The APOIKOZOA is a monophyletic clade of opisthokont eukaryotes consisting of the Animalia and Choanoflagellatea . The sister-group relationship between the choanoflagellates and animals has important implications for the origin of the animals. It was identified in 2015 by Graham Budd and Sören Jensen
A close relationship between choanoflagellates and animals has long been recognised , dating back at least to the 1840s. A particularly striking and famous similarity between the single-celled choanoflagellates and multicellular animals is provided by the collar cells of sponges and the overall morphology of the choanoflagellate cell, and this has been confirmed by multiple molecular analyses since. This proposed homology has however been thrown into some doubt recently by the still controversial suggestion that ctenophores and not sponges are the sister group to all other animals. More recent genomic work has suggested that choanoflagellates possess some of the important genetic machinery necessary for the multicellularity of animals.
The name "Apoikozoa", derived from the ancient Greek for "colony" and "animal", refers to the ability of both the animals and (some) choanoflagellates to form multicellular units. While animals are permanently multicellular (apart from their reproductive cells), 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 entire clade , or whether it was independently derived within the animals and choanoflagellates. In either case, these two groups are the only heterotrophs known to form colonies.
Although the last common ancestor of the Apoikozoa 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 the Ediacaran 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.
* ^ King, N .; Westbrook, M.J.; Young, S.L.; Kuo, A.; Abedin, M.; Chapman, J.; Fairclough, S.; Hellsten, U.; Isogai, Y.; Letunic, I. (February 14, 2008). "The genome of the choanoflagellate _Monosiga brevicollis_ and the origin of metazoans" . _Nature_. 451 (7180): 783–8. PMC 2562698 _. PMID 18273011 . doi :10.1038/nature06617 . CS1 maint: Multiple names: authors list (link ) * ^ Budd, G. E. & Jensen, S. (2015). "The origin of the animals and a 'Savannah' hypothesis for early bilaterian evolution". Biological Reviews_: n/a. PMID 26588818 . doi :10.1111/brv.12239 . CS1 maint: Multiple names: authors list (link ) * ^ Ryan JF (December 13, 2013). "The genome of the ctenophore _Mnemiopsis leidyi_ and its implications for cell type evolution" . _Science_. 342 (6164): 1242592. PMC 3920664 _. PMID 24337300 . doi :10.1126/science.1242592 . * ^ Pisani, D., Pett, W., Dohrmann, M., Feuda, R., Rota-Stabelli, O., Philippe, H., Lartillot, N., & Wörheide, G. (December 15, 2015). "Genomic data do not support comb jellies as the sister group to all other animals" . Proceedings of the National Academy of Sciences_. 112 (50): 15402. PMC 4687580 . PMID 26621703 . doi :10.1073/pnas.1518127112 . CS1 maint: Multiple names: authors list (link )
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