Did the notochord evolve from an ancient axial muscle? The axochord hypothesis

Brunet, Thibaut; Lauri, Antonella; Arendt, Detlev

doi:10.1002/bies.201500027

Cited by 30 publications

(40 citation statements)

References 122 publications

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“…Both axochord and notochord are located between the axial blood vessel and the central nervous system. In this context, the ventromedial muscle between the ventral blood vessel and the ventral nerve cord would be the best structural candidate for a notochord homolog in enteropneust hemichordates [39 ]. It would be intriguing to examine this hypothesis using histological and molecular developmental studies of ventromedial muscles of various hemichordates.…”

Section: The Stomochord and The Pygochordmentioning

confidence: 97%

Hemichordate models

Tagawa

2016

Current Opinion in Genetics & Development

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Hemichordates are marine animals with two different lifestyles. The solitary, free-living enteropneusts or acorn worms resemble polychaetes or earthworms, while the tiny, colonial, sessile pterobranchs are similar to bryozoans and phoronids. Hemichordates, together with echinoderms, comprise the clade Ambulacraria and are a sister group to the Chordata. As adults, they exhibit cardinal chordate characters, such as gill slits. Their embryogenesis and dipleurula-type (tornaria) larvae are very similar to those of echinoderms. Recent advances in comparative genomics and molecular developmental biology of hemichordates, especially the vermiform enteropneusts, have shed light on deuterostome ancestors. This paper briefly reviews the numerous recent studies on the Phylum Hemichordata.

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Section: The Stomochord and The Pygochordmentioning

confidence: 97%

Hemichordate models

Tagawa

2016

Current Opinion in Genetics & Development

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“…Therefore, the pygochord was sometimes completely ignored and regarded just as an extension of ventral mesentery or gut epithelium (Benito & Pardos, 1997). Second, the position of the pygochord relative to the blood vessel along DV axis is not consistent with the DV inversion hypothesis (Brunet, Lauri, & Arendt, 2015). In the ventral part of the acorn worm, the gut, pygochord, blood vessel, and nerve cord are located in a dorsal to ventral direction, while in chordates the gut, blood vessel, notochord, and nerve cord are located in a ventral to dorsal direction.…”

Section: Introductionmentioning

confidence: 83%

Regeneration of the acorn worm pygochord with the implication for its convergent evolution with the notochord

2018

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The origin of the notochord is a central issue in chordate evolution. This study examined the development of the acorn worm pygochord, a putative homologue of the notochord. Because the pygochord differentiates only after metamorphosis, the developmental was followed process by inducing regeneration after artificial amputation in Ptychodera flava. It was found that although the regeneration of the posterior part of the body did not proceed via formation of an obvious regeneration bud, pygochord regeneration was observed within a few weeks, possibly via trans‐differentiation of endoderm cells. The expression of the fibrillary collagen gene (Fcol) and elav in the pygochord during regeneration was detected. This indicates that pygochord cells are not part of gut epithelial cells, but that they differentiated as a distinct cell type. Our gene expression analyses do not provide supporting evidence for the homology between the pygochord and notochord, but rather favored the convergent evolution between them.

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“…Recently, a hypothesis emerged that suggested homology of a ventromedian longitudinal muscle occurring in aculiferan mollusks, annelids, and several other protostomes with the notochord of chordates (Lauri et al., 2014; Brunet et al., 2015). This reasoning was based, among others, on the shared expression of a specific set of genes, including twist , in the cells forming this muscular “axochord” in annelids as well as in the developing notochord of chordates.…”

Section: Discussionmentioning

confidence: 99%

Cell Proliferation Pattern and Twist Expression in an Aplacophoran Mollusk Argue Against Segmented Ancestry of Mollusca

et al. 2016

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The study of aplacophoran mollusks (i.e., Solenogastres or Neomeniomorpha and Caudofoveata or Chaetodermomorpha) has traditionally been regarded as crucial for reconstructing the morphology of the last common ancestor of the Mollusca. Since their proposed close relatives, the Polyplacophora, show a distinct seriality in certain organ systems, the aplacophorans are also in the focus of attention with regard to the question of a potential segmented ancestry of mollusks. To contribute to this question, we investigated cell proliferation patterns and the expression of the twist ortholog during larval development in solenogasters. In advanced to late larvae, during the outgrowth of the trunk, a pair of longitudinal bands of proliferating cells is found subepithelially in a lateral to ventrolateral position. These bands elongate during subsequent development as the trunk grows longer. Likewise, expression of twist occurs in two laterally positioned, subepithelial longitudinal stripes in advanced larvae. Both, the pattern of proliferating cells and the expression domain of twist demonstrate the existence of extensive and long‐lived mesodermal bands in a worm‐shaped aculiferan, a situation which is similar to annelids but in stark contrast to conchiferans, where the mesodermal bands are usually rudimentary and ephemeral. Yet, in contrast to annelids, neither the bands of proliferating cells nor the twist expression domain show a separation into distinct serial subunits, which clearly argues against a segmented ancestry of mollusks. Furthermore, the lack of twist expression during the development of the ventromedian muscle argues against homology of a ventromedian longitudinal muscle in protostomes with the notochord of chordates.

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Did the notochord evolve from an ancient axial muscle? The axochord hypothesis

Cited by 30 publications

References 122 publications

Hemichordate models

Hemichordate models

Regeneration of the acorn worm pygochord with the implication for its convergent evolution with the notochord

Cell Proliferation Pattern and Twist Expression in an Aplacophoran Mollusk Argue Against Segmented Ancestry of Mollusca

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