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Deuterostome

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For the cell biology protein structure, seeDeuterosome.

Deuterostomes
Temporal range: EarliestCambrian–Present (PossibleEdiacaranrecord, 557 Ma[1])
Diversity of deuterostomes
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Grobben,1908
Clades

Deuterostomes(fromGreek:lit. 'mouth second') arebilateriananimalsof thesuperphylumDeuterostomia(/ˌdjuːtərəˈstoʊmi.ə/),[3][4]typically characterized by theiranusforming before themouthduringembryonic development.Deuterostomia is further divided into 4phyla:Chordata,Echinodermata,Hemichordata,and the extinctVetulicoliaknown fromCambrianfossils. The extinctcladeCambroernidais also thought to be a member of Deuterostomia.

In deuterostomy, the developing embryo's first opening (theblastopore) becomes the anus andcloaca,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 amongprotostomesas well.[5]This group is also known asenterocoelomates,because theircoelomdevelops throughenterocoely.

Deuterostomia's sister clade isProtostomia,animals that develop mouth first and whosedigestive tractdevelopment is more varied. Protostomia includes theecdysozoans(panarthropods,nematoids,penis worms,mud dragonsetc.) andspiralians(mollusks,annelids,flatworms,rotifers,arrow worms,etc.), as well as the extinctKimberella.Together with Protostomia and theiroutgroupXenacoelomorpha,they constitute the largeinfrakingdomBilateria,i.e. animals withbilateral symmetryand threegerm layers.

Systematics[edit]

History[edit]

Initially, Deuterostomia included the phylaBrachiopoda,[6]Bryozoa,[7]Chaetognatha,[8]andPhoronida[6]based on morphological and embryological characteristics. However, Deuterostomia was redefined in 1995 based on DNA molecular sequence analyses, leading to the removal of thelophophorateswhich was later combined with other protostome animals to form the superphylumLophotrochozoa.[9]Thearrow wormsmay also be deuterostomes,[8]but molecular studies have placed them in the protostomes more often.[10][11]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 throughhorizontal gene transfer.[12]

Classification[edit]

See also:List of bilateral animal orders

This is the generally agreed uponphylogenyof the deuterostomes:

There is a possibility that Ambulacraria is the sister clade toXenacoelomorpha,and could form theXenambulacrariagroup.[13][14][15]

Notable characteristics[edit]

Early development differences between deuterostomes versus protostomes. In deuterostomes, blastula divisions occur as radial cleavage because they occur parallel or perpendicular to the major polar axis. In protostomes the cleavage is spiral because division planes are oriented obliquely to the polar major axis. During gastrulation, deuterostome embryos' anus is given first by the blastopore while the mouth is formed secondarily, and vice versa for the protostomes

In both deuterostomes and protostomes, azygotefirst develops into a hollow ball of cells, called ablastula.In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is calledradial cleavage,and also occurs in certain protostomes, such as thelophophorates.

Most deuterostomes displayindeterminate 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 ofidentical twins.

In deuterostomes themesodermforms asevaginationsof the developed gut that pinch off to form thecoelom.This process is calledenterocoely.

Another feature present in both the Hemichordata and Chordata is pharyngotremy — the presence of spiracles orgill slitsinto thepharynx,which is also found in some primitive fossilechinoderms(mitrates).[16][17]

A hollow nerve cord is found in all chordates, includingtunicates(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 thehemichordatesand thechordateshave a thickening of theaorta,homologous to the chordateheart,which contracts to pump blood. This suggests a presence in the deuterostome ancestor of the three groups, with theechinodermshaving secondarily lost it.[citation needed]

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.[18]

Formation of mouth and anus[edit]

Main article:Embryological origins of the 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[citation needed],during the fourth week of development, and the anus forms four weeks later, temporarily forming acloaca.

Origins and evolution[edit]

Bilateria,one of the five major lineages of animals, is split into two groups; theprotostomesand deuterostomes. Deuterostomes consist of chordates (which include the vertebrates) and ambulacrarians.[19]It seems likely that the555million year oldKimberellawas a member of the protostomes.[20][21]That implies that the protostome and deuterostome lineages split some time beforeKimberellaappeared — at least558million years ago,and hence well before the start of the Cambrian538.8million years ago,[19]i.e.during the later part of theEdiacaranPeriod (circa 635-539 Mya, around the end of globalMarinoan glaciationin the lateNeoproterozoic). 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.[22]

The last common ancestor of the deuterostomes had lost allinnexindiversity.[23]

Fossils of one major deuterostome group, theechinoderms(whose modern members includesea stars,sea urchinsandcrinoids), are quite common from the start of Series 2 of the Cambrian,521million years ago.[24]The MidCambrianfossilRhabdotubus johanssonihas been interpreted as apterobranchhemichordate.[25]Opinions differ about whether theCheng gian g faunafossilYunnanozoon,from the earlier Cambrian, was a hemichordate or chordate.[26][27]Another Cheng gian g 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.[27]HaikouichthysandMyllokunmingia,also from the Cheng gian g fauna, are regarded asfish.[28][29]Pikaia,discovered much earlier but from the Mid CambrianBurgess Shale,is also regarded as a primitive chordate.[30]

On the other hand, fossils of early chordates are very rare, as non-vertebrate chordates have nobone tissueor teeth, and fossils of no Post-Cambrian non-vertebrate chordates are known aside from thePermian-agedPaleobranchiostoma,trace fossils of theOrdoviciancolonial tunicateCatellocaula,and various Jurassic-aged and Tertiary-aged spicules tentatively attributed to ascidians.

Phylogeny[edit]

Below is aphylogenetic treeshowing consensus relationships among deuterostome taxa. Phylogenomic evidence suggests the enteropneust family,Torquaratoridae,fall within thePtychoderidae.The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources.[31][32]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.[33]


Bilateria

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 asCentroneuralia), or the Xenacoelomorpha are re-positioned next to Ambulacraria within the Deuterostomia as in the above diagram.[32][34][35][36][37][38][39][40]

Fossil record[edit]

Deuterostomes have a rich fossil record with thousands of fossil species being found throughout thePhanerozoic.The earliest undisputed deuterostomes are forms such as the early chordatePikaiaand the early echinodermGogia,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 thetunicate-like organismsFinkoellaandAusiafrom theEdiacaranperiod. While these may in fact be tunicates, others have interpreted them ascnidarians[41]orsponges,[42]and as such their true affinity remains uncertain.

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Further reading[edit]

External links[edit]

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