Journal articles: '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 (November 7, 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.

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2

Zeng, Liyun, and Billie J. Swalla. "Molecular phylogeny of the protochordates: chordate evolution." Canadian Journal of Zoology 83, no. 1 (January 1, 2005): 24–33. http://dx.doi.org/10.1139/z05-010.

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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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3

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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4

Su, Yi-Hsien, Yi-Chih Chen, Hsiu-Chi Ting, Tzu-Pei Fan, Ching-Yi Lin, Kuang-Tse Wang, and Jr-Kai Yu. "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 (June 12, 2019): 12925–32. http://dx.doi.org/10.1073/pnas.1901919116.

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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.

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5

Ruppert, Edward E. "Key characters uniting hemichordates and chordates: homologies or homoplasies?" Canadian Journal of Zoology 83, no. 1 (January 1, 2005): 8–23. http://dx.doi.org/10.1139/z04-158.

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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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6

Holland, Nicholas D. "Chordates." Current Biology 15, no. 22 (November 2005): R911—R914. http://dx.doi.org/10.1016/j.cub.2005.11.008.

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7

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 (October 29, 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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8

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 (June 16, 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.

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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 (October 29, 2001): 1599–613. http://dx.doi.org/10.1098/rstb.2001.0918.

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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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10

Eyal-Giladi, H. "Establishment of the axis in chordates: facts and speculations." Development 124, no. 12 (June 15, 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.

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11

Harada, Y., H. Yasuo, and N. Satoh. "A sea urchin homologue of the chordate Brachyury (T) gene is expressed in the secondary mesenchyme founder cells." Development 121, no. 9 (September 1, 1995): 2747–54. http://dx.doi.org/10.1242/dev.121.9.2747.

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Chordates are thought to have emerged from some common ancestor of deuterostomes by organizing shared anatomical and embryological features including a notochord, a dorsal nerve cord and pharyngeal gill slits. Because the notochord is the most prominent feature of chordates and because the Brachyury (T) gene is essential for notochord formation, the T gene is a key molecular probe with which to explore the origin and evolution of chordates. We investigated whether the sea urchin (echinoderm) conserves the T gene and, if so, where the sea urchin T gene is expressed. A cDNA clone for the sea urchin T (HpTa) gene contained a long open reading frame that encodes a polypeptide of 434 amino acids. Although the overall degree of amino acid identity was not very high (52%, sea urchin/mouse), in the T domain of the N terminus the amino acid identity was 73% (sea urchin/mouse). The HpTa gene is present as a single copy per haploid genome. As with the chordate T gene, the expression of HpTa is transient, being first detected in the swimming blastula, maximally transcribed in the gastrula, decreasing at the prism larval stage and barely detectable at the pluteus larval stage. HpTa transcripts were found in the secondary mesenchyme founder cells, vegetal plate of the mesenchyme blastula, extending tip of the invaginating archenteron and, finally, the secondary mesenchyme cells at the late-gastrula stage.(ABSTRACT TRUNCATED AT 250 WORDS)

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12

Sui, Zihao, Zhihan Zhao, and Bo Dong. "Origin of the Chordate Notochord." Diversity 13, no. 10 (September 25, 2021): 462. http://dx.doi.org/10.3390/d13100462.

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The phylum of Chordata is defined based on the discovery of a coelom-like dorsal notochord in ascidian and amphioxus embryos. Chordata can be classified into three subphylums, Cephalochordata, Urochordata, and Vertebrata, united by the presence of a notochord at some point during development. The origin of the notochord, the signature anatomical structure of chordates, has been under debate since the publication of Alexander Kovalevsky’s work in the mid-19th century that placed ascidians close to the vertebrates on the phylogenetic tree. During the late 20th century, the development of molecular and genetic tools in biology brought about a revival of studies on the evolutionary path of notochord development. Two main hypotheses for the origin of the notochord were proposed, the de novo theory and the axochord theory. The former states that notochord has developed de novo from the mid-dorsal archenteron of a chordate ancestor with simple morphology and no central nervous system nor notochord homolog. The putative notochord along the dorsal side of the animal is proposed to take on the signal functions later from the endoderm and ectoderm. An alternative hypothesis, the axochord theory, proposes that notochord has evolved from the mid-line muscle tissue, the so-called axochord, in annelids. Structural and molecular evidence point to the midline muscle of annelids as a distant homolog of the notochord. This hypothesis thus suggests a notochord-like structure in the urbilaterian ancestor, opposed to the consensus that notochord is a chordate-specific feature. In this review, we introduce the history of the formation of these views and summarize the current understandings of embryonic development, molecular profile, and gene regulatory networks of notochord and notochord-like structures.

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13

Simões-costa, Marcos S., Ana Paula Azambuja, and José Xavier-Neto. "The search for non-chordate retinoic acid signaling: lessons from chordates." Journal of Experimental Zoology Part B: Molecular and Developmental Evolution 310B, no. 1 (2007): 54–72. http://dx.doi.org/10.1002/jez.b.21139.

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14

RAINERI, MARGHERITA. "Are protochordates chordates?" Biological Journal of the Linnean Society 87, no. 2 (February 13, 2006): 261–84. http://dx.doi.org/10.1111/j.1095-8312.2006.00574.x.

