Chordates evolved a unique body plan within deuterostomes and are Because hemichordates are the sister group of echinoderms, the Echinoderms and hemichordates are sister groups, while tunicate relationships are still not .. the formation of acellular vertebrae in teleost fish (Ekanayake and Hall. taxon relationship of hemichordates and the phylogeny of deuterostomes are only ysis of chordates and deuterostomes is presented based on these and other .. stage in several species of fishes, described a pair of centri- oles in the center. The main difference between Hemichordata and Chordata is that . Chordata: Mammals, birds, reptiles, amphibians, fish, amphioxus, and.
However, Lowe et al.
Chordate - Wikipedia
These results suggest that the deuterostome ancestor may have had an epidermal nerve net rather than a CNS, yet the anterior-posterior patterning of the ectoderm was already present Holland In contrast, three of the chordate characteristics are present in hemichordates, a ventral postanal tail, an endostyle, and pharyngeal gill slits.
A ventral postanal tail is present in juvenile harrimaniid hemichordate worms Bateson ; Burdon-Jones ; Cameronand it has been proposed to be homologous to the chordate postanal tail due to posterior Hox gene expression data Lowe et al. Furthermore, they share specific amino acid motifs with the echinoderms Petersonstrongly supporting their sister-group relationship Zeng and Swalla It is interesting that a postanal tail has only been reported in species of the direct-developing Harrimaniidae Bateson ; Burdon-Jones ; Cameronnot in the Ptychoderidae Urata and Yamaguchi These two families are paraphyletic with 18S rDNA analyses, with harrimaniids a sister group to the colonial pterobranchs Halanych ; Cameron, Garey, and Swalla ; however, theses two families are monophyletic when a 28S rDNA Winchell et al.
A postanal tail not being present in the indirect-developing ptychoderid acorn worms means that either it was lost in ptychoderids or the direct-developing harrimaniid worms more closely resemble the chordate ancestor. The endostyle is an iodine-binding organ present in the tunicate and cephalochordate pharynx and is considered by many researchers to be a homologous organ to the vertebrate thyroid Sasaki et al. The hemichordate epibranchial ridge, like the chordate endostyle, is composed of specialized secretory cells Ruppert, Cameron, and Frickand these cells bind iodine.
However, iodine binding is not restricted to this region but instead occurs all throughout the pharynx Ruppert Likewise, the hemichordate homolog of the gene NK2. Based on these data, it appears that the endostyle function in chordates is accomplished broadly by the pharynx in hemichordates. The most convincing homology between hemichordates and chordates is the pharyngeal gills. Hollandlamprey Ogasawara et al. The expression of the most anterior Hox gene, Hox1, is first seen in vertebrates in the second pharyngeal slit and the expression of Hox1 in hemichordates is seen at the level between the first and second gill slits, suggesting that the location of the gill slits along the anterior-posterior axis is also homologous Lowe et al.
Additionally, the pharyngeal skeletal elements of hemichordates and cephalochordates are strikingly similar in appearance Hyman ; Schaeffer Both have been reported to contain fibrillar collagen, as seen by transmission electron microscopy TEM studies Rahr ; Pardos and Benito Pharyngeal gills appear to function primarily for feeding and possibly for oxygen absorption, and thus in burrowing animals, cartilaginous elements may play an important structural role.
All deuterostomes with gills, except for tunicates, have structural elements associated with pharyngeal gills. Phylogeny results have shown that it is likely that the ancestral deuterostome was a worm that had pharyngeal gills supported by a collagenous acellular skeleton Cameron, Garey, and Swalla ; Bourlat et al.
Neither Xenoturbella nor Echinodermata has been reported to express this transcription factor. The most abundant and studied members of the collagen family are the fibrillar collagens. Each fibrillar collagen possesses the following characteristics: In this article, we construct gene trees with invertebrate and vertebrate triple helical domain sequences to show that hemichordates and lancelets have at least one fibrillar collagen and that it is similar to type I and type II vertebrate collagens.
We have been testing the hypothesis that hemichordates have a pharyngeal cartilage similar to vertebrates, containing collagen and proteoglycans in the matrix. Here, we show that the matrix contains at least one fibrillar collagen and probably contains proteoglycans as well, even if it is not cellular.
The extracellular matrix ECM that comprises the gill bar skeleton in both hemichordates and cephalochordates is probably secreted by the endoderm, unlike the vertebrate pharyngeal cartilage which is neural crest derived. We hypothesize that the earliest gill cartilages were acellular, and these were gradually replaced by neural crest cells in vertebrates.
