Deuterostome

Deuterostomes (from Greek: lit. 'mouth second') are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into 4 phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is also thought to be a member of Deuterostomia.

In deuterostomy, the developing embryo's first opening (the blastopore) becomes the anus and cloaca, while the mouth is formed at a different site later on. This was initially the group's distinguishing characteristic, but deuterostomy has since been discovered among protostomes as well. This group is also known as enterocoelomates, because their coelom develops through enterocoely.

Deuterostomia's sister clade is Protostomia, animals that develop mouth first and whose digestive tract development is more varied. Protostomia includes the ecdysozoans (panarthropods, nematoids, penis worms, mud dragons etc.) and spiralians (mollusks, annelids, flatworms, rotifers, arrow worms, etc.), as well as the extinct Kimberella. Together with Protostomia and their outgroup Xenacoelomorpha, they constitute the large infrakingdom Bilateria, i.e. animals with bilateral symmetry and three germ layers.

History
Initially, Deuterostomia included the phyla Brachiopoda, Bryozoa, Chaetognatha, and Phoronida based on morphological and embryological characteristics. However, Deuterostomia was redefined in 1995 based on DNA molecular sequence analyses, leading to the removal of the lophophorates which was later combined with other protostome animals to form the superphylum Lophotrochozoa. The arrow worms may also be deuterostomes, but molecular studies have placed them in the protostomes more often. Genetic studies have also revealed that deuterostomes have more than 30 genes not found in any other animal groups, but which yet are present in some marine algae and prokaryotes. This could mean they are very ancient genes that were lost in other organisms, or that a common ancestor acquired them through horizontal gene transfer.

Classification
This is the generally agreed upon phylogeny of the deuterostomes:
 * Superphylum Deuterostomia
 * Phylum Chordata
 * Subphylum Cephalochordata (lancelets)
 * Clade Olfactores
 * Subphylum Tunicata (tunicates)
 * Subphylum Vertebrata
 * Superclass Agnatha (jawless fish)
 * Infraphylum Gnathostomata (jawed fish)
 * Class Chondrichthyes (cartilaginous fish)
 * Superclass Osteichthyes (bony fish - includes tetrapods)
 * Clade Ambulacraria
 * Phylum Hemichordata
 * Class Planctosphaeroidea
 * Class Enteropneusta (acorn worms)
 * Class Pterobranchia
 * Phylum Echinodermata
 * Subphylum Asterozoa
 * Class Asteroidea (starfish)
 * Class Ophiuroidea (brittle stars)
 * Subphylum Blastozoa †
 * Subphylum Crinozoa (sea lillies and extinct relatives)
 * Subphylum Echinozoa
 * Echinoidea (sea urchins)
 * Holothuriodea (sea cucumbers)

There is a possibility that Ambulacraria is the sister clade to Xenacoelomorpha, and could form the Xenambulacraria group.

Notable characteristics


In both deuterostomes and protostomes, a zygote first develops into a hollow ball of cells, called a blastula. In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is called radial cleavage, and also occurs in certain protostomes, such as the lophophorates.

Most deuterostomes display indeterminate cleavage, in which the developmental fate of the cells in the developing embryo is not determined by the identity of the parent cell. Thus, if the first four cells are separated, each can develop into a complete small larva; and if a cell is removed from the blastula, the other cells will compensate. This is the source of identical twins.

In deuterostomes the mesoderm forms as evaginations of the developed gut that pinch off to form the coelom. This process is called enterocoely.

Another feature present in both the Hemichordata and Chordata is pharyngotremy — the presence of spiracles or gill slits into the pharynx, which is also found in some primitive fossil echinoderms (mitrates).

A hollow nerve cord is found in all chordates, including tunicates (in the larval stage). Some hemichordates also have a tubular nerve cord. In the early embryonic stage, it looks like the hollow nerve cord of chordates.

Both the hemichordates and the chordates have a thickening of the aorta, homologous to the chordate heart, which contracts to pump blood. This suggests a presence in the deuterostome ancestor of the three groups, with the echinoderms having secondarily lost it.