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15

RAINERI, MARGHERITA. "Are protochordates chordates?" Biological Journal of the Linnean Society 87, no. 4 (April 18, 2006): 637. http://dx.doi.org/10.1111/j.1095-8312.2006.00620.x.

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16

McMenamin. "Cambrian Chordates and Vetulicolians." Geosciences 9, no. 8 (August 11, 2019): 354. http://dx.doi.org/10.3390/geosciences9080354.

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Deuterostomes make a sudden appearance in the fossil record during the early Cambrian. Two bilaterian groups, the chordates and the vetulicolians, are of particular interest for understanding early deuterostome evolution, and the main objective of this review is to examine the Cambrian diversity of these two deuterostome groups. The subject is of particular interest because of the link to vertebrates, and because of the enigmatic nature of vetulicolians. Lagerstätten in China and elsewhere have dramatically improved our understanding of the range of variation in these ancient animals. Cephalochordate and vertebrate body plans are well established at least by Cambrian Series 2. Taken together, roughly a dozen chordate genera and fifteen vetulicolian genera document part of the explosive radiation of deuterostomes at the base of the Cambrian. The advent of deuterostomes near the Cambrian boundary involved both a reversal of gut polarity and potentially a two-sided retinoic acid gradient, with a gradient discontinuity at the midpoint of the organism that is reflected in the sharp division of vetulicolians into anterior and posterior sections. A new vetulicolian (Shenzianyuloma yunnanense nov. gen. nov. sp.) with a laterally flattened, polygonal anterior section provides significant new data regarding vetulicolians. Its unsegmented posterior region (‘tail’) bears a notochord and a gut trace with diverticula, both surrounded by myotome cones.

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17

Nielsen, Claus. "Origin of the chordate central nervous system - and the origin of chordates." Development Genes and Evolution 209, no. 3 (February 24, 1999): 198–205. http://dx.doi.org/10.1007/s004270050244.

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18

Locascio, A., F. Aniello, A. Amoroso, M. Manzanares, R. Krumlauf, and M. Branno. "Patterning the ascidian nervous system: structure, expression and transgenic analysis of the CiHox3 gene." Development 126, no. 21 (November 1, 1999): 4737–48. http://dx.doi.org/10.1242/dev.126.21.4737.

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Hox genes play a fundamental role in the establishment of chordate body plan, especially in the anteroposterior patterning of the nervous system. Particularly interesting are the anterior groups of Hox genes (Hox1-Hox4) since their expression is coupled to the control of regional identity in the anterior regions of the nervous system, where the highest structural diversity is observed. Ascidians, among chordates, are considered a good model to investigate evolution of Hox gene, organisation, regulation and function. We report here the cloning and the expression pattern of CiHox3, a Ciona intestinalis anterior Hox gene homologous to the paralogy group 3 genes. In situ hybridization at the larva stage revealed that CiHox3 expression was restricted to the visceral ganglion of the central nervous system. The presence of a sharp posterior boundary and the absence of transcript in mesodermal tissues are distinctive features of CiHox3 expression when compared to the paralogy group 3 in other chordates. We have investigated the regulatory elements underlying CiHox3 neural-specific expression and, using transgenic analysis, we were able to isolate an 80 bp enhancer responsible of CiHox3 activation in the central nervous system (CNS). A comparative study between mouse and Ciona Hox3 promoters demonstrated that divergent mechanisms are involved in the regulation of these genes in vertebrates and ascidians.

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Onuma, Takeshi A., Momoko Hayashi, Fuki Gyoja, Kanae Kishi, Kai Wang, and Hiroki Nishida. "A chordate species lacking Nodal utilizes calcium oscillation and Bmp for left–right patterning." Proceedings of the National Academy of Sciences 117, no. 8 (February 6, 2020): 4188–98. http://dx.doi.org/10.1073/pnas.1916858117.

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Larvaceans are chordates with a tadpole-like morphology. In contrast to most chordates of which early embryonic morphology is bilaterally symmetric and the left–right (L–R) axis is specified by the Nodal pathway later on, invariant L–R asymmetry emerges in four-cell embryos of larvaceans. The asymmetric cell arrangements exist through development of the tailbud. The tail thus twists 90° in a counterclockwise direction relative to the trunk, and the tail nerve cord localizes on the left side. Here, we demonstrate that larvacean embryos have nonconventional L–R asymmetries: 1) L- and R-cells of the two-cell embryo had remarkably asymmetric cell fates; 2) Ca2+ oscillation occurred through embryogenesis; 3) Nodal, an evolutionarily conserved left-determining gene, was absent in the genome; and 4) bone morphogenetic protein gene (Bmp) homolog Bmp.a showed right-sided expression in the tailbud and larvae. We also showed that Ca2+ oscillation is required for Bmp.a expression, and that BMP signaling suppresses ectopic expression of neural genes. These results indicate that there is a chordate species lacking Nodal that utilizes Ca2+ oscillation and Bmp.a for embryonic L–R patterning. The right-side Bmp.a expression may have arisen via cooption of conventional BMP signaling in order to restrict neural gene expression on the left side.

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Morov, Arseniy R., Tharcisse Ukizintambara, Rushan M. Sabirov, and Kinya Yasui. "Acquisition of the dorsal structures in chordate amphioxus." Open Biology 6, no. 6 (June 2016): 160062. http://dx.doi.org/10.1098/rsob.160062.

