Hemichordates: Development

Abstract

Hemichordates are marine invertebrates consisting of two distinct groups: the solitary enteropneusts, or acorn worms (comprising a few hundred species), and the colonial and tube‐dwelling pterobranchs (comprising 20 or so species). Phylogenetically, hemichordates are the sister group to echinoderms, together composing the Ambulacraria, and share some features with chordates. Hemichordates are thus considered key organisms for addressing the origins of deuterostome and chordate body plans. Unlike the other deuterostome models including echinoderms and chordates, however, information about the developmental biology of this group is limited. Recent improvements in the accessibility of embryos, functional tool development and genomic resources from several model organisms have yielded important information on the cellular and genetic mechanisms of embryogenesis and organogenesis in hemichordates. Comparisons of hemichordates and other deuterostomes permit identification of the common ancestor of deuterostomes and help elucidate the origin of the chordate body plan. Key Concepts Hemichordate developmental biology is key to elucidating the origin of chordates and the early evolution of deuterostomes. Descriptive and comparative studies using novel model species provide new insights into hemichordate evolution and diversity. Establishment of genetic resources and experimental methods drive the molecular developmental biology of hemichordates. Hemichordates and chordates show a conserved anteroposterior patterning mechanism. Hemichordate research indicates that inversion of the dorsoventral axis occurred in the chordate lineage. Gill slits of hemichordates and chordates are almost certainly homologous. The stomochord of hemichordates and the notochord of chordates share some genetic features, but are unlikely to be homologous. The origin of the tubular nervous system predates the diversification of hemichordates and chordates, but the homology of the collar nerve cord and neural tube still requires resolution.

Neurons and Glia Cells in Marine Invertebrates: An Update

“…Due their bilateral symmetry as adults, hemichordates show analogous regeneration patterns with the chordates. This has been proved at molecular level, where gene expression analysis revealed that Hemicordata regeneration requires an independent stem-cell endogenous transdifferentiation, and particular genetic mechanisms (Lowe et al, 2003;Tagawa, 2015, 2018;Miyamoto and Wada, 2015). In Deuterostome, Hemichordate are the only known to regenerate the complete NS of the anterior head-like structure, as Ptychodera flava that has the ability to regenerate their entire NS in 1-4 days (Humphreys et al, 2010;Luttrell et al, 2016).…”