The highly modified nervous system of echinoderms obscures much about their ancestry, but several facts suggest that all present deuterostomes evolved from a common ancestor that had pharyngeal gill slits, a hollow nerve cord, circular and longitudinal muscles and a segmented body.

Formation of mouth and anus
The defining characteristic of the deuterostome is the fact that the blastopore (the opening at the bottom of the forming gastrula) becomes the anus, whereas in protostomes the blastopore becomes the mouth. The deuterostome mouth develops at the opposite end of the embryo, from the blastopore, and a digestive tract develops in the middle, connecting the two.

In many animals, these early development stages later evolved in ways that no longer reflect these original patterns. For instance, humans have already formed a gut tube at the time of formation of the mouth and anus. Then the mouth forms first, during the fourth week of development, and the anus forms four weeks later, temporarily forming a cloaca.

Origins and evolution


Bilateria, one of the five major lineages of animals, is split into two groups; the protostomes and deuterostomes. Deuterostomes consist of chordates (which include the vertebrates) and ambulacrarians. It seems likely that the Kimberella was a member of the protostomes. That implies that the protostome and deuterostome lineages split some time before Kimberella appeared — at least, and hence well before the start of the Cambrian , i.e. during the later part of the Ediacaran Period (circa 635-539 Mya, around the end of global Marinoan glaciation in the late Neoproterozoic). It has been proposed that the ancestral deuterostome, before the chordate/ambulacrarian split, could have been a chordate-like animal with a terminal anus and pharyngeal openings but no gill slits, with active suspension feeding strategy.

The last common ancestor of the deuterostomes had lost all innexin diversity.

Fossils of one major deuterostome group, the echinoderms (whose modern members include sea stars, sea urchins and crinoids), are quite common from the start of Series 2 of the Cambrian,. The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate. Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate. Another Chengjiang fossil, Haikouella lanceolata, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes — although it also had short tentacles round its mouth. Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish. Pikaia, discovered much earlier but from the Mid Cambrian Burgess Shale, is also regarded as a primitive chordate.

On the other hand, fossils of early chordates are very rare, as non-vertebrate chordates have no bone tissue or teeth, and fossils of no Post-Cambrian non-vertebrate chordates are known aside from the Permian-aged Paleobranchiostoma, trace fossils of the Ordovician colonial tunicate Catellocaula, and various Jurassic-aged and Tertiary-aged spicules tentatively attributed to ascidians.

Phylogeny
Below is a phylogenetic tree showing consensus relationships among deuterostome taxa. Phylogenomic evidence suggests the enteropneust family, Torquaratoridae, fall within the Ptychoderidae. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources. The approximate dates for each radiation into a new clade are given in millions of years ago (Mya). Not all dates are consistent, as of date ranges only the center is given.

Support for the clade Deuterostomia is not unequivocal. In particular, the Ambulacraria are sometimes shown to be related to the Xenacoelomorpha. If true, this raises two possibilities: either the Ambulacraria are taken out of the deuterostome-protostome dichotomy (in which case the grouping Deuterostomia dissolves, with Chordata and Protostomia grouped together as Centroneuralia), or the Xenacoelomorpha are re-positioned next to Ambulacraria within the Deuterostomia as in the above diagram.

Fossil record
Deuterostomes have a rich fossil record with thousands of fossil species being found throughout the Phanerozoic. The earliest undisputed deuterostomes are forms such as the early chordate Pikaia and the early echinoderm Helicoplacus, each from about 515 million years ago. There are also a few earlier fossils that may represent deuterostomes, but these remain debated. The earliest of these disputed fossils are the tunicate-like organisms Finkoella and Ausia from the Ediacaran period. While these may in fact be tunicates, others have interpreted them as cnidarians or sponges, and as such their true affinity remains uncertain. Another Ediacaran fossil, Arkarua, may represent the earliest echinoderm, while Yanjiahella from the early Cambrian (Fortunian) period is another notable stem group echinoderm.