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Acquisition of dorsal structures, such as notochord and hollow nerve cord, is likely to have had a profound influence upon vertebrate evolution. Dorsal formation in chordate development thus has been intensively studied in vertebrates and ascidians. However, the present understanding does not explain how chordates acquired dorsal structures. Here we show that amphioxus retains a key clue to answer this question. In amphioxus embryos, maternal nodal mRNA distributes asymmetrically in accordance with the remodelling of the cortical cytoskeleton in the fertilized egg, and subsequently lefty is first expressed in a patch of blastomeres across the equator where wnt8 is expressed circularly and which will become the margin of the blastopore. The lefty domain co-expresses zygotic nodal by the initial gastrula stage on the one side of the blastopore margin and induces the expression of goosecoid , not-like, chordin and brachyury1 genes in this region, as in the oral ectoderm of sea urchin embryos, which provides a basis for the formation of the dorsal structures. The striking similarity in the gene regulations and their respective expression domains when comparing dorsal formation in amphioxus and the determination of the oral ectoderm in sea urchin embryos suggests that chordates derived from an ambulacrarian-type blastula with dorsoventral inversion.

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Fodor, Alexander C. A., Megan M. Powers, Kristin Andrykovich, Jiatai Liu, Elijah K. Lowe, C. Titus Brown, Anna Di Gregorio, Alberto Stolfi, and Billie J. Swalla. "The Degenerate Tale of Ascidian Tails." Integrative and Comparative Biology 61, no. 2 (June 28, 2021): 358–69. http://dx.doi.org/10.1093/icb/icab022.

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Abstract Ascidians are invertebrate chordates, with swimming chordate tadpole larvae that have distinct heads and tails. The head contains the small brain, sensory organs, including the ocellus (light) and otolith (gravity) and the presumptive endoderm, while the tail has a notochord surrounded by muscle cells and a dorsal nerve cord. One of the chordate features is a post-anal tail. Ascidian tadpoles are nonfeeding, and their tails are critical for larval locomotion. After hatching the larvae swim up toward light and are carried by the tide and ocean currents. When competent to settle, ascidian tadpole larvae swim down, away from light, to settle and metamorphose into a sessile adult. Tunicates are classified as chordates because of their chordate tadpole larvae; in contrast, the sessile adult has a U-shaped gut and very derived body plan, looking nothing like a chordate. There is one group of ascidians, the Molgulidae, where many species are known to have tailless larvae. The Swalla Lab has been studying the evolution of tailless ascidian larvae in this clade for over 30 years and has shown that tailless larvae have evolved independently several times in this clade. Comparison of the genomes of two closely related species, the tailed Molgula oculata and tailless Molgula occulta reveals much synteny, but there have been multiple insertions and deletions that have disrupted larval genes in the tailless species. Genomics and transcriptomics have previously shown that there are pseudogenes expressed in the tailless embryos, suggesting that the partial rescue of tailed features in their hybrid larvae is due to the expression of intact genes from the tailed parent. Yet surprisingly, we find that the notochord gene regulatory network is mostly intact in the tailless M. occulta, although the notochord does not converge and extend and remains as an aggregate of cells we call the “notoball.” We expect that eventually many of the larval gene networks will become evolutionarily lost in tailless ascidians and the larval body plan abandoned, with eggs developing directly into an adult. Here we review the current evolutionary and developmental evidence on how the molgulids lost their tails.

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Roots, Betty I. "The phylogeny of invertebrates and the evolution of myelin." Neuron Glia Biology 4, no. 2 (May 2008): 101–9. http://dx.doi.org/10.1017/s1740925x0900012x.

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Current concepts of invertebrate phylogeny are reviewed. Annelida and Arthropoda, previously regarded as closely related, are now placed in separate clades. Myelin, a sheath of multiple layers of membranes around nerve axons, is found in members of the Annelida, Arthropoda and Chordata. The structure, composition and function of the sheaths in Annelida and Arthropoda are examined and evidence for the separate evolutionary origins of myelin in the three clades is presented. That myelin has arisen independently at least three times, namely in Annelids, Arthropodas and Chordates, provides a remarkable example of convergent evolution.

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Ehrenborg, Richard, and Miguel Mendez. "Schröder parenthesizations and chordates." Journal of Combinatorial Theory, Series A 67, no. 2 (August 1994): 127–39. http://dx.doi.org/10.1016/0097-3165(94)90008-6.

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Esposito, Alfonso, Luca Ambrosino, Silvano Piazza, Salvatore D’Aniello, Maria Luisa Chiusano, and Annamaria Locascio. "Evolutionary Adaptation of the Thyroid Hormone Signaling Toolkit in Chordates." Cells 10, no. 12 (December 2, 2021): 3391. http://dx.doi.org/10.3390/cells10123391.

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The specification of the endostyle in non-vertebrate chordates and of the thyroid gland in vertebrates are fundamental steps in the evolution of the thyroid hormone (TH) signaling to coordinate development and body physiology in response to a range of environmental signals. The physiology and biology of TH signaling in vertebrates have been studied in the past, but a complete understanding of such a complex system is still lacking. Non-model species from non-vertebrate chordates may greatly improve our understanding of the evolution of this complex endocrine pathway. Adaptation of already existing proteins in order to perform new roles is a common feature observed during the course of evolution. Through sequence similarity approaches, we investigated the presence of bona fide thyroid peroxidase (TPO), iodothyronine deiodinase (DIO), and thyroid hormone receptors (THRs) in non-vertebrate and vertebrate chordates. Additionally, we determined both the conservation and divergence degrees of functional domains at the protein level. This study supports the hypothesis that non-vertebrate chordates have a functional thyroid hormone signaling system and provides additional information about its possible evolutionary adaptation.