Therefore, the deuterostome ancestor would have been a benthic worm with gill cartilages, a mouth, and the ability to filter feed. Cephalochordates may have lost their mouth and evolved a velum, while vertebrates lost the ability to filter feed. We next show evidence that invertebrate deuterostome cartilage is acellular, with an ECM of collagen and proteoglycans. Worms were dug with shovels and transported back to the laboratory in ml falcon tubes filled with seawater.
Sites in which to dig for the worms were recognized by small circular fecal casts that the animals deposit on the surface of the mud. Seven-micrometer sections were made on a Spencer microtome and mounted on gelatin-subbed slides. Photographs were taken using Cool Snap Version 1. This monoclonal antibody is specific for vertebrate type II collagen, has a broad species specificity, and does not cross-react with vertebrate nonfibrillar collagens Linsenmayer and Hendrix Collagen Phylogenetic analysis All known Homo sapiens fibrillar collagen amino acid sequences were downloaded from GenBank, and they have the following accession numbers: Invertebrate accession numbers are as follows for all known fibrillar collagens in GenBank— AAC Tunicate fibrillar collagen sequences were downloaded from the Ciona genome Web site http: The mosquito Anopheles gambiae and bee Apis mellifera fibrillar collagen sequences were found via Blast on the Ensembl genome Web site http: For all collagen sequences found via Blast, only those having a triple helical sequence in the same gene or genomic vicinity of a highly conserved noncollagenous domain known to be present in all fibrillar collagens were used.
As jaws, limbs, and other body parts have evolved in vertebrates, so have the muscles that operate them. Nervous system and sense organs The anterior end of the main nerve cord in chordates is enlarged to form at least the suggestion of a brain, but a brain is well developed only in vertebrates.
Tunicate larvae have visual organs sensitive to light and sense organs responsive to the direction of gravity. Pigment spots and light receptors in the nerve cord of lancelets detect sudden changes in light intensity. The eyes and other sense organs of vertebrates are more elaborate and complex.
The presence in cephalochordates and vertebrates of a nervous system with segmentally repeated nerves arising from the dorsal hollow nerve cord is suggestive of a common ancestry. The tunicate nervous system does not have the segmentally repeated nerves. The brains of all vertebrates are greatly enlarged and subdivided into functionally specialized regions. Digestion and nutrition Both tunicates and cephalochordates are filter feeders of small particles of food suspended in the water.
Beating cilia hairlike cellular extensions on the gill slits draw a current of water into the mouth and through the pharynx, where a sheet of mucussecreted by the endostyle a glandular organ lying below the two rows of gill slitsfilters suspended food particles from the water. Cilia lining the pharynx move the food-rich sheet of mucus upward over the gill slits, and it is then rolled up and transported to the posterior part of the gut.
The water current passes into the atrium and exits through the atrial opening. The difference is that the food consists of somewhat larger particles that have been deposited on the bottom detritusand, instead of the feeding current being driven by cilia, the pharyngeal musculature pumps water and food particles across the gill slits.
The earliest fishes probably fed on detritus, and a sucking action is retained by their extant representatives lampreys and hagfishes. With the development of jaws, it became possible for the vertebrates to capture and seize larger food items.
The lower digestive tract of the primitive chordate is a simple tube with a saclike stomach. There are only indications of the specialized areas and of glandlike structures, such as the liver and pancreas, that occur in vertebrates.
Excretion The excretion of wastes and the control of the chemical composition of the internal environment are largely effected by kidneys, although other parts of the body, including the gills, may play an important role.
Tunicates and cephalochordates have a salt content essentially the same as seawater, but vertebrates, even marine species, have body fluids of low salt content, with the exception of hagfishes. A possible explanation is that the vertebrates evolved in fresh water, but it seems reasonable that hagfishes branched off while still marine and that the freshwater form evolved later. Respiration A primitive chordate gill is present in tunicates and cephalochordates, where it serves in both respiration and feeding.
The vertebrate gill may retain some role in feeding, although the current is now produced by the action of muscles, not cilia. The gills became reduced in number in various lineages, and they were strengthened by supporting elements, some of which evolved into jaws. Lungs, already present in fishes, became the main respiratory organs of terrestrial vertebrates.
Circulatory system The circulatory system in chordates has a characteristic pattern. In tunicates and vertebrates the blood is propelled by a distinct heart; in cephalochordates, by contraction of the blood vessels.
Unoxygenated blood is driven forward via a vessel called the ventral aorta. It then passes through a series of branchial arteries in the gills, where gas exchange takes place, and the oxygenated blood flows to the body, much of it returning to its origin via a dorsal aorta. The blood of vertebrates passes through the tissues via tiny vessels called capillaries. In tunicates and cephalochordates, capillaries are absent and the blood passes through spaces in the tissues instead.
