Chordate origins and evolution

Swalla, Billie J.; Xavier‐Neto, José

doi:10.1002/dvg.20477

Cited by 12 publications

(7 citation statements)

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“…Of course “Phylogenetic speculation is expected of every study of Enteropneusta!” (Bullock, 1944) and speculations on the evolution of the chordate nervous system abound (Garstang, 1928; Nübler‐Jung and Arendt, 1996; Nielsen, 1999; Holland, 2003; Lacalli, 2006, 2008, 2010). Most of the recent publications discuss modern molecular ontogenetic data in the light of the latest phylogenetic tree based on molecular data (Holland, 2003; Brown et al, 2008; Swalla and Xavier‐Neto, 2008; Holland, 2009; Nomaksteinsky et al, 2009). In our view, this practice ignores the wealth of information that is present in comparative data on any other level, although some authors have shown that, e.g., phylogenetic information content on morphological levels is generally higher than in sequence data (Giribet, 2010).…”

Section: Discussionmentioning

confidence: 99%

Ontogeny of the collar cord: Neurulation in the hemichordateSaccoglossus kowalevskii

Kaul

Stach

2010

Journal of Morphology

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The chordate body plan is characterized by a central notochord, a pharynx perforated by gill pores, and a dorsal central nervous system. Despite progress in recent years, the evolutionary origin of each of theses characters remains controversial. In the case of the nervous system, two contradictory hypotheses exist. In the first, the chordate nervous system is derived directly from a diffuse nerve net; whereas, the second proposes that a centralized nervous system is found in hemichordates and, therefore, predates chordate evolution. Here, we document the ontogeny of the collar cord of the enteropneust Saccoglossus kowalevskii using transmission electron microscopy and 3D-reconstruction based on completely serially sectioned stages. We demonstrate that the collar cord develops from a middorsal neural plate that is closed in a posterior to anterior direction. Transversely oriented ependymal cells possessing myofilaments mediate this morphogenetic process and surround the remnants of the neural canal in juveniles. A mid-dorsal glandular complex is present in the collar. The collar cord in juveniles is clearly separated into a dorsal saddle-like region of somata and a ventral neuropil. We characterize two cell types in the somata region, giant neurons and ependymal cells. Giant neurons connect via a peculiar cell junction that seems to function in intercellular communication. Synaptic junctions containing different vesicle types are present in the neuropil. These findings support the hypotheses that the collar cord constitutes a centralized element of the nervous system and that the morphogenetic process in the ontogeny of the collar cord is homologous to neurulation in chordates. Moreover, we suggest that these similarities are indicative of a close phylogenetic relationship between enteropneusts and chordates.

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Section: Discussionmentioning

confidence: 99%

Ontogeny of the collar cord: Neurulation in the hemichordateSaccoglossus kowalevskii

Kaul

Stach

2010

Journal of Morphology

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“…It appears less likely that a metameric organization around a well-developed CNS of ancient metazoans devolved to form the partially centralized epithelial nerve nets found in diploblasts (Matus, et al, 2007b, Satterlie, 2011) and the simple CNS found in some basal bilaterians (Brown, et al, 2008, Fritzsch and Glover, 2006, Harzsch and Muller, 2007, Raikova, et al, 2000). A non-metamerically organized nervous system with no specialized sensory organs seems to be the most parsimonious assumption for diploblasts and, by logical extension, bilaterian ancestors (Bourlat, et al, 2006, Budd, 2001, Satoh, 2008, Swalla and Xavier-Neto, 2008). The different patterns of nervous system of extant deuterostomes are considered here to be independently derived from such bilaterian ancestors, reflecting either independent formation of a CNS (acorn worms, cephalochordates, urochordates, vertebrates) or show a transformation into a pentameric nerve net (echinoderms).…”

Section: Introductionmentioning

confidence: 99%

Evolving gene regulatory networks into cellular networks guiding adaptive behavior: an outline how single cells could have evolved into a centralized neurosensory system

et al. 2014

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Understanding the evolution of the neurosensory system of man, able to reflect on its own origin, is one of the major goals of comparative neurobiology. Details of the origin of neurosensory cells, their aggregation into central nervous systems and associated sensory organs, their localized patterning into remarkably different cell types aggregated into variably sized parts of the central nervous system begin to emerge. Insights at the cellular and molecular level begin to shed some light on the evolution of neurosensory cells, partially covered in this review. Molecular evidence suggests that high mobility group (HMG) proteins of pre-metazoans evolved into the definitive Sox [SRY (sex determining region Y)-box] genes used for neurosensory precursor specification in metazoans. Likewise, pre-metazoan basic helix-loop-helix (bHLH) genes evolved in metazoans into the group A bHLH genes dedicated to neurosensory differentiation in bilaterians. Available evidence suggests that the Sox and bHLH genes evolved a cross-regulatory network able to synchronize expansion of precursor populations and their subsequent differentiation into novel parts of the brain or sensory organs. Molecular evidence suggests metazoans evolved patterning gene networks early and not dedicated to neuronal development. Only later in evolution were these patterning gene networks tied into the increasing complexity of diffusible factors, many of which were already present in pre-metazoans, to drive local patterning events. It appears that the evolving molecular basis of neurosensory cell development may have led, in interaction with differentially expressed patterning genes, to local network modifications guiding unique specializations of neurosensory cells into sensory organs and various areas of the central nervous system.

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“…Understanding chordate origins and evolution has been a preeminent challenge for biologists over the last two centuries (Swalla and Xavier-Neto 2008). The phylum Chordata consists of three distinct lineages: cephalochordates (lancelets), tunicates or urochordates (sea squirts, salps and appendicularians) and craniates (i.e., cyclostomes and vertebrates).…”

Section: Introductionmentioning

confidence: 99%

Accelerated Evolutionary Rate of Housekeeping Genes in Tunicates

et al. 2010

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Phylogenomics has recently revealed that tunicates represent the sister-group of vertebrates in the newly defined clade Olfactores. However, phylogenomic and comparative genomic studies have also suggested that tunicates are characterized by an elevated rate of molecular evolution and a high degree of genomic divergence. Despite the recurrent interest in the group, the picture of tunicate peculiar evolutionary dynamics is still fragmentary, as it mainly lies in studies focusing on only a few model species. In order to expand the available genomic data for the group, we used the high-throughput 454 technology to sequence the partial transcriptome of a previously unsampled tunicate, Microcosmus squamiger. This allowed us to get further insights into tunicate-accelerated evolution through a comparative analysis based on pertinent phylogenetic markers, i.e., a core of 35 housekeeping genes conserved across bilaterians. Our results showed that tunicates evolved on average about two times faster than the other chordates, yet the degree of this acceleration varied extensively upon genes and upon lineages. Appendicularia and Aplousobranchia were detected as the most divergent groups which were also characterized by highly heterogeneous substitution rates across genes. Finally, an estimation of the d (N)/d (S) ratio in three pairs of closely related taxa within Olfactores did not reveal strong differences between the tunicate and vertebrate lineages suggesting that for this set of housekeeping genes, the accelerated evolution of tunicates is plausibly due to an elevated mutation rate rather than to particular selective effects.

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Chordate origins and evolution

Abstract: No abastract.

Cited by 12 publications

References 50 publications

Ontogeny of the collar cord: Neurulation in the hemichordateSaccoglossus kowalevskii

Ontogeny of the collar cord: Neurulation in the hemichordateSaccoglossus kowalevskii

Evolving gene regulatory networks into cellular networks guiding adaptive behavior: an outline how single cells could have evolved into a centralized neurosensory system

Accelerated Evolutionary Rate of Housekeeping Genes in Tunicates

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