Coelomogenesis during the abbreviated development of the echinoid Heliocidaris erythrogramma and the developmental origin of the echinoderm pentameral body plan

Formation of Coelomic Cavities during Abbreviated Development of the Brittle Star Ophioplocus esmarki

AbstractIn brittle stars, the coelomic cavities that form during embryogenesis contribute to most of the internal organ systems of the juvenile. In the ancestral mode of development, the coelomic cavities begin with bilateral symmetry and play a minor role in the function of the ophiopluteus larva. However, the coelomic cavities undergo extensive changes during metamorphosis to set up the body systems of the juvenile brittle star. Many lineages of brittle stars have evolved life histories without the ophiopluteus larva. The non-feeding vitellaria larva has rapid development of juvenile structures. This work demonstrates the modifications to the origin and early development of the coelomic cavities in a vitellaria larva. Much of the archenteron forms an unpaired axocoel, hydrocoel, and somatocoel. The posterior-most portion of the archenteron forms the rudiment of the juvenile stomach. The right somatocoel and a portion of the left somatocoel form as invaginations of the lateral ectoderm. Later morphogenesis of the axocoel, the hydrocoel, and the two somatocoels is similar to what has been shown for brittle stars with an ophiopluteus larva. Confocal microscopy and three-dimensional modeling were used to show new details for the later morphogenesis of the coelomic cavities. The stone canal originates as an outgrowth of the hydrocoel between lobes 4 and 5. The hydrocoel lobes have minimal migration after they meet to complete the ring canal. The right somatocoel contributes to a component of the axial complex and perihemal system. A detailed description is given for how the left somatocoel contributes to multiple organ systems.

An easy and rapid staining method for confocal microscopic observation and reconstruction of three-dimensional images of echinoderm larvae and juveniles

The morphologies of the internal organs of echinoderm larvae and juveniles are difficult to study using conventional optical microscopes because of their structural complexity and opaqueness. This paper describes an easy and rapid protocol involving Nile blue staining followed by benzyl alcohol/benzyl benzoate (BABB) clearing to overcome this limitation. This method was developed for a three-dimensional (3D) analysis of the internal structures of advanced larvae and juveniles of echinoderms (the sea lily Metacrinus rotundus, the sea urchin Hemicentrotus pulcherrimus, and the sand dollar Scaphechinus mirabilis) and is suitable for obtaining serial optical images by confocal microscopy without the use of specific antibodies or special reagents for labeling. Nile blue is an easy-to-use stain that offers several advantages for confocal microscopy such as it can stain various tissues with strong fluorescent signals without substantial bleaching during observation. We found that the strong fluorescence signal of Nile blue quickly yielded clear high-resolution optical section images for 3D reconstruction. BABB clearing rendered opaque larvae highly transparent. The clearing procedure was also easy and quick. During the process, agarose embedding prior to staining and clearing was found to be critical for handling the samples of less than 500-μm length and stabilizing their orientations. To conclude, the protocol described is useful for performing a rapid and accurate 3D morphological analysis of echinoderm larvae and juveniles.

Post-metamorphic skeletal growth in the sea urchin Paracentrotus lividus and implications for body plan evolution

Background

Understanding the molecular and cellular processes that underpin animal development are crucial for understanding the diversity of body plans found on the planet today. Because of their abundance in the fossil record, and tractability as a model system in the lab, skeletons provide an ideal experimental model to understand the origins of animal diversity. We herein use molecular and cellular markers to understand the growth and development of the juvenile sea urchin (echinoid) skeleton.

Results

We developed a detailed staging scheme based off of the first ~ 4 weeks of post-metamorphic life of the regular echinoid Paracentrotus lividus. We paired this scheme with immunohistochemical staining for neuronal, muscular, and skeletal tissues, and fluorescent assays of skeletal growth and cell proliferation to understand the molecular and cellular mechanisms underlying skeletal growth and development of the sea urchin body plan.

Conclusions

Our experiments highlight the role of skeletogenic proteins in accretionary skeletal growth and cell proliferation in the addition of new metameric tissues. Furthermore, this work provides a framework for understanding the developmental evolution of sea urchin body plans on macroevolutionary timescales.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13227-021-00174-1.

Early development of the feeding larva of the sea urchin Heliocidaris tuberculata: role of the small micromeres

The two modes of development in sea urchins are direct development, in which the adult develops directly from the gastrula to the adult and does not feed, and indirect development, in which the adult develops indirectly through a feeding larva. In this account of the indirect, feeding larva of Heliocidaris tuberculata, the question raised is whether an evolutionary difference of unequal cell divisions contributes to the development of feeding structures in the indirect larva. In indirect development, the cell divisions at the fourth and fifth cell cycles of the zygote are unequal, with four small micromeres formed at the vegetal pole at the fifth cell division. In direct development, these cell divisions are not unequal. From their position at the head of the archenteron, the small micromeres are strategically located to contribute to the feeding tissues of the larva and the adult of H. tuberculata.

Expression of genes and proteins of the pax-six-eya-dach network in the metamorphic sea urchin: Insights into development of the enigmatic echinoderm body plan and sensory structures

BACKGROUND

Photoreception-associated genes of the Pax-Six-Eya-Dach network (PSEDN) are deployed for many roles in addition to photoreception development. In this first study of PSEDN genes during development of the pentameral body in sea urchins, we investigated their spatial expression in Heliocidaris erythrogramma.

