Holozoa
Clade including animals and their closest single-celled relatives.
Holozoa is a biological grouping that brings together animals and their nearest single-celled relatives, while leaving out fungi and everything else. The name comes from Greek words meaning "whole animal." This clade contains over 1.5 million species, though not all get their energy by consuming other organisms in the typical sense—many, like parasitic tapeworms and ichthyosporeans, absorb nutrients directly from their hosts. Only a few hundred of these species are single-celled, with the total number of described unicellular holozoans (choanoflagellates, filastereans, ichthyosporeans) being well under 300. The group includes animals (Metazoa) and several protist lineages: Choanoflagellata, Filasterea, Pluriformea, and Ichthyosporea. Holozoa sits within the larger supergroup Opisthokonta, which also includes fungi. An older name, Choanofila, was used for a similar grouping, but it’s no longer favored because it was paraphyletic—it included choanoflagellates and sometimes sponges but excluded other animals, not because it left out animals entirely.
The single-celled members of Holozoa are key to figuring out how multicellular animals evolved from unicellular ancestors. Recent studies of their genomes have clarified the relationships among these lineages and offered clues about the origins of multicellularity. Some fossils once thought to be early animals have been reinterpreted as holozoan protists.
Holozoa is defined in a branch-based way: it includes all descendants of the common ancestor shared by humans (an animal) that are more closely related to us than to the fungus *Neurospora crassa*. Besides animals, the clade contains mostly unicellular protists with varied forms: choanoflagellates, filastereans, ichthyosporeans, and the genera *Corallochytrium*, *Syssomonas*, and *Tunicaraptor*.
Choanoflagellates, with about 250 species, are the closest living relatives of animals. They are free-living, single-celled or colonial flagellates that feed on bacteria using a collar of microvilli. This collar looks very much like the collar cells of sponges, a similarity that has been noted since the 1800s. The colonial choanoflagellate *Proterospongia* was once thought to be linked to sponge origins. The relationships of the other single-celled holozoans only started to become clear in the 1990s.
Ichthyosporea (also called Mesomycetozoea) includes around 40 species
- Composition
- Metazoa (animals), Choanoflagellata, Filasterea, Pluriformea, Ichthyosporea, and Tunicaraptor
- Number of species
- More than 1.5 million, including a few hundred unicellular species (well under 300 described)
- Clade definition
- All relatives of Homo sapiens but not Neurospora crassa
- Divergence time
- Around 1070 million years ago from opisthokont ancestor
- Key subgroups
- Filozoa (choanoflagellates, animals, filastereans) and Choanozoa (choanoflagellates and animals)
Quick Facts
- Taxon
- Holozoa
- Subdivision
- Pluriformea / Ichthyosporea (=Mesomycetozoea) / Filozoa / Filasterea / Choanozoa / Choanoflagellata / Animalia / Incertae sedis / Tunicaraptor / Bicellum
Facts from the source article.
Lore & Background
Holozoa is a clade that includes animals and their closest relatives, as well as their common ancestor, but excludes fungi. It is defined on a branch-based approach as the clade encompassing all relatives of Homo sapiens (an animal), but not Neurospora crassa (a fungus). Holozoa, besides animals, primarily comprises unicellular protist lineages of varied morphologies such as choanoflagellates, filastereans, ichthyosporeans, and the distinct genera Corallochytrium, Syssomonas, and Tunicaraptor. Choanoflagellata, with around 250 species, are the closest living relatives of animals. They are free-living unicellular or colonial flagellates that feed on bacteria using a characteristic 'collar' of microvilli. The collar of choanoflagellates closely resembles sponge collar cells, leading to theories since the 19th century about their relatedness to sponges.
Reader's Guide
Holozoa is significant because it encompasses the evolutionary transition from single-celled to multicellular life, specifically the origin of animals. Recent genomic studies have shed light on the evolutionary relationships between the various holozoan lineages, revealing insights into the origins of multicellularity. The genetic content of single-celled holozoans has revealed that many genetic characteristics previously thought as unique to animals can also be found in these unicellular relatives, suggesting that the origin of multicellular animals did not happen solely because of the appearance of new genes, but because of pre-existing genes that were adapted or utilized in new ways. For example, adhesion proteins, ECM-related proteins, and some signal transduction proteins and transcription factors present in animals are also found in unicellular holozoans. The fossil record includes a billion-year-old freshwater microscopic fossil named Bicellum brasieri, possibly the earliest known holozoan, showing two differentiated cell types. Proposed Ediacaran fossil 'embryos' have been reinterpreted as non-animal protists within Holozoa.
Did You Know?
- Holozoa includes more than 1.5 million species of purely heterotrophic organisms, including around 300 unicellular species.
- Choanoflagellates, with around 250 species, are the closest living relatives of animals.
- The first sequenced unicellular holozoan genome was that of Monosiga brevicollis, a choanoflagellate, measuring around 41.6 mega–base-pairs.
- A billion-year-old fossil named Bicellum brasieri is possibly the earliest known holozoan.
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