What is a segment?

Hannibal, Roberta L.; Patel, Nipam H.

doi:10.1186/2041-9139-4-35

Cited by 51 publications

(42 citation statements)

References 64 publications

(108 reference statements)

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“…However, monophyly has not been established for this group [10,11,16,17,19], and therefore the assignment of particular synapomorphic characters within cycloneuralian organ systems, such as digestive, muscular, and nervous tissues, must also remain tentative [15,20,21]. Yet, the kinorhynchs may be the most derived taxon within Scalidophora, or even among the cycloneuralians, since they are the only animals with clearly segmented internal and external structures, including the cuticle, paired and unpaired spines, gland cells, sensory spots, muscles and neurons that collectively represent developmental products of mesodermal and ectodermal tissues along the anterior-posterior axis [14,22,23]. Most importantly, because of their relative position within Ecdysozoa, as a separate and distinct lineage from Panarthropoda, the segmented body plan of kinorhynchs may represent a fascinating example of convergent evolution, and an independent contrast of both the development and function of segmentation not only within Ecdysozoa, but also among other metazoans where segmentation is recognized.…”

Section: Introductionmentioning

confidence: 99%

Comparative myoanatomy of Echinoderes (Kinorhyncha): a comprehensive investigation by CLSM and 3D reconstruction

et al. 2014

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IntroductionKinorhyncha is a clade of marine invertebrate meiofauna. Their body plan includes a retractable introvert bearing rings of cuticular spines, and a limbless trunk with distinct segmentation of nervous, muscular and epidermal organ systems. As derived members within the basal branch of Ecdysozoa, kinorhynchs may provide an important example of convergence on the evolution of segmentation within one of three bilaterian superclades. We describe the myoanatomy of Echinoderes, the most specious kinorhynch genus, and build upon historical studies of kinorhynch ultrastructure and gross morphology. This is the first multi-species comparison of a complete organ system by confocal microscopy and three-dimensional reconstruction within Kinorhyncha.ResultsMyoanatomy of adult Echinoderes is composed of the following: Head with two mouth cone circular muscles, nine pairs of oral style muscles, ten introvert retractors, one introvert circular muscle, and fourteen introvert circular muscle retractors; Neck with one circular muscle; Trunk showing distinct pairs of ventral and dorsal muscles within segments 1–10, dorsoventral muscles within segments 3–10, diagonal muscles within segments 1–8, longitudinal fibers spanning segments 1–9, three pairs of terminal spine muscles, and one pair of male penile spine muscles; Gut showing a pharynx with ten alternating rings of radial and circular muscle fibers enclosed in a complex sheath of protractors and retractors, an orthogonal grid of longitudinal and circular fibers surrounding the intestine, and paired hindgut dilators.ConclusionsMyoanatomy is highly conserved between species of Echinoderes. Interspecific variation is observed in the arrangement and number of introvert fibers and the composition of pharyngeal muscles. Segmented trunk musculature facilitates the movements of articulated cuticular plates along the anterior-posterior axis. Intersegmental muscle fibers assist with dorsoventral and lateral trunk movements. Protractors, retractors and circular muscles coordinate eversion and retraction of the introvert and mouth cone, and relocation of the pharynx during locomotion and feeding behaviors. Pairs of posterior fibers suggest independent movements of terminal spines, and male penile spines. Within Scalidophora, myoanatomy is more similar between Kinorhyncha and Loricifera, than either group is to Priapulida. Kinorhynch myoanatomy may reflect a convergent transition from vermiform to segmented body plans during the early radiation of Ecdysozoa.

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Section: Introductionmentioning

confidence: 99%

Comparative myoanatomy of Echinoderes (Kinorhyncha): a comprehensive investigation by CLSM and 3D reconstruction

et al. 2014

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“…With an ancient fossil record, unique body plan, and new position within Annelida, sipunculans will likely inspire many important studies on the origin, evolution and/or loss of segmentation in one of only three segmented lineages within Metazoa (Seaver, 2003;Huang et al, 2004;Struck et al, 2011;Hannibal and Patel, 2013). Do the non-segmented sipunculans possess the basic repertoire of 'segmentation' genes found in other annelids, arthropods and vertebrates?…”

Section: Future Directionsmentioning

confidence: 99%

Sipuncula: an emerging model of spiralian development and evolution

Boyle¹,

Rice²

2014

Int. J. Dev. Biol.

