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Academic literature on the topic 'Chordates'
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Journal articles on the topic "Chordates"
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Satoh, Noriyuki, Daniel Rokhsar, and Teruaki Nishikawa. "Chordate evolution and the three-phylum system." Proceedings of the Royal Society B: Biological Sciences 281, no. 1794 (2014): 20141729. http://dx.doi.org/10.1098/rspb.2014.1729.
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Traditional metazoan phylogeny classifies the Vertebrata as a subphylum of the phylum Chordata, together with two other subphyla, the Urochordata (Tunicata) and the Cephalochordata. The Chordata, together with the phyla Echinodermata and Hemichordata, comprise a major group, the Deuterostomia. Chordates invariably possess a notochord and a dorsal neural tube. Although the origin and evolution of chordates has been studied for more than a century, few authors have intimately discussed taxonomic ranking of the three chordate groups themselves. Accumulating evidence shows that echinoderms and hemichordates form a clade (the Ambulacraria), and that within the Chordata, cephalochordates diverged first, with tunicates and vertebrates forming a sister group. Chordates share tadpole-type larvae containing a notochord and hollow nerve cord, whereas ambulacrarians have dipleurula-type larvae containing a hydrocoel. We propose that an evolutionary occurrence of tadpole-type larvae is fundamental to understanding mechanisms of chordate origin. Protostomes have now been reclassified into two major taxa, the Ecdysozoa and Lophotrochozoa, whose developmental pathways are characterized by ecdysis and trochophore larvae, respectively. Consistent with this classification, the profound dipleurula versus tadpole larval differences merit a category higher than the phylum. Thus, it is recommended that the Ecdysozoa, Lophotrochozoa, Ambulacraria and Chordata be classified at the superphylum level, with the Chordata further subdivided into three phyla, on the basis of their distinctive characteristics.
2
Zeng, Liyun, and Billie J. Swalla. "Molecular phylogeny of the protochordates: chordate evolution." Canadian Journal of Zoology 83, no. 1 (2005): 24–33. http://dx.doi.org/10.1139/z05-010.
Full textAbstract:
The deuterostomes are a monophyletic group of multicellular animals that include the Chordata, a phylum that exhibits a unique body plan within the metazoans. Deuterostomes classically contained three phyla, Echinodermata, Hemichordata, and Chordata. Protochordata describes two invertebrate chordate subphyla, the Tunicata (Urochordata) and the Cephalochordata. Tunicate species are key to understanding chordate origins, as they have tadpole larvae with a chordate body plan. However, molecular phylogenies show only weak support for the Tunicata as the sister-group to the rest of the chordates, suggesting that they are highly divergent from the Cephalochordata and Vertebrata. We believe that members of the Tunicata exhibit a unique adult body plan and should be considered a separate phylum rather than a subphylum of Chordata. The molecular phylogeny of the deuterostomes is reviewed and discussed in the context of likely morphological evolutionary scenarios and the possibility is raised that the ancestor of the Tunicata was colonial. In this scenario, the colonial tadpole larva would more resemble an ancestral chordate than the solitary tadpole larva. In contrast, the true chordates (vertebrates and cephalochordates) would have evolved from filter-feeding benthic worms with cartilaginous gill slits, similar to extant enteropneust hemichordates.
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Jefferies, R. P. S. "How chordates and echinoderms separated from each other and the problem of dorso-ventral inversion." Paleontological Society Papers 3 (October 1997): 249–66. http://dx.doi.org/10.1017/s1089332600000280.
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It is now possible to reconstruct what happened when the chordates and echinoderms first split from each other. This involves a three-way comparison among: 1) the solute Coleicarpus, which is probably a stem-group dexiothete; 2) the Cincta, which seem to be the least crownward known echinoderms; and 3) the solute Castericystis, which is a stem-group chordate, probably the least crownward known. Counter-torsion of the tail, by which the effects of dexiothetism were nullified in the tail, took place in two phases, firstly in the fore tail and later in the hind tail. Echinoderms and chordates are descended from ancestors that were attached to, or lay on, the sea floor and were therefore much more liable to attack from above than beneath. This probably explains why the main nerve trunk in chordates is dorsal, rather than being ventral as in protostomes.
