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Background: Head or anterior body part regeneration is commonly associated with protostome, but not deuterostome invertebrates. However, it has been shown that the solitary hemichordate Ptychodera flava possesses the remarkable capacity to regenerate their entire nervous system, including their dorsal neural tube and their anterior head‐like structure, or proboscis. Hemichordates, also known as acorn worms, are marine invertebrate deuterostomes that have retained chordate traits that were likely present in the deuterostome ancestor, placing these animals in a vital position to study regeneration and chordate evolution. All acorn worms have a tripartite body plan, with an anterior proboscis, middle collar region, and a posterior trunk. The collar houses a hollow, dorsal neural tube in ptychoderid hemichordates and numerous chordate genes involved in brain and spinal cord development are expressed in a similar anterior–posterior spatial arrangement along the body axis. Results: We have examined anterior regeneration in the hemichordate Ptychodera flava and report the spatial and temporal morphological changes that occur. Additionally, we have sequenced, assembled, and analyzed the transcriptome for eight stages of regenerating P. flava, revealing significant differential gene expression between regenerating and control animals. Conclusions: Importantly, we have uncovered developmental steps that are regeneration‐specific and do not strictly follow the embryonic program. Developmental Dynamics 245:1159–1175, 2016. © 2016 The Authors. Developmental Dynamics published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists, Key findings We describe internal and external regeneration morphology from day 1 through day 15 of anterior regeneration.We detail the transcriptome for eight stages of anterior regeneration describing hundreds of putative genes.Anterior regeneration does not strictly follow the embryonic program.There is temporal plasticity during regeneration, as well as different modes of acquiring the same anterior structures.

Severe injury to the central nervous system of chordates often results in permanent and irreversible mental and physical challenges. While some chordates are able to repair and/or regenerate portions of their nervous system, no chordate has been shown to be able to regenerate all regions of its central nervous system after catastrophic injury or amputation. Some hemichordates, on the other hand, are able to efficiently regenerate all neural structures, including their dorsal, hollow neural tube after complete ablation. Solitary hemichordates are marine acorn worms and a sister group to the echinoderms. The hemichordate Ptychodera flava progresses from a pelagic, feeding tornaria larva to a tripartite benthic worm with an anterior proboscis, a middle collar region, and a long posterior trunk. The adult worm regenerates all body parts when bisected in the trunk, but it was unknown whether the regeneration process was present in tornaria larvae. Now, we show that P. flava larvae are capable of robust regeneration after bisection through the sagittal, coronal, and axial planes. We also use antibody staining to show that the apical sensory organ regenerates a rich, serotonin-positive complex of cells within two weeks after amputation. Cells labeled with 5-ethynyl-2'-deoxyuridine confirm that regeneration is occurring through epimorphic processes as new cells are added at the cut site and throughout the regenerating tissue. This study verifies that P. flava larvae can be used for future functional studies aimed at identifying the genetic and morphological mechanisms controlling central nervous system regeneration in a stem deuterostome.

Hemichordates are the sister group to echinoderms, not chordates. Phylogenomics suggest that the tunicates are the sister group to vertebrates, while lancelets are more distantly related to vertebrates. Hox genes are expressed in an anterior to posterior manner in hemichordates and chordates. Tunicates have lost the middle Hox genes and show rather different tissue expression patterns. Echinoderms have a rearranged Hox cluster and show limited co-linearity of expression in the somatapleura. Pharyngeal gill slits in hemichordates and chordates are homologous. Pharyngeal gill bars develop similarly in hemichordates and lancelets, but differ from vertebrates in that they are acellular and endodermal in origin. Post-anal tails and endostyles in hemichordates and chordates are likely to be homologous. Chordates specify neural and non-neural ectoderm, while all ectoderm is neural in hemichordates. Hemichordates make a dorsal neural tube in their neck region. Recent gene expression data suggest the stomochord may be the notochord homolog in hemichordates. Tunicates contain migratory cells that develop into pigment cells, so may contain neural crest cells. Tunicates also have sensory placodes, and form a secondary anus after metamorphosis, so the excurrent siphon develops from the otic placode.

Enteropneust hemichordates share several characteristics with chordates, such as a Hox-specified anterior-posterior axis, pharyngeal gill slits, a dorsal central nervous system (CNS), and a juvenile postanal tail. Ptychoderid hemichordates, such as the indirect-developer Ptychodera flava, have feeding larvae and a remarkable capacity to regenerate their CNS. We compared neurulation of ptychoderid hemichordates and chordates using histological analyses, and found many similarities in CNS development. In ptychoderid hemichordates, which lack a notochord, the proboscis skeleton develops from endoderm after neurulation. The position of the proboscis skeleton directly under the nerve cord suggests that it serves a structural role similar to the notochord of chordates. These results suggest that either the CNS preceded evolution of the notochord or that the notochord has been lost in hemichordates. The evolution of the notochord remains ambiguous, but it may have evolved from endoderm, not mesoderm.