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Hudson, Clare, and Hitoyoshi Yasuo. "Neuromesodermal Lineage Contribution to CNS Development in Invertebrate and Vertebrate Chordates." Genes 12, no. 4 (April 17, 2021): 592. http://dx.doi.org/10.3390/genes12040592.

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Ascidians are invertebrate chordates and the closest living relative to vertebrates. In ascidian embryos a large part of the central nervous system arises from cells associated with mesoderm rather than ectoderm lineages. This seems at odds with the traditional view of vertebrate nervous system development which was thought to be induced from ectoderm cells, initially with anterior character and later transformed by posteriorizing signals, to generate the entire anterior-posterior axis of the central nervous system. Recent advances in vertebrate developmental biology, however, show that much of the posterior central nervous system, or spinal cord, in fact arises from cells that share a common origin with mesoderm. This indicates a conserved role for bi-potential neuromesoderm precursors in chordate CNS formation. However, the boundary between neural tissue arising from these distinct neural lineages does not appear to be fixed, which leads to the notion that anterior-posterior patterning and neural fate formation can evolve independently.

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Matviichuk, Oleksandr, Oksana Shevchuk, Olena Khodanitska, Olesia Tkachuk, Stepan Polyvany, and Inna Stepanenko. "Prospects for the preservation of hydrophilic zoocenosis under the conditions of the urbanized landscape of Vinnytsia." Personality and Environmental Issues, no. 3 (2023): 49–55. http://dx.doi.org/10.31652/2786-6033-2023-1(3)-49-55.

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The formation of a network of protected objects under the conditions of the urbanized landscape is one of the effective measures for implementing the concept of sustainable urban development. The transformation of natural ecosystems within settlements causes significant changes in the structure of zoocenoses: some species disappear from the territory; others are forced to adapt to new conditions. Reducing the level of anthropogenic pressure in the created protected objects due to the limitation of certain types of economic activity will allow preserving the species wealth of fauna even under the conditions of the urban landscape. Work was carried out on the study of the taxonomic wealth of the vertebrate fauna of one of the tracts of the city of Vinnytsia for the further preparation of the scientific justification for the creation of a protected object within its boundaries. To study the seasonal structure of zoocenoses of chordate animals, the tract 'Brigantyna' was chosen, which is located on the left bank of the Sabariv reservoir (Southern Bug river), the mouth of the Tyazhilivka river and the surrounding area. The total area of the studied territory is about 5.1 hectares. Generally accepted methods were used to study the species composition of the chordate animals of the tract. During all seasonal periods of 2020-2022, the seasonal structure and nature of topical connections of representatives of 5 classes of Chordata were studied within the facility: class Actinopterygii, Amphibia, Reptilia, Aves, Mammalia. For the first time, the taxonomic and ecological structure of the zoocenoses of the tract were analyzed, in its species structure representatives were identified, which are subject to protection at the local or global level. Within the studied territory, as a result of the records, the presence of representatives of 99 species of animals of the Chordata type was found: class Actinopterygii (16 species), Amphibia (4 species), Reptilia (4 species), Aves (66 species), Mammalia (9 species). Taxonomically, they are united in 79 genera, 42 families and 21 orders. The vast majority of discovered chordates are typical representatives of the fauna of Eastern Podillia. The creation of a protected object within the surveyed tract will contribute to the preservation of the biological diversity of the urban zoocenosis.

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Lambert, Charles C. "Historical introduction, overview, and reproductive biology of the protochordates." Canadian Journal of Zoology 83, no. 1 (January 1, 2005): 1–7. http://dx.doi.org/10.1139/z04-160.

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This issue of the Canadian Journal of Zoology exhaustively reviews most major aspects of protochordate biology by specialists in their fields. Protochordates are members of two deuterostome phyla that are exclusively marine. The Hemichordata, with solitary enteropneusts and colonial pterobranchs, share a ciliated larva with echinoderms and appear to be closely related, but they also have many chordate-like features. The invertebrate chordates are composed of the exclusively solitary cephalochordates and the tunicates with both solitary and colonial forms. The cephalochordates are all free-swimming, but the tunicates include both sessile and free-swimming forms. Here I explore the history of research on protochordates, show how views on their relationships have changed with time, and review some of their reproductive and structural traits not included in other contributions to this special issue.

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Mazet, Francoise. "The Evolution of Sensory Placodes." Scientific World JOURNAL 6 (2006): 1841–50. http://dx.doi.org/10.1100/tsw.2006.314.

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The vertebrate cranial sensory placodes are ectodermal embryonic patches that give rise to sensory receptor cells of the peripheral paired sense organs and to neurons in the cranial sensory ganglia. Their differentiation and the genetic pathways that underlay their development are now well understood. Their evolutionary history, however, has remained obscure. Recent molecular work, performed on close relatives of the vertebrates, demonstrated that some sensory placodes (namely the adenohypophysis, the olfactory, and accoustico-lateralis placodes) first evolved at the base of the chordate lineage, while others might be specific to vertebrates. Combined with morphological and cellular fate data, these results also suggest that the sensory placodes of the ancestor of all chordates differentiated into a wide range of structures, most likely to fit the lifestyle and environment of each species.