RESULTS

Expression of PSEDN genes in the hydrocoele of early (Dach, Eya, Six1/2) and/or late (Pax6, Six3/6) larvae, and the five hydrocoele lobes, the first morphological expression of pentamery, supports a role in body plan development. Pax6, Six1/2, and Six3/6 were localized to the primary and/or secondary podia and putative sensory/neuronal cells. Six1/2 and Six3/6 were expressed in the neuropil region in the terminal disc of the podia. Dach was localized to spines. Sequential up-regulation of gene expression as new podia and spines formed was evident. Rhabdomeric opsin and pax6 protein were localized to cells in the primary podia and spines.

CONCLUSIONS

Our results support roles for PSEDN genes in development of the pentameral body plan, contributing to our understanding of how the most unusual body plan in the Bilateria may have evolved. Development of sensory cells within the Pax-Six expression field is consistent with the role of these genes in sensory cell development in diverse species. Developmental Dynamics 247:239-249, 2018. © 2017 Wiley Periodicals, Inc.

Nodal and BMP expression during the transition to pentamery in the sea urchin Heliocidaris erythrogramma: insights into patterning the enigmatic echinoderm body plan

BACKGROUND

The molecular mechanisms underlying the development of the unusual echinoderm pentameral body plan and their likeness to mechanisms underlying the development of the bilateral plans of other deuterostomes are of interest in tracing body plan evolution. In this first study of the spatial expression of genes associated with Nodal and BMP2/4 signalling during the transition to pentamery in sea urchins, we investigate Heliocidaris erythrogramma, a species that provides access to the developing adult rudiment within days of fertilization.

RESULTS

BMP2/4, and the putative downstream genes, Six1/2, Eya, Tbx2/3 and Msx were expressed in the earliest morphological manifestation of pentamery during development, the five hydrocoele lobes. The formation of the vestibular ectoderm, the specialized region overlying the left coelom that forms adult ectoderm, involved the expression of putative Nodal target genes Chordin, Gsc and BMP2/4 and putative BMP2/4 target genes Dlx, Msx and Tbx. The expression of Nodal, Lefty and Pitx2 in the right ectoderm, and Pitx2 in the right coelom, was as previously observed in other sea urchins.

CONCLUSION

That genes associated with Nodal and BMP2/4 signalling are expressed in the hydrocoele lobes, indicates that they have a role in the developmental transition to pentamery, contributing to our understanding of how the most unusual body plan in the Bilateria may have evolved. We suggest that the Nodal and BMP2/4 signalling cascades might have been duplicated or split during the evolution to pentamery.

Analysis of coelom development in the sea urchin Holopneustes purpurescens yielding a deuterostome body plan

An analysis of early coelom development in the echinoid Holopneustes purpurescens yields a deuterostome body plan that explains the disparity between the pentameral plan of echinoderms and the bilateral plans of chordates and hemichordates, the three major phyla of the monophyletic deuterostomes. The analysis shows an early separation into a medial hydrocoele and lateral coelomic mesoderm with an enteric channel between them before the hydrocoele forms the pentameral plan of five primary podia. The deuterostome body plan thus has a single axial or medial coelom and a pair of lateral coeloms, all surrounding an enteric channel, the gut channel. Applied to the phyla, the medial coelom is the hydrocoele in echinoderms, the notochord in chordates and the proboscis coelom in hemichordates: the lateral coeloms are the coelomic mesoderm in echinoderms, the paraxial mesoderm in chordates and the lateral coeloms in hemichordates. The plan fits frog and chick development and the echinoderm fossil record, and predicts genes involved in coelomogenesis as the source of deuterostome macroevolution.

Summary: A common body plan for echinoderms, chordates and hemichordates resolves the apparent morphological disparity between the pentameral and the bilateral body plans of these major deuterostome phyla.

Oral-aboral identity displayed in the expression of HpHox3 and HpHox11/13 in the adult rudiment of the sea urchin Holopneustes purpurescens

Hox genes are noted for their roles in specifying axial identity in bilateral forms. In the radial echinoderms, the axis whose identity Hox genes might specify remains unclear. From the expression of Hox genes in the development of the sea urchin Holopneustes purpurescens reported here and that reported previously, we clarify the axis that might be specified by Hox genes in echinoderms. The expression of HpHox11/13 here is described at three developmental stages. The expression is around the rim of the blastopore in gastrulae, in the archenteron wall and adjacent mesoderm in early vestibula larvae, and in a patch of mesoderm close to the archenteron wall in later vestibula larvae. The retained expression of HpHox11/13 in the patch of mesoderm in the later vestibula larvae is, we suggest, indicative of a posterior or an aboral growth zone. The expression of HpHox3 at the echinoid-rudiment stage, in contrast, is in oral mesoderm beneath the epineural folds, concentrated in sites where the first three adult spines form. With the expression of HpHox5 and HpHox11/13 reported previously, the expressions here support the role of Hox genes in specifying oral-aboral identity in echinoderms. How such specification and a posterior growth zone add support to a concept of the structural homology between echinoderms and chordates is discussed.