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Su, Yi-Hsien, Yi-Chih Chen, Hsiu-Chi Ting, et al. "BMP controls dorsoventral and neural patterning in indirect-developing hemichordates providing insight into a possible origin of chordates." Proceedings of the National Academy of Sciences 116, no. 26 (2019): 12925–32. http://dx.doi.org/10.1073/pnas.1901919116.
Full textAbstract:
A defining feature of chordates is the unique presence of a dorsal hollow neural tube that forms by internalization of the ectodermal neural plate specified via inhibition of BMP signaling during gastrulation. While BMP controls dorsoventral (DV) patterning across diverse bilaterians, the BMP-active side is ventral in chordates and dorsal in many other bilaterians. How this phylum-specific DV inversion occurs and whether it is coupled to the emergence of the dorsal neural plate are unknown. Here we explore these questions by investigating an indirect-developing enteropneust from the hemichordate phylum, which together with echinoderms form a sister group of the chordates. We found that in the hemichordate larva, BMP signaling is required for DV patterning and is sufficient to repress neurogenesis. We also found that transient overactivation of BMP signaling during gastrulation concomitantly blocked mouth formation and centralized the nervous system to the ventral ectoderm in both hemichordate and sea urchin larvae. Moreover, this mouthless, neurogenic ventral ectoderm displayed a medial-to-lateral organization similar to that of the chordate neural plate. Thus, indirect-developing deuterostomes use BMP signaling in DV and neural patterning, and an elevated BMP level during gastrulation drives pronounced morphological changes reminiscent of a DV inversion. These findings provide a mechanistic basis to support the hypothesis that an inverse chordate body plan emerged from an indirect-developing ancestor by tinkering with BMP signaling.
5
Ruppert, Edward E. "Key characters uniting hemichordates and chordates: homologies or homoplasies?" Canadian Journal of Zoology 83, no. 1 (2005): 8–23. http://dx.doi.org/10.1139/z04-158.
Full textAbstract:
Four chordate characters — dorsal hollow nerve cord, notochord, gill slits, and endostyle — are compared morphologically, molecularly, and functionally with similar structures in hemichordates to assess their putative homologies. The dorsal hollow nerve cord and enteropneust neurocord are probably homoplasies. The neurocord (= collar cord) may be an autapomorphy of Enteropneusta that innervates a unique pair of muscles, the perihemal coelomic muscles. Despite the apparent lack of organ-level homology, chordates and enteropneusts share a common pattern of neurulation that preserves a "contact innervation" between neuro- and myo-epithelia, which may be the primitive deuterostome pattern of neuromuscular innervation. The chordate notochord and hemichordate stomochord are probably homoplasies. Other potential notochord antecedents in hemichordates are examined, but no clear homolog is identified. The comparative morphology of notochords suggests that the "stack-of-coins" developmental stage, retained into adulthood only by cephalochordates, is the plesiomorphic notochord form. Hemichordate and chordate gill slits are probably homologs, but only at the level of simple ciliated circular or oval pores, lacking a skeleton, as occur in adults of Cephalodiscus spp., developmentally in some enteropneusts, and in many urochordates. Functional morphology, I125-binding experiments, and genetic data suggest that endostylar function may reside in the entire pharyngeal lining of Enteropneusta and is not restricted to a specialized midline structure as in chordates. A cladistic analysis of Deuterostomia, based partly on homologs discussed in this paper, indicates a sister-taxon relationship between Urochordata and Vertebrata, with Cephalochordata as the plesiomorphic clade.
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Holland, Nicholas D. "Chordates." Current Biology 15, no. 22 (2005): R911—R914. http://dx.doi.org/10.1016/j.cub.2005.11.008.
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Sordino, Paolo, Lisa Belluzzi, Rosaria De Santis, and William C. Smith. "Developmental genetics in primitive chordates." Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 356, no. 1414 (2001): 1573–82. http://dx.doi.org/10.1098/rstb.2001.0919.
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Recent advances in the study of the genetics and genomics of urochordates testify to a renewed interest in this chordate subphylum, believed to be the most primitive extant chordate relatives of the vertebrates. In addition to their primitive nature, many features of their reproduction and early development make the urochordates ideal model chordates for developmental genetics. Many urochordates spawn large numbers of transparent and externally developing embryos on a daily basis. Additionally, the embryos have a defined and well–characterized cell lineage until the end of gastrulation. Furthermore, the genomes of the urochordates have been estimated to be only 5–10% of the size of the vertebrates and to have fewer genes and less genetic redundancy than vertebrates. Genetic screens, which are powerful tools for investigating developmental mechanisms, have recently become feasible due to new culturing techniques in ascidians. Because hermaphrodite ascidians are able to self–fertilize, recessive mutations can be detected in a single generation. Several recent studies have demonstrated the feasibility of applying modern genetic techniques to the study of ascidian biology.
