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Did the Notochord Evolve from an Ancient Axial Muscle?

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Did the Notochord Evolve from an Ancient Axial Muscle? Insights & Perspectives Did the notochord evolve from an Hypotheses ancient axial muscle? The axochord hypothesis Thibaut Brunet, Antonella Lauri and Detlev Arendt* The origin of the notochord is one of the key remaining mysteries of our Regarding notochord evolution, we can evolutionary ancestry. Here, we present a multi-level comparison of the thus ask: what structure in the last chordate notochord to the axochord, a paired axial muscle spanning the ventral common bilaterian ancestor gave rise to the chordate notochord? This structure midline of annelid worms and other invertebrates. At the cellular level, necessarily existed, but its nature and comparative molecular profiling in the marine annelids P. dumerilii and C. teleta complexity – representing a simple reveals expression of similar, specific gene sets in presumptive axochordal and population of cells, a certain tissue, or notochordal cells. These cells also occupy corresponding positions in a even a distinct organ such as a specific conserved anatomical topology and undergo similar morphogenetic move- muscle – remain to be defined. Which structures in non-chordate lineages has ments. At the organ level, a detailed comparison of bilaterian musculatures it given rise to? The answers to these reveals that most phyla form axochord-like muscles, suggesting that such a questions are currently unclear [2, 3]. muscle was already present in urbilaterian ancestors. Integrating comparative The notochord has variously been pro- evidence at the cell and organ level, we propose that the notochord evolved by posed to be related to the stomochord of modification of a ventromedian muscle followed by the assembly of an axial enteropneusts [4, 5]; to the hydrocele of complex supporting swimming in vertebrate ancestors. echinoderms [6]; to a longitudinal stiff- ening ofthegut [7, 8];and, by one author, to a ventral midline muscle in annelid Keywords: worms [9]. However, none of these axochord; evo-devo; evolution; mesoderm; musculature; notochord; urbilateria . homology proposals has gained wide- Additional supporting information may be found in the online version of this spread acceptance. : article at the publisher’s web-site. How can homology of two structures be experimentally tested? The nature of possible supporting evidence is sum- marized by Remane’s triple homology Introduction and comparison of homologous struc- criteria [10, 11]: (i) specific quality: tures in modern forms,allowing inference similarity in structural detail [12]; (ii) The reconstruction of last common ances- of the most likely ancestral states. Homo- position: they should have the same tors of modern groups is one of the key logy refers to structures in two modern relative position within the body; (iii) challenges in evolutionary biology. It is species that have been inherited from continuity: they should be present in possible by two methods: observation of their last common ancestor. It applies phylogenetically intermediate groups. fossils (which, for early bilaterian evolu- at all levels of biological organization – Later, Hennig, building on these cri- tion, represent a still patchy record [1]), genes, cell types, tissues, and organs. teria [12], emphasized their importance as pre-requisites for homology (as the DOI 10.1002/bies.201500027 cladistics school did after him [13]). He extended the continuity criterion Developmental Biology Unit, European Molecular † Present address: Institute for Biological and by emphasizing the need to test for Biology Laboratory, Heidelberg, Germany Medical Imaging and Institute of Developmental absence or presence of a character along Genetics, Helmholtz Zentrum Munchen,€ Neuher- the branches of a phylogenetic tree to *Corresponding author: berg, Germany. Detlev Arendt infer ancestral states. According to the E-mail: [email protected] cladistic approach, a character is only 836 www.bioessays-journal.com Bioessays 37: 836–850, ß 2015 The Authors. Bioessays published by WILEY Periodicals, Inc. