Sessile lifestyle and origin of chordates

Williams, Jennifer

doi:10.1080/03014223.1996.9518072

Cited by 9 publications

(8 citation statements)

References 52 publications

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“…Recent phylogenetic analyses led to the conclusion that tunicates, and not the cephalochordates (among which the lancelet, Amphioxus) as previously believed, are the sister group of vertebrates (Delsuc et al 2006). These novel findings were clearly in opposition to evolutionary scenarios, which postulated a sessile, ascidean-like ancestor to the chordates (e.g., Williams 1996).…”

Section: Box 1 Tunicates the Sister Group Of Vertebratescontrasting

confidence: 57%

Thaliaceans, The Neglected Pelagic Relatives of Ascidians: A Developmental and Evolutionary Enigma

Piette¹,

Lemaire²

2015

The Quarterly Review of Biology

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Most developmental biologists equate tunicates to the sessile ascidians, including Ciona intestinalis, and the pelagic appendicularians, in particular Oikopleura dioica. However, there exists a third group of tunicates with a pelagic lifestyle, the thaliaceans, which include salps, pyrosomes, and doliolids. Although thaliaceans have raised the curiosity offamous zoologists since the 18th century, the difficulty of observing and experimentally manipulating them has led to many controversies and speculations about their life cycles and developmental strategies, the phylogenetic relationship within the group and with other tunicates, and the drivers of speciation in these widely distributed animals living in a seemingly uniform environment. Here, we take a historical perspective to summarize 250 years of work on this intriguing group of animals, and explore how modern genomics and imaging approaches are starting to solve fascinating evolutionary and developmental riddles. Recent molecular analyses support previous morphological evidence that ascidians are not monophyletic and that thaliaceans evolved from a sessile ascidian-like ancestor. In parallel, preliminary live-imaging and gene-expression data offer exciting entry points to understand how the adoption of a pelagic lifestyle led to drastic modifications in the morphology, embryology, and life cycle of these tunicates, compared to their sessile ancestor.

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Section: Box 1 Tunicates the Sister Group Of Vertebratescontrasting

confidence: 57%

Thaliaceans, The Neglected Pelagic Relatives of Ascidians: A Developmental and Evolutionary Enigma

Piette¹,

Lemaire²

2015

The Quarterly Review of Biology

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“…In fact, appendicularians had been textbook examples for neoteny, based on the rapid development and it was precisely this bias that led to the christening of the taxon with its alternative name – Larvacea (Herdman, ; Garstang, ; Lacalli, ; Stach et al., ; see reviews in: Gee, ; Ruppert, 1997a). The fact that appendicularians resembled ascidian larvae and the observation that their intestine followed a U‐shaped course seemed to support the neoteny hypothesis, because U‐shaped guts in planktonic organisms were interpreted as characters inherited from a sessile ancestor (Williams, ). These characteristics were more easily reconciled with the hypothesis based on a combination of molecular and morphological data that Appendicularia and Aplousobranchiata form sister groups (Stach and Turbeville, ).…”

Section: Discussionmentioning

confidence: 96%

Phylogenetic analysis of phenotypic characters of Tunicata supports basal Appendicularia and monophyletic Ascidiacea

2019

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With approximately 3000 marine species, Tunicata represents the most disparate subtaxon of Chordata. Molecular phylogenetic studies support Tunicata as sister taxon to Craniota, rendering it pivotal to understanding craniate evolution. Although successively more molecular data have become available to resolve internal tunicate phylogenetic relationships, phenotypic data have not been utilized consistently. Herein these shortcomings are addressed by cladistically analyzing 117 phenotypic characters for 49 tunicate species comprising all higher tunicate taxa, and five craniate and cephalochordate outgroup species. In addition, a combined analysis of the phenotypic characters with 18S rDNA-sequence data is performed in 32 OTUs. The analysis of the combined data is congruent with published molecular analyses. Successively up-weighting phenotypic characters indicates that phenotypic data contribute disproportionally more to the resulting phylogenetic hypothesis. The strict consensus tree from the analysis of the phenotypic characters as well as the single most parsimonious tree found in the analysis of the combined dataset recover monophyletic Appendicularia as sister taxon to the remaining tunicate taxa. Thus, both datasets support the hypothesis that the last common ancestor of Tunicata was free-living and that ascidian sessility is a derived trait within Tunicata. "Thaliacea" is found to be paraphyletic with Pyrosomatida as sister taxon to monophyletic Ascidiacea and the relationship between Doliolida and Salpida is unresolved in the analysis of morphological characters; however, the analysis of the combined data reconstructs Thaliacea as monophyletic nested within paraphyletic "Ascidiacea". Therefore, both datasets differ in the interpretation of the evolution of the complex holoplanktonic life history of thaliacean taxa. According to the phenotypic data, this evolution occurred in the plankton, whereas from the combined dataset a secondary transition into the plankton from a sessile ascidian is inferred. Besides these major differences, both analyses are in accord on many phylogenetic groupings, although both phylogenetic reconstructions invoke a high degree of homoplasy. In conclusion, this study represents the first serious attempt to utilize the potential phylogenetic information present in phenotypic characters to elucidate the inter-relationships of this diverse marine taxon in a consistent cladistic framework.