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Butts, Thomas, Peter W. H. Holland, and David E. K. Ferrier. "Ancient homeobox gene loss and the evolution of chordate brain and pharynx development: deductions from amphioxus gene expression." Proceedings of the Royal Society B: Biological Sciences 277, no. 1699 (2010): 3381–89. http://dx.doi.org/10.1098/rspb.2010.0647.
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Homeobox genes encode a large superclass of transcription factors with widespread roles in animal development. Within chordates there are over 100 homeobox genes in the invertebrate cephalochordate amphioxus and over 200 in humans. Set against this general trend of increasing gene number in vertebrate evolution, some ancient homeobox genes that were present in the last common ancestor of chordates have been lost from vertebrates. Here, we describe the embryonic expression of four amphioxus descendants of these genes— AmphiNedxa, AmphiNedxb, AmphiMsxlx and AmphiNKx7 . All four genes are expressed with a striking asymmetry about the left–right axis in the pharyngeal region of neurula embryos, mirroring the pronounced asymmetry of amphioxus embryogenesis. AmphiMsxlx and AmphiNKx7 are also transiently expressed in an anterior neural tube region destined to become the cerebral vesicle. These findings suggest significant rewiring of developmental gene regulatory networks occurred during chordate evolution, coincident with homeobox gene loss. We propose that loss of otherwise widely conserved genes is possible when these genes function in a confined role in development that is subsequently lost or significantly modified during evolution. In the case of these homeobox genes, we propose that this has occurred in relation to the evolution of the chordate pharynx and brain.
9
Schilling, Thomas F., and Robert D. Kinght. "Origins of anteroposterior patterning and Hox gene regulation during chordate evolution." Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 356, no. 1414 (2001): 1599–613. http://dx.doi.org/10.1098/rstb.2001.0918.
Full textAbstract:
All chordates share a basic body plan and many common features of early development. Anteroposterior (AP) regions of the vertebrate neural tube are specified by a combinatorial pattern of Hox gene expression that is conserved in urochordates and cephalochordates. Another primitive feature of Hox gene regulation in all chordates is a sensitivity to retinoic acid during embryogenesis, and recent developmental genetic studies have demonstrated the essential role for retinoid signalling in vertebrates. Two AP regions develop within the chordate neural tube during gastrulation: an anterior ‘forebrain–midbrain’ region specified by Otx genes and a posterior ‘hindbrain–spinal cord’ region specified by Hox genes. A third, intermediate region corresponding to the midbrain or midbrain–hindbrain boundary develops at around the same time in vertebrates, and comparative data suggest that this was also present in the chordate ancestor. Within the anterior part of the Hox –expressing domain, however, vertebrates appear to have evolved unique roles for segmentation genes, such as Krox–20 , in patterning the hindbrain. Genetic approaches in mammals and zebrafish, coupled with molecular phylogenetic studies in ascidians, amphioxus and lampreys, promise to reveal how the complex mechanisms that specify the vertebrate body plan may have arisen from a relatively simple set of ancestral developmental components.
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Eyal-Giladi, H. "Establishment of the axis in chordates: facts and speculations." Development 124, no. 12 (1997): 2285–96. http://dx.doi.org/10.1242/dev.124.12.2285.
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A master plan for the early development of all chordates is proposed. The radial symmetry of the chordate ovum is changed at or after fertilization into a bilateral symmetry by an external signal. Until now two alternative triggers, sperm entry and gravity, have been demonstrated. It is suggested that a correlation exists between the amount of yolk stored in the egg and the mechanism used for axialization. The speed at which axialization of the embryo proper takes place depends on the translocation speed of maternal determinants from the vegetal pole towards the future dorsoposterior side of the embryo. On arrival at their destination, the activated determinants form, in all chordates, an induction center homologous to the amphibian ‘Nieuwkoop center’, which induces the formation of ‘Spemann's organizer’. On the basis of the above general scenario, a revision is proposed of the staging of some embryonic types, as well as of the identification of germ layer and the spaces between them.