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Holland, Linda Z. "The origin and evolution of chordate nervous systems." Philosophical Transactions of the Royal Society B: Biological Sciences 370, no. 1684 (December 19, 2015): 20150048. http://dx.doi.org/10.1098/rstb.2015.0048.

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In the past 40 years, comparisons of developmental gene expression and mechanisms of development (evodevo) joined comparative morphology as tools for reconstructing long-extinct ancestral forms. Unfortunately, both approaches typically give congruent answers only with closely related organisms. Chordate nervous systems are good examples. Classical studies alone left open whether the vertebrate brain was a new structure or evolved from the anterior end of an ancestral nerve cord like that of modern amphioxus. Evodevo plus electron microscopy showed that the amphioxus brain has a diencephalic forebrain, small midbrain, hindbrain and spinal cord with parts of the genetic mechanisms for the midbrain/hindbrain boundary, zona limitans intrathalamica and neural crest. Evodevo also showed how extra genes resulting from whole-genome duplications in vertebrates facilitated evolution of new structures like neural crest. Understanding how the chordate central nervous system (CNS) evolved from that of the ancestral deuterostome has been truly challenging. The majority view is that this ancestor had a CNS with a brain that gave rise to the chordate CNS and, with loss of a discrete brain, to one of the two hemichordate nerve cords. The minority view is that this ancestor had no nerve cord; those in chordates and hemichordates evolved independently. New techniques such as phylostratigraphy may help resolve this conundrum.

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Bertrand, Stéphanie, Mohamed R. Belgacem, and Hector Escriva. "Nuclear hormone receptors in chordates." Molecular and Cellular Endocrinology 334, no. 1-2 (March 1, 2011): 67–75. http://dx.doi.org/10.1016/j.mce.2010.06.017.

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Blieck, Alain. "At the orgin of chordates." Geobios 25, no. 1 (January 1992): 101–13. http://dx.doi.org/10.1016/s0016-6995(09)90039-0.

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Lacalli, Thurston C. "Vetulicolians?are they deuterostomes? chordates?" BioEssays 24, no. 3 (February 28, 2002): 208–11. http://dx.doi.org/10.1002/bies.10064.

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Garcia-Fernàndez, Jordi, Salvatore D'Aniello, and Hector Escrivà. "Organizing chordates with an organizer." BioEssays 29, no. 7 (2007): 619–24. http://dx.doi.org/10.1002/bies.20596.

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Singh, Rajani. "Is the Tubular Nervous System Related with the Development of Skeletal Muscle in Chordates? – A Review." Journal of Morphological Sciences 35, no. 04 (November 6, 2018): 207–11. http://dx.doi.org/10.1055/s-0038-1675617.

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AbstractMany theories and arguments have been proposed regarding the ancestors of the vertebrates and the factors that lead to the evolution of the tubular nervous system. Invertebrates had simpler smooth muscles. Vertebrates acquired additional skeletal muscles. The skeletal muscles were found to be associated with a new type of tubular nervous system. There were three stages in the evolution of the nervous system. The most primitive was the network type, in which there was neither a polarization nor a centralization of neurons. The second stage was characterized by the evolution of a ganglionic nervous system. Then, the tubular type of nervous system appeared for the first time in chordates. Therefore, the author hypothesizes that the skeletal muscle developed simultaneously with the tubular nervous system. The chorda mesoderm and, thereby, the skeletal muscle, induced the formation of a tubular nervous system in chordates. In the present article, the author aims to analyze the nervous system, starting from invertebrates and moving on to chordates.

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SYVANEN, MICHAEL, and JONATHAN DUCORE. "WHOLE GENOME COMPARISONS REVEALS A POSSIBLE CHIMERIC ORIGIN FOR A MAJOR METAZOAN ASSEMBLAGE." Journal of Biological Systems 18, no. 02 (June 2010): 261–75. http://dx.doi.org/10.1142/s0218339010003408.

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The availability of whole genome sequences from multiple metazoan phyla is making it possible to determine their phylogeny. We have found that a sea urchin and human define a clade that excludes a tunicate, contradicting both classical and recent molecular studies that place the tunicate and vertebrate in the Chordate phylum. Intriguingly, by means of a novel four taxa analysis, we have partitioned the 2000 proteins responsible for this assignment into two groups. One group, containing about 40% of the proteins, supports the classical assemblage of the tunicate with vertebrates, while the remaining group places the tunicate outside of the chordate assemblage. The existence of these two phylogenetic groups is robustly maintained in five, six and nine taxa analyses. These results suggest that major horizontal gene transfer events occurred during the emergence of one of the metazoan phyla. The simplest explanation is that the modern tunicate (as represented by Ciona intestinalis) began as a hybrid between a primitive vertebrate and some other organism, perhaps from an extinct and unidentified protostome phylum, at a time close to but after the diversification of the chordates and echinoderms and before the lineages leading to Drosophila melanogaster and Caenorhabditis elegans diverged.

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Schmidt, Pascal, Eva Leman, Ronan Lagadec, Michael Schubert, Sylvie Mazan, and Ram Reshef. "Evolutionary Transition in the Regulation of Vertebrate Pronephros Development: A New Role for Retinoic Acid." Cells 11, no. 8 (April 12, 2022): 1304. http://dx.doi.org/10.3390/cells11081304.