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“…In contrast, the sessile pterobranchs have anterior tentacles and a concentration of neurons at the base of the tentacles that has been termed a brain, as well as several concentrations of neurites and associated neurons extending into the tentacles, the stalk and between the gill slits [ 99 ]. Although Romer and others argued that pterobranchs were basal hemichordates [ 100 , 101 ], recent molecular phylogenetic analyses do not distinguish which family is basal [ 102 , 103 ], leaving open the possibility that pterobranchs are derived. Indeed, fossil tube-dwelling enteropneusts from the Cambrian were recently discovered [ 104 ].…”

Section: Reviewmentioning

confidence: 99%

Evolution of bilaterian central nervous systems: a single origin?

Holland

Carvalho

Escrivà

et al. 2013

EvoDevo

155

214

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The question of whether the ancestral bilaterian had a central nervous system (CNS) or a diffuse ectodermal nervous system has been hotly debated. Considerable evidence supports the theory that a CNS evolved just once. However, an alternative view proposes that the chordate CNS evolved from the ectodermal nerve net of a hemichordate-like ancestral deuterostome, implying independent evolution of the CNS in chordates and protostomes. To specify morphological divisions along the anterior/posterior axis, this ancestor used gene networks homologous to those patterning three organizing centers in the vertebrate brain: the anterior neural ridge, the zona limitans intrathalamica and the isthmic organizer, and subsequent evolution of the vertebrate brain involved elaboration of these ancestral signaling centers; however, all or part of these signaling centers were lost from the CNS of invertebrate chordates. The present review analyzes the evidence for and against these theories. The bulk of the evidence indicates that a CNS evolved just once – in the ancestral bilaterian. Importantly, in both protostomes and deuterostomes, the CNS represents a portion of a generally neurogenic ectoderm that is internalized and receives and integrates inputs from sensory cells in the remainder of the ectoderm. The expression patterns of genes involved in medio/lateral (dorso/ventral) patterning of the CNS are similar in protostomes and chordates; however, these genes are not similarly expressed in the ectoderm outside the CNS. Thus, their expression is a better criterion for CNS homologs than the expression of anterior/posterior patterning genes, many of which (for example, Hox genes) are similarly expressed both in the CNS and in the remainder of the ectoderm in many bilaterians. The evidence leaves hemichordates in an ambiguous position – either CNS centralization was lost to some extent at the base of the hemichordates, or even earlier, at the base of the hemichordates + echinoderms, or one of the two hemichordate nerve cords is homologous to the CNS of protostomes and chordates. In any event, the presence of part of the genetic machinery for the anterior neural ridge, the zona limitans intrathalamica and the isthmic organizer in invertebrate chordates together with similar morphology indicates that these organizers were present, at least in part, at the base of the chordates and were probably elaborated upon in the vertebrate lineage.

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Sessile lifestyle and origin of chordates

Cited by 9 publications

References 52 publications

Thaliaceans, The Neglected Pelagic Relatives of Ascidians: A Developmental and Evolutionary Enigma

Thaliaceans, The Neglected Pelagic Relatives of Ascidians: A Developmental and Evolutionary Enigma

Phylogenetic analysis of phenotypic characters of Tunicata supports basal Appendicularia and monophyletic Ascidiacea

Evolution of bilaterian central nervous systems: a single origin?

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