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Sipuncula is an ancient clade of unsegmented marine worms that develop through a conserved pattern of unequal quartet spiral cleavage. They exhibit putative character modifications, including conspicuously large first-quartet micromeres and prototroch cells, postoral metatroch with exclusive locomotory function, paired retractor muscles and terminal organ system, and a U-shaped digestive architecture with left-right asymmetric development. Four developmental life history patterns are recognized, and they have evolved a unique metazoan larval type, the pelagosphera. When compared with other quartet spiral-cleaving models, sipunculan development is understudied, challenging and typically absent from evolutionary interpretations of spiralian larval and adult body plan diversity. If spiral cleavage is appropriately viewed as a flexible character complex, then understudied clades and characters should be investigated. We are pursuing sipunculan models for modern molecular, genetic and cellular research on evolution of spiralian development. Protocols for whole mount gene expression studies are established in four species. Molecular labeling and confocal imaging techniques are operative from embryogenesis through larval development. Nextgeneration sequencing of developmental transcriptomes has been completed for two species with highly contrasting life history patterns, Phascolion cryptum (direct development) and Nephasoma pellucidum (indirect planktotrophy). Looking forward, we will attempt intracellular lineage tracing and fate-mapping studies in a proposed model sipunculan, Themiste lageniformis. Importantly, with the unsegmented Sipuncula now repositioned within the segmented Annelida, sipunculan worms have become timely and appropriate models for investigating the potential for flexibility in spiralian development, including segmentation. We briefly review previous studies, and discuss new observations on the spiralian character complex within Sipuncula. KEY WORDS: pelagosphera, metatroch, unequal, U-shaped, ectomesoderm Ancient worms with a new statusSipuncula is a clade of unsegmented, coelomate marine worms. They have colonized benthic habitats worldwide from intertidal zones to abyssal plains in polar, temperate and tropical environments, and they have evolved a larval form that is unique among all metazoans, the pelagosphera (Mueller, 1850;Hatschek, 1883;Gerould, 1906;Rice, 1967). The adult body plan consists of a relatively large posterior trunk region with a single undivided coelomic cavity, and a retractable introvert with an array of tentacles on the anterior end that typically surrounds the mouth. The mouth leads to a U-shaped digestive system that terminates with an anus on the dorsal-anterior side of the trunk. Adult sipunculans have a centralized nervous system with a single, median, unsegmented Int. J. Dev. Biol. 58: 485-499 (2014) doi: 10.1387/ijdb.140095mb ventral nerve cord. There is comprehensive information on their reproductive biology (Rice, 1975b;Rice, 1989), anatomy (Rice, 1973(R...

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“…Irrespective of the taxa included, the state of almost every character considered varies greatly among annelids making ground pattern reconstruction a difficult task. Although there is general agreement that Annelida are organisms with a multiple repetition of identically organised segments Struck, 2011;Hannibal and Patel, 2013), there are certain taxa in which even this so-called key-character is virtually absent: e.g., Echiura, Sipuncula, Diurodrilus Wanninger et al, 2005;Worsaae and Rouse, 2008;Nielsen, 2012;Golombek et al, 2013). The number of segments varies between species and may comprise between only 6 or fewer (e. g. Parapodrilus psammophilus Westheide, 1965) to more than 1,000 segments (e. g. Eunice aphroditois (Pallas, 1788)) resulting in body lengths varying from less than 600 µm to about 6 m (see Paxton, 2000).…”

Section: Introductionmentioning

confidence: 99%

Systematics, evolution and phylogeny of Annelida – a morphological perspective

Purschke¹,

Bleidorn²,

Struck³

2014

Mem. Mus. Vic.

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Purschke, G., Bleidorn, C. and Struck, T. 2014. Systematics, evolution and phylogeny of Annelida -a morphological perspective . Memoirs of Museum Victoria 71: 247-269.Annelida, traditionally divided into Polychaeta and Clitellata, is an evolutionary ancient and ecologically important group today usually considered to be monophyletic. However, there is a long debate regarding the in-group relationships as well as the direction of evolutionary changes within the group. This debate is correlated to the extraordinary evolutionary diversity of this group. Although annelids may generally be characterised as organisms with multiple repetitions of identically organised segments and usually bearing certain other characters such as a collagenous cuticle, chitinous chaetae or nuchal organs, none of these are present in every subgroup. This is even true for the annelid key character, segmentation. The first morphology-based cladistic analyses of polychaetes showed Polychaeta and Clitellata as sister groups. The former were divided into Scolecida and Palpata comprising Aciculata and Canalipalpata. This systematisation definitely replaced the old concept of dividing polychaetes into Errantia and Sedentaria, whereas the group Archiannelida had already been abandoned. The main critics came from a contradicting hypothesis relying on scenario based on plausibility considerations regarding Clitellata as highly derived annelids nesting within polychaetes and rendering the latter paraphyletic. In this hypothesis the absences of typical polychaete characters were regarded as losses rather than as primary absences. However, to date attempts to unambiguously identify the sister group of Clitellata on the basis of morphological characters have failed. Thus, two hypotheses on the last common annelid ancestor have been put forward either being an oligochaete-like burrowing animal or a parapodia-bearing epibenthic worm. These attempts to understand the major transitions in annelid evolution are reviewed and discussed in the light of new morphological evidence such as photoreceptor cell and eye evolution as well as the evolution of the nervous system and musculature. We also discuss the plausibility of these scenarios with regard to recent advances in molecular phylogenetic analyses.

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What is a segment?

Cited by 51 publications

References 64 publications

Comparative myoanatomy of Echinoderes (Kinorhyncha): a comprehensive investigation by CLSM and 3D reconstruction

Comparative myoanatomy of Echinoderes (Kinorhyncha): a comprehensive investigation by CLSM and 3D reconstruction

Sipuncula: an emerging model of spiralian development and evolution

Systematics, evolution and phylogeny of Annelida – a morphological perspective

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