Choanozoa is aclade ofopisthokonteukaryotes consisting of thechoanoflagellates (Choanoflagellatea) and theanimals (Animalia, Metazoa). Thesister-group relationship between the choanoflagellates and animals has important implications for the origin of the animals.[2] The clade was identified in 2015 byGraham Budd and Sören Jensen, who used the nameApoikozoa.[3] The 2018 revision of the classification first proposed by the International Society of Protistologists in 2012 recommends the use of the name Choanozoa.[4]
A close relationship between choanoflagellates and animals has long beenrecognised, dating back at least to the 1840s. A particularly striking and famous similarity between the single-celled choanoflagellates andmulticellular animals is provided by thecollar cells ofsponges and the overall morphology of the choanoflagellate cell. The relationship has since been confirmed by multiple molecular analyses. This proposedhomology was however thrown into some doubt in 2013 by the still controversial suggestion thatctenophores, and not sponges, are the sister group to all other animals.[5][6] More recent genomic work has suggested that choanoflagellates possess some of the important genetic machinery necessary for themulticellularity found in animals.[7]
Asynonym for the Choanozoa, Apoikozoa, derives from the ancient Greek for "colony" and "animal", referring to the ability of both animals and (some) choanoflagellates to form multicellular units.[4] While animals are permanently multicellular, the colony-building choanoflagellates are only sometimes so, which raises the question of whether or not the colony-building ability in both groups was present at the base of the entireclade, or whether it was independently derived within the animals and choanoflagellates.
The name "Choanozoa" was first used by protozoologistThomas Cavalier-Smith in 1991 to refer to a group of basalprotists that later proved not to form aclade. This group had the rank ofphylum and contained allopisthokont protists while excluding bothfungi andanimals, making the groupparaphyletic. Its classification was the following:[8]
The International Society of Protistologists rejected the use of this name for the paraphyletic group. Instead, since 2017, the nameChoanozoa is considered appropriate for the clade that unites choanoflagellates and animals, since theGreekchoanē (χοάνη), meaning 'funnel', refers to the collar, which is asynapomorphy (i.e. a unique characteristic) of the clade. A synonym of this clade,Apoikozoa, was used in previous years; however, it was rejected as being neither formally defined nor appropriate, since it refers to the ability to formcolonies, a characteristic not unique to this clade.[4]
Although the last common ancestor of the Choanozoa cannot be reconstructed with certainty, Budd and Jensen suggest that these organisms formed benthic colonies that competed for space amongst other mat-forming organisms known to have existed during theEdiacaran Period some 635–540 million years ago. As such they would form an important link between the unicellular ancestors of the animals and the enigmatic"Ediacaran" organisms known from this interval, thus allowing some sort of reconstruction of the earliest animals and their ecology.[3] In the following cladogram, an indication is given of approximately how many million years ago (Mya) the clades diverged into newer clades.[9][10][11] (Note that the later Budd and Jensen paper gives significantly younger dates. See alsoKimberella.) The Holomycota tree follows Tedersoo et al.[12]
^Brunet, Thibaut; Larson, Ben T.; Linden, Tess A.; Vermeij, Mark J. A.; McDonald, Kent; King, Nicole (18 October 2019). "Light-regulated collective contractility in a multicellular choanoflagellate".Science.366 (6463):326–334.Bibcode:2019Sci...366..326B.doi:10.1126/science.aay2346.PMID31624206.
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