Scientific name Apoikozoa | Higher classification Filozoa | |
The Apoikozoa is a monophyletic group of opisthokont eukaryotes that was erected in 2015 by Graham Budd and Sören Jensen to name the clade consisting of the Animalia and Choanoflagellatea. The sister-group relationship between the choanoflagellates and animals has important implications for the origin of the animals.
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Introduction
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.
Evolutionary implications
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.