Holozoa
Clade including animals and their closest single-celled relatives, excluding fungi.
Holozoa is a biological clade that groups together animals and their nearest single-celled relatives, while leaving out fungi and everything else. This clade contains over 1.5 million species, all of which are heterotrophic—they get energy from other organisms—and includes roughly 300 unicellular species. Its main subgroups are Metazoa (animals) and several protist lineages: Choanoflagellata, Filasterea, Pluriformea, and Ichthyosporea. Holozoa belongs to the larger eukaryotic supergroup Opisthokonta, which also includes fungi. The single-celled members of Holozoa are key to studying how multicellular animals evolved from unicellular ancestors.
- Composition
- Animals, Choanoflagellata, Filasterea, Pluriformea, Ichthyosporea, and Tunicaraptor
- Number of species
- More than 1.5 million, including around 300 unicellular species
- Closest living relatives of animals
- Choanoflagellates (around 250 species)
- Divergence from opisthokont ancestor
- Around 1070 million years ago
- First sequenced unicellular holozoan gen
- Monosiga brevicollis (41.6 Mbp, ~9200 coding genes)
- Key subgroups
- Filozoa (choanoflagellates, animals, filastereans) and Choanozoa (choanoflagellates and animals)
Lore & Background
Holozoa is defined on a branch-based approach as the clade encompassing all relatives of Homo sapiens (an animal) but not Neurospora crassa (a fungus). Its composition includes unicellular protist lineages of varied morphologies: choanoflagellates, filastereans, ichthyosporeans, and the distinct genera Corallochytrium, Syssomonas, and Tunicaraptor. Choanoflagellates, with around 250 species, are the closest living relatives of animals and possess a characteristic 'collar' of microvilli that closely resembles sponge collar cells. The affinities of other single-celled holozoans only began to be recognized in the 1990s.
Reader's Guide
Holozoa is significant because it provides the evolutionary framework for understanding the transition from single-celled to multicellular life, particularly the origin of animals. Genomic studies of unicellular holozoans have revealed that many genetic characteristics previously thought unique to animals—such as adhesion proteins (cadherin, integrin), ECM-related proteins (laminins, collagens), and certain transcription factors (p53, T-box)—are present in these single-celled relatives. This suggests that the origin of multicellular animals involved co-option of pre-existing genes rather than solely the appearance of new ones. The fossil record includes Bicellum brasieri, a billion-year-old freshwater fossil possibly representing the earliest known holozoan, showing two differentiated cell types. Additionally, proposed Ediacaran fossil 'embryos' from the Doushantuo Formation have been reinterpreted as non-animal holozoan protists. Uncertainty remains about the relationships of basal groups like Ichthyosporea and Pluriformea, and the position of the newly discovered Tunicaraptor is still unresolved.
Did You Know?
- The name Holozoa comes from Ancient Greek for 'whole animal'.
- Choanofila was previously used for a similar group but is now discouraged because it excludes animals and is paraphyletic.
- