Click
here to close Hello! We notice that
you are using Internet Explorer, which is not supported by Echinobase
and may cause the site to display incorrectly. We suggest using a
current version of Chrome,
FireFox,
or Safari.
???displayArticle.abstract???
A challenge for evolutionary developmental (evo-devo) biology is to expand the breadth of research organisms used to investigate how animal diversity has evolved through changes in embryonic development. New experimental systems should couple a relevant phylogenetic position with available molecular tools and genomic resources. As a phylum of the sister group to chordates, echinoderms extensively contributed to our knowledge of embryonic patterning, organ development and cell-type evolution. Echinoderms display a variety of larval forms with diverse shapes, making them a suitable group to compare the evolution of embryonic developmental strategies. However, because of the laboratory accessibility and the already available techniques, most studies focus on sea urchins and sea stars mainly. As a comparative approach, the field would benefit from including information on other members of this group, like the sea cucumbers (holothuroids), for which little is known on the molecular basis of their development. Here, we review the spawning and culture methods, the available morphological and molecular information, and the current state of genomic and transcriptomic resources on sea cucumbers. With the goal of making this system accessible to the broader community, we discuss how sea cucumber embryos and larvae can be a powerful system to address the open questions in evo-devo, including understanding the origins of bilaterian structures.
Fig. 1. Deuterostomes phylogenetic tree. Phylogenetic relationship of deuterostomes with a focus on living echinoderms. Cartoons represent for each class the typical adult and planktonic larva body plans
Fig. 2. Life cycle of sea cucumbers. Schematic showing the two types of reproductive strategies of Holothurians. Planktotrophic species have transparent embryos and develop through a feeding larva stage, while lecithotrophic species have yolky embryos that do not develop a complete digestive system and do not feed
Fig. 3. Development and morphology of Holothuroidea, focus on H. tubulosa. a Spawning female (G, gonopore; E, eggs); b female gonads; c arrested oocyte, note the nucleus in the center; d mature oocyte, note that the nucleus is not visible anymore; e 2-cell stage; f 4-cell stage; g early gastrula and h late gastrula; i early and j late auricularia larvae; k fully developed doliolaria; l early pentactula larva. m Juvenile and n adult (b–i scale bar = 40 μm; j–m scale bar = 100 μm). H, hydropore; HS, hyaline spheres; T, tentacles; P, podia. a, m are modified from [126], b, c, d, e, f, g, h, i, j, k, I, n are original pictures taken in the Annunziata’s laboratory
Fig. 4. Cartoons showing the morphology of gastrula, auricularia, doliolaria and pentactula larvae. In the gastrula, mesenchymal cells migrate from the archenteron (future gut) while it elongates anteriorly. The first neurons appear at this stage. Auricularia larvae are characterized by a functional digestive system, the presence of the hydrocoel, the left and right somatocoels and hyaline spheres. At this stage, the nervous system increases in complexity. Doliolaria is a transitional barrel-shaped larva that does not feed. It has larger hyaline spheres compared to the auricularia. Adult organs are formed in the pentactula, the last stage before the juvenile. In figure, a green continuous line indicates the neurons underneath the ciliary band. To make the auricularia cartoon clearer and show distribution of neurons we omitted the ciliary band that follows the same pattern
Fig. 5. Square plot summarizing all the gene expression data available for sea cucumber embryos and larvae. Colors represent the species where gene expression was investigated. Tph and Synaptotagmin expression has been inferred based on the localization of the proteins through immunohistochemistry
Fig. 6. Comparison of the main cell types in the sea cucumber auricularia larva with the sea star bipinnaria and sea urchin pluteus larvae. The cartoons depict the main characterized cell types in echinoderm larvae. Sea cucumber larvae have unique features that distinguish them from other echinoderms, like the presence of hyaline spheres (for lipid storage), one or more short skeletal rods, or ossicles, that do not extend (number varies based on the genus), a single hydrocoel that appears on the left side of the stomach and an extensive neuronal network that do not show long axonal projections towards the internal structures