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The anterior-posterior (AP) axis in chordates is regulated by a conserved set of genes and signaling pathways, including Hox genes and retinoic acid (RA), which play well-characterized roles in the organization of the chordate body plan. The intermediate mesoderm (IM), which gives rise to all vertebrate kidneys, is an example of a tissue that differentiates sequentially along this axis. Yet, the conservation of the spatiotemporal regulation of the IM across vertebrates remains poorly understood. In this study, we used a comparative developmental approach focusing on non-conventional model organisms, a chondrichthyan (catshark), a cyclostome (lamprey), and a cephalochordate (amphioxus), to assess the involvement of RA in the regulation of chordate and vertebrate pronephros formation. We report that the anterior expression boundary of early pronephric markers (Pax2 and Lim1), positioned at the level of somite 6 in amniotes, is conserved in the catshark and the lamprey. Furthermore, RA, driving the expression of Hox4 genes like in amniotes, regulates the anterior pronephros boundary in the catshark. We find no evidence for the involvement of this regulatory hierarchy in the AP positioning of the lamprey pronephros and the amphioxus pronephros homolog, Hatschek’s nephridium. This suggests that despite the conservation of Pax2 and Lim1 expressions in chordate pronephros homologs, the responsiveness of the IM, and hence of pronephric genes, to RA- and Hox-dependent regulation is a gnathostome novelty.

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Morris, Valerie B. "Origins of radial symmetry identified in an echinoderm during adult development and the inferred axes of ancestral bilateral symmetry." Proceedings of the Royal Society B: Biological Sciences 274, no. 1617 (April 17, 2007): 1511–16. http://dx.doi.org/10.1098/rspb.2007.0312.

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How the radial body plan of echinoderms is related to the bilateral body plan of their deuterostome relatives, the hemichordates and the chordates, has been a long-standing problem. Now, using direct development in a sea urchin, I show that the first radially arranged structures, the five primary podia, form from a dorsal and a ventral hydrocoele at the oral end of the archenteron. There is a bilateral plane of symmetry through the podia, the mouth, the archenteron and the blastopore. This adult bilateral plane is thus homologous with the bilateral plane of bilateral metazoans and a relationship between the radial and bilateral body plans is identified. I conclude that echinoderms retain and use the bilateral patterning genes of the common deuterostome ancestor. Homologies with the early echinoderms of the Cambrian era and between the dorsal hydrocoele, the chordate notochord and the proboscis coelom of hemichordates become evident.

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Kiełbówna, Leokadia, and Izabela Jędrzejowska. "How is Myogenesis Initiated in Chordates?" Folia Biologica 60, no. 3 (July 30, 2012): 107–19. http://dx.doi.org/10.3409/fb60_3-4.107-119.

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Williams, J. B. "Sessile lifestyle and origin of chordates." New Zealand Journal of Zoology 23, no. 2 (January 1996): 111–33. http://dx.doi.org/10.1080/03014223.1996.9518072.

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Gerhart, John, Christopher Lowe, and Marc Kirschner. "Hemichordates and the origin of chordates." Current Opinion in Genetics & Development 15, no. 4 (August 2005): 461–67. http://dx.doi.org/10.1016/j.gde.2005.06.004.

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Bedois, Alice M. H., Hugo J. Parker, and Robb Krumlauf. "Retinoic Acid Signaling in Vertebrate Hindbrain Segmentation: Evolution and Diversification." Diversity 13, no. 8 (August 23, 2021): 398. http://dx.doi.org/10.3390/d13080398.

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In metazoans, Hox genes are key drivers of morphogenesis. In chordates, they play important roles in patterning the antero-posterior (A-P) axis. A crucial aspect of their role in axial patterning is their collinear expression, a process thought to be linked to their response to major signaling pathways such as retinoic acid (RA) signaling. The amplification of Hox genes following major events of genome evolution can contribute to morphological diversity. In vertebrates, RA acts as a key regulator of the gene regulatory network (GRN) underlying hindbrain segmentation, which includes Hox genes. This review investigates how the RA signaling machinery has evolved and diversified and discusses its connection to the hindbrain GRN in relation to diversity. Using non-chordate and chordate deuterostome models, we explore aspects of ancient programs of axial patterning in an attempt to retrace the evolution of the vertebrate hindbrain GRN. In addition, we investigate how the RA signaling machinery has evolved in vertebrates and highlight key examples of regulatory diversification that may have influenced the GRN for hindbrain segmentation. Finally, we describe the value of using lamprey as a model for the early-diverged jawless vertebrate group, to investigate the elaboration of A-P patterning mechanisms in the vertebrate lineage.

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Cone, Angela C., and Robert W. Zeller. "Using ascidian embryos to study the evolution of developmental gene regulatory networks." Canadian Journal of Zoology 83, no. 1 (January 1, 2005): 75–89. http://dx.doi.org/10.1139/z04-165.

