction and quality monitoring. Our results demonstrate the method’s efficacy by identifying distinct histone modification patterns and analyzing differentially expressed genes in oocytes with and without granulosa cells. We also successfully profiled divergent histone modifications in oocytes derived from wild-type and huluwa mutants. These advancements overcome technical challenges in epigenetic research on zebrafish oocytes and establish a solid foundation for exploring the epigenetic regulatory mechanisms of maternal contribution.

Multi-omics revealed that DCP1A and SPDL1 determine embryogenesis defects in postovulatory ageing oocytes.

Growing evidence indicates that the deterioration of egg quality caused by postovulatory ageing significantly hampers embryonic development. However, the molecular mechanisms by which postovulatory ageing leads to a decline in oocyte quality have not been fully characterized. In this study, we observed an accelerated decay of maternal mRNAs through RNA-seq analyses in postovulatory-aged (PostOA) oocytes. We noted that these downregulated mRNAs should be degraded during the 2-cell stage. Proteomic analyses revealed that the degradation of maternal mRNAs is associated with the accumulation of DCP1A. The injection of exogenous Dcp1a mRNA or siRNA into MII stage oocytes proved that DCP1A could accelerate the degradation of maternal mRNAs. Additionally, we also found that SPDL1 is crucial for maintaining spindle/chromosome structure and chromosome euploidy in PostOA oocytes. Spdl1-mRNA injection remarkably recovered the meiotic defects in PostOA oocytes. Collectively, our findings provide valuable insights into the molecular mechanisms underlying postovulatory ageing.

Transcriptomic and Metabolomic Analysis Reveals Multifaceted Impact of Gcn5 Knockdown in Drosophila Development

Background: General control nonderepressible 5 (Gcn5) is a lysine acetyltransferase (KAT) that is evolutionarily conserved across eukaryotes, with two homologs (Kat2a and Kat2b) identified in humans and one (Gcn5) in Drosophila. Gcn5 contains a P300/CBP-associated factor (PCAF) domain, a Gcn5-N-acetyltransferase (GNAT) domain, and a Bromodomain, allowing it to regulate gene expression through the acetylation of both histone and non-histone proteins. In Drosophila, Gcn5 is crucial for embryonic development, with maternal Gcn5 supporting early development. However, the functional mechanisms of Gcn5 after the depletion of maternal deposits remain unclear. Methods: Our study employed the Gal4/UAS-RNAi system to achieve whole-body or heart-specific Gcn5 knockdown in Drosophila and selected 96-hour-old surviving larvae for transcriptomic and metabolomic analyses. Results: Omics results revealed that Gcn5 knockdown significantly impacts various metabolic pathways, as well as lysosomes, non-homologous end-joining, Toll and Imd signaling pathways, and circadian rhythms, among others. Furthermore, defects in chitin synthesis may be associated with impaired pupation. Additionally, heart-specific Gcn5 knockdown affected cardiac physiology but appeared to have a potential protective effect against age-related cardiac decline. Conclusions: These findings deepen our understanding of Gcn5’s roles in Drosophila development and provide valuable insights for developing Gcn5-targeted therapies, particularly considering its involvement in various human diseases.

Co-option of neck muscles supported the vertebrate water-to-land transition

A major event in vertebrate evolution was the separation of the skull from the pectoral girdle and the acquisition of a functional neck, transitions that required profound developmental rearrangements of the musculoskeletal system. The neck is a hallmark of the tetrapod body plan and allows for complex head movements on land. While head and trunk muscles arise from distinct embryonic mesoderm populations, the origins of neck muscles remain elusive. Here, we combine comparative embryology and anatomy to reconstruct the mesodermal contribution to neck evolution. We demonstrate that head/trunk-connecting muscle groups have conserved mesodermal origins in fishes and tetrapods and that the neck evolved from muscle groups present in fishes. We propose that expansions of mesodermal populations into head and trunk domains during embryonic development underpinned the emergence and adaptation of the tetrapod neck. Our results provide evidence for the exaptation of archetypal muscle groups in ancestral fishes, which were co-opted to acquire novel functions adapted to a terrestrial lifestyle.