Hormones In vertebrates, endocrine glands those of internal secretion produce hormones that regulate many physiological activities.
In tunicates and cephalochordates, organs have been identified that correspond in anatomical position to the pituitary gland of vertebrates, but which hormones, if any, they secrete is uncertain. In vertebrates, the thyroid gland produces thyroxine, an iodine-containing hormone that helps regulate metabolism.
The thyroid is a modified endostyle, as can be illustrated by larval lampreys in which the thyroid still secretes mucus for use in feeding. The endostyles of lancelets take up iodine and form thyroxine, but the thyroxine formed may not function as a hormone in the lancelets themselves.
Features of defense and aggression Tunicates largely rely upon the passive defense afforded by their heavy tunic. Lancelets move rapidly through the substrate, and their well-developed locomotory apparatus evolved largely to provide a means of escaping predators. Vertebrates have ceased to feed on detritus brought to them by water currents. They have shifted to consuming larger foodstuffs and to actively locating, pursuing, and subduing what they eat.
Evolution and paleontology Many scientists maintain that chordates originated sometime earlier than million years ago; that is, they predate the fossil record.
Such early representatives were soft-bodied and therefore left a poor fossil record. The oldest known fossil chordate is Pikaia gracilens, a primitive cephalochordate dated to approximately million years ago. There is disagreement over whether older animals—such as Yunnanozoon lividum and Haikouella both of which date to million years ago and possess several chordate features —should be considered chordates.
An extensive vertebrate fossil record begins about million years ago. Embryological evidence places the phylum Chordata within the deuterostomes bilaterally symmetrical animals with undeterminate cleavage and whose mouth does not arise from the blastoporewhich also includes the phyla Hemichordata, Echinodermata, and Chaetognatha.
The closest relatives of the chordates are probably the hemichordates, since these animals possess gill slits and other features not found in other animal phyla.
A slightly more remote relationship to the echinoderms is inferred on the basis of resemblances between the larvae in some groups of hemichordates and echinoderms. The derivation of chordates from certain fossil echinoderms has been argued on the basis of features such as what appear to be gill slits. Theories that derive them from other phyla e.
Whether the first ancestral chordate was more like a tunicate or a cephalochordate has been extensively debated. The classical theory is that the ancestor was like a cephalochordate and that one lineage became attached to hard surfaces and evolved into tunicates, whereas another remained unattached and evolved into vertebrates.
An alternative theory is that the ancestor was like a tunicate and that the other two subphyla arose by modification of the tadpole larva. There is some preference for the classical theory because it provides the most satisfactory way of accounting for the similarities between chordates and hemichordates of the subphylum Enteropneusta.
Within the chordates, the tunicates probably branched off before the common ancestor of cephalochordates and vertebrates arose, for the latter resemble each other in some details of neuroanatomy and biochemistry. Classification Annotated classification Phylum Chordata Deuterostomatous eucoelomates; gill clefts; endostyle or its derivative in pharynx; notochord; hollow dorsal nerve cord; tail posterior and dorsal to anus.
Subphylum Tunicata or Urochordata; tunicates Notochord, when present, restricted to tail; body covered with tunic, but sometimes only cuticle; atrium, absent in Appendiculariadorsal and often paired in embryonic development; heart present; generally sessile attached as adults; see below Tunicates. Class Ascidiacea sea squirts Sessile; benthic; solitary or colonial within a common tunic.
Class Appendicularia larvacea Free-swimming; pelagic; resembles tadpole larvae of ascidians; 1 pair of gill slits; no distinct atrium. Class Thaliacea Pelagic; forms aggregations or colonies.
Subphylum Cephalochordata or Acrania; lancelets Notochord extends entire body length, with tip anterior to nerve cord; atrium a single cavity with single, ventral opening; segments well developed; head poorly developed; no paired fins; no heart; see below Cephalochordates.
Subphylum Vertebrata or Craniata; vertebrates Notochord extends to the back of a well-developed head; no atrium; segments well developed; paired fins or limbs usually present; heart present; see below Vertebrates.
Critical appraisal This outline gives the major groups of chordates. Modern systematic biology attempts to arrange groups of organisms in a way that suggests the genealogical relationships branching sequences and therefore presents an epitome of evolutionary history. It also may attempt to show where there are important differences among the various groups.
These goals often conflict. In a purely genealogical system, each group must correspond to a single lineage clade composed of the common ancestor and all of its descendants.