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Ascidians are ideally positioned taxonomically at the base of the chordate tree to provide a point of comparison for developmental regulatory mechanisms that operate among protostomes, non-chordate deuterostomes, invertebrate chordates, and vertebrates. In this review, we propose a model for the gene regulatory network that gives rise to the ascidian notochord. The purpose of this model is not to clarify all of the interactions between molecules of this network, but to provide a working schematic of the regulatory architecture that leads to the specification of endoderm and the patterning of mesoderm in ascidian embryos. We describe a series of approaches, both computational and biological, that are currently being used, or are in development, for the study of ascidian embryo gene regulatory networks. It is our belief that the tools now available to ascidian biologists, in combination with a streamlined mode of development and small genome size, will allow for more rapid dissection of developmental gene regulatory networks than in more complex organisms such as vertebrates. It is our hope that the analysis of gene regulatory networks in ascidians can provide a basic template which will allow developmental biologists to superimpose the modifications and novelties that have arisen during deuterostome evolution.

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Peinado, Gabriel, Tomás Osorno, María del Pilar Gomez, and Enrico Nasi. "Calcium activates the light-dependent conductance in melanopsin-expressing photoreceptors of amphioxus." Proceedings of the National Academy of Sciences 112, no. 25 (June 8, 2015): 7845–50. http://dx.doi.org/10.1073/pnas.1420265112.

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Melanopsin, the photopigment of the “circadian” receptors that regulate the biological clock and the pupillary reflex in mammals, is homologous to invertebrate rhodopsins. Evidence supporting the involvement of phosphoinositides in light-signaling has been garnered, but the downstream effectors that control the light-dependent conductance remain unknown. Microvillar photoreceptors of the primitive chordate amphioxus also express melanopsin and transduce light via phospholipase-C, apparently not acting through diacylglycerol. We therefore examined the role of calcium in activating the photoconductance, using simultaneous, high time-resolution measurements of membrane current and Ca2+ fluorescence. The light-induced calcium rise precedes the onset of the photocurrent, making it a candidate in the activation chain. Moreover, photolysis of caged Ca elicits an inward current of similar size, time course and pharmacology as the physiological photoresponse, but with a much shorter latency. Internally released calcium thus emerges as a key messenger to trigger the opening of light-dependent channels in melanopsin-expressing microvillar photoreceptors of early chordates.

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Datta, D., S. Nath Talapatra, and S. Swarnakar. "Bioactive Compounds from Marine Invertebrates for Potential Medicines - An Overview." International Letters of Natural Sciences 34 (February 2015): 42–61. http://dx.doi.org/10.18052/www.scipress.com/ilns.34.42.

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The present review deals with the bioactive compounds of the marine non-chordates. The potent medicinal usage of the bioactive compounds viz. steroids, terpenoids, isoprenoid and non-isoprenoid compounds, quinones, brominated compounds, nitrogen heterocyclics and nitrogen-sulphur heterocyclics from marine non-chordates have been compiled. Various literatures survey revealed that the bioactive compounds isolated in recent past from the marine poriferans, cnidarians, annelids, arthropods, molluscs and echinoderms could be rich sources of therapeutic agents having antibacterial, antiinflamatory, anticarcinogenic properties. In overall, the present study will be benefitted to know global drug discovery researches on bioactive compounds from marine organisms for students, scholars, scientists, pharmaceutical sector, and government regulating authorities as new challenging technology in clinical applications through medicines.

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Datta, D., S. Nath Talapatra, and S. Swarnakar. "Bioactive Compounds from Marine Invertebrates for Potential Medicines - An Overview." International Letters of Natural Sciences 34 (February 17, 2015): 42–61. http://dx.doi.org/10.56431/p-i22ej9.

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The present review deals with the bioactive compounds of the marine non-chordates. The potent medicinal usage of the bioactive compounds viz. steroids, terpenoids, isoprenoid and non-isoprenoid compounds, quinones, brominated compounds, nitrogen heterocyclics and nitrogen-sulphur heterocyclics from marine non-chordates have been compiled. Various literatures survey revealed that the bioactive compounds isolated in recent past from the marine poriferans, cnidarians, annelids, arthropods, molluscs and echinoderms could be rich sources of therapeutic agents having antibacterial, antiinflamatory, anticarcinogenic properties. In overall, the present study will be benefitted to know global drug discovery researches on bioactive compounds from marine organisms for students, scholars, scientists, pharmaceutical sector, and government regulating authorities as new challenging technology in clinical applications through medicines.

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Locascio, Annamaria, Giovanni Annona, Filomena Caccavale, Salvatore D’Aniello, Claudio Agnisola, and Anna Palumbo. "Nitric Oxide Function and Nitric Oxide Synthase Evolution in Aquatic Chordates." International Journal of Molecular Sciences 24, no. 13 (July 6, 2023): 11182. http://dx.doi.org/10.3390/ijms241311182.

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Nitric oxide (NO) is a key signaling molecule in almost all organisms and is active in a variety of physiological and pathological processes. Our understanding of the peculiarities and functions of this simple gas has increased considerably by extending studies to non-mammal vertebrates and invertebrates. In this review, we report the nitric oxide synthase (Nos) genes so far characterized in chordates and provide an extensive, detailed, and comparative analysis of the function of NO in the aquatic chordates tunicates, cephalochordates, teleost fishes, and amphibians. This comprehensive set of data adds new elements to our understanding of Nos evolution, from the single gene commonly found in invertebrates to the three genes present in vertebrates.

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Li, Mengyang, Zhan Gao, Dongrui Ji, and Shicui Zhang. "Functional Characterization of GH-Like Homolog in Amphioxus Reveals an Ancient Origin of GH/GH Receptor System." Endocrinology 155, no. 12 (December 1, 2014): 4818–30. http://dx.doi.org/10.1210/en.2014-1377.