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. .....Insights & Perspectives T. Brunet et al. Hypotheses Figure 1. The bilaterian phylogenetic tree, after [15]. Levels of homology (cells and organs) are parsimoniousinterpretationaccordingto indicated, together with the taxa for which evidence is available as well as the nature of that Hennig’s cladistic approach would be evidence. The three branches known to separate annelids and chordates are colored. The that it existed in bilaterian ancestors. As axochord hypothesis implies conservation of a ventromedian muscle along at least these three we will outline in the second part of this branches, and possibly other branches within Lophotrochozoa once their phylogeny is solved. review, these comparative data make a strong case that such a muscle was considered homologous if its distributed cell type level, hence suggesting that indeed present. Future developmental presence in a clade supports its likely the notochordal cells might have arisen and genetic studies will reveal its cellular existence in the last common ancestor. from ancient contractile cells in the characteristics and will allow further (This method is called “ancestral state ventral midline. For obvious reasons, testing of the continuity criterion. reconstruction,” see below.) such detailed developmental and We recently published a detailed molecular investigations have so far comparison of cell types between only covered few species, and more remote groups: those that assemble into species need to be examined to test for Are the axochord and the a ventromedian muscle in the annelid presence/absence (iii) of these genetic notochord homologous? worms Platynereis dumerilii and Cap- and developmental traits in the bilat- itella teleta and those that form the erian tree. Molecular profiling and notochord in chordates [14]. We found However, continuity can already be developmental data support strong similarities between these cells tested at the tissue/organ level, as a vast cell type-level homology of in terms of gene expression, morpho- repertoire of anatomical data is available axochordal and notochordal genetic movements, and position in the to test for the presence of ventromedian cells in annelids and chordates bodyplan. Following the criteria of muscles in various bilaterians (Fig. 1). If a structural similarity (i) and topology ventromedian muscle were present in the Similar to the notochord, the P. dumer- (ii), our data suggest homology at the majority of bilaterian lineages, the most ilii axochord develops by convergence- Bioessays 37: 836–850, ß 2015 The Authors. Bioessays published by WILEY Periodicals, Inc. 837 T. Brunet et al. Insights & Perspectives..... Hypotheses Figure 2. The axochord in annelids. A: Development of the axochord in Platynereis dumerilii following [14]. Red cells are axochordal cells; green cells are presumptive ventral oblique muscles; blue cells give rise to the rest of the mesoderm; foregut is in grey; dotted circle is the mouth. B: Development of the axochord in Capitella teleta following [14]. C: Ancestral state reconstruction for annelids. The tree follows [22, 23]. Only one group of known phylogenetic position, sipunculids, lacks an axochord (the other family, Sphaerodoridae, has not been included in any phylogenomic analysis). extension of mesodermal cells towards transcription factors (brachyury, foxA, investigated in amphioxus are also the midline [14] (Fig. 2A). These cells foxD, twist, soxD, soxE) and eight general notochord markers (apart from differentiate into a rod of tissue located effector genes (colA1, colA2, chordin, soxE). Since this combined signature is between the central nervous system noggin, netrin, slit and hedgehog) that found nowhere else in the body, its co- and the axial blood vessel, serve as uniquely defines it, and also uniquely option from another expression territory an attachment band for transverse define the vertebrate notochord. is unlikely: its convergent acquisition muscles, and likely secrete a collagen- Together, these 13 genes represent the would require multiple, independent rich extracellular matrix (as suggested most complete and most evolutionarily and identical co-option events. This is by the expression of the genes colA1 and stable molecular profile for notochordal unparsimonious, since the known colA2). These histological, morphoge- cells that can be put forward after an instances of convergent cell type evo- netic and positional properties are unbiased screening of the vertebrate lution have involved the independent reminiscent of those of the chordate literature – thus avoiding arbitrary production of similar cellular pheno- notochord [16]. Moreover, the axochord “cherry-picking” of markers [17]. All types by completely different molecular expresses a specific combination of six the genes of this list that have been components [18, 19]. 838 Bioessays 37: 836–850, ß 2015 The Authors. Bioessays published by WILEY Periodicals, Inc. .....Insights & Perspectives T. Brunet et al. It is worth noting that the molecular histological data is challenging, as converge in late development and form profile of the axochord includes a they are often of insufficient resolution a proper axochord before hatching [14] notochord-like combination of signal- to observe axochord-like structures, (Fig. 2B). Axochord development thus Hypotheses ing molecules (noggin, hedgehog,
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