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Amphioxus belongs to the subphylum cephalochordata, an extant representative of the most basal chordates. Despite many studies on the endocrine system of amphioxus, no evidence showed the presence of pituitary hormones. In this study, we clearly demonstrated the existence of a functional GH-like hormone in amphioxus, which is able to bind purified GH receptors, stimulate IGF-I expression, promote growth rate of fish, and rescue embryonic defects caused by a shortage of GH. We also showed the presence of a GH/prolactin-like-binding protein containing the entire hormone binding domain of GH/prolactin receptors in amphioxus, which is widely expressed among tissues, and interacts with the GH-like hormone. It is clear from these results that the GH/GH receptor-like system is present in amphioxus and, hence, in all classes of chordates. Notably, the GH-like hormone appears to be the only member of the vertebrate pituitary hormones family in amphioxus, suggesting that the hormone is the ancestral peptide that originated first in the molecular evolution of the pituitary hormones family in chordates. These data collectively suggest that a vertebrate-like neuroendocrine axis setting has already emerged in amphioxus, which lays a foundation for subsequent formation of hypothalamic-pituitary system in vertebrates.

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Wada, H., H. Saiga, N. Satoh, and P. W. Holland. "Tripartite organization of the ancestral chordate brain and the antiquity of placodes: insights from ascidian Pax-2/5/8, Hox and Otx genes." Development 125, no. 6 (March 15, 1998): 1113–22. http://dx.doi.org/10.1242/dev.125.6.1113.

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Ascidians and vertebrates belong to the Phylum Chordata and both have dorsal tubular central nervous systems. The structure of the ascidian neural tube is extremely simple, containing less than 400 cells, among which less than 100 cells are neurons. Recent studies suggest that, despite its simple organization, the mechanisms patterning the ascidian neural tube are similar to those of the more complex vertebrate brain. Identification of homologous regions between vertebrate and ascidian nervous systems, however, remains to be resolved. Here we report the expression of HrPax-258 gene: an ascidian homologue of vertebrate Pax-2, Pax-5 and Pax-8 genes. Molecular phylogenetic analyses indicate that HrPax-258 is descendant from a single precursor gene that gave rise to the three vertebrate genes. The expression pattern of HrPax-258 suggests that this subfamily of Pax genes has conserved roles in regional specification of the brain. Comparison with expression of ascidian Otx (Hroth) and a Hox gene (HrHox1) by double-staining in situ hybridizations indicate that the ascidian brain region can be subdivided into three regions; the anterior region marked by Hroth probably homologous to the vertebrate forebrain and midbrain, the middle region marked by HrPax-258 probably homologous to the vertebrate anterior hindbrain (and maybe also midbrain) and the posterior region marked by Hox genes which is homologous to the vertebrate hindbrain and spinal cord. Later expression of HrPax-258 in atrial primordia implies that basal chordates such as ascidians have already acquired a sensory organ that develops from epidermal thickenings (placodes) and expresses HrPax-258; we suggest it is homologous to the vertebrate ear. Therefore, placodes are not likely to be a newly acquired feature in vertebrates, but may have already been possessed by the earliest chordates.

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Bozzo, Matteo, Simone Costa, Valentina Obino, Tiziana Bachetti, Emanuela Marcenaro, Mario Pestarino, Michael Schubert, and Simona Candiani. "Functional Conservation and Genetic Divergence of Chordate Glycinergic Neurotransmission: Insights from Amphioxus Glycine Transporters." Cells 10, no. 12 (December 2, 2021): 3392. http://dx.doi.org/10.3390/cells10123392.

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Glycine is an important neurotransmitter in vertebrates, performing both excitatory and inhibitory actions. Synaptic levels of glycine are tightly controlled by the action of two glycine transporters, GlyT1 and GlyT2, located on the surface of glial cells and neurons, respectively. Only limited information is available on glycinergic neurotransmission in invertebrates, and the evolution of glycinergic neurotransmission is poorly understood. Here, by combining phylogenetic and gene expression analyses, we characterized the glycine transporter complement of amphioxus, an important invertebrate model for studying the evolution of chordates. We show that amphioxus possess three glycine transporter genes. Two of these (GlyT2.1 and GlyT2.2) are closely related to GlyT2 of vertebrates, whereas the third (GlyT) is a member of an ancestral clade of deuterostome glycine transporters. GlyT2.2 expression is predominantly non-neural, whereas GlyT and GlyT2.1 are widely expressed in the amphioxus nervous system and are differentially expressed, respectively, in neurons and glia. Vertebrate glycinergic neurons express GlyT2 and glia GlyT1, suggesting that the evolution of the chordate glycinergic system was accompanied by a paralog-specific inversion of gene expression. Despite this genetic divergence between amphioxus and vertebrates, we found strong evidence for conservation in the role glycinergic neurotransmission plays during larval swimming, the implication being that the neural networks controlling the rhythmic movement of chordate bodies may be homologous.

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Huilin, LUO, HU Shixue, and CHEN Liangzhong. "New Early Cambrian Chordates from Haikou, Kunming." Acta Geologica Sinica - English Edition 75, no. 4 (September 7, 2010): 345–48. http://dx.doi.org/10.1111/j.1755-6724.2001.tb00051.x.

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Journal articles on the topic 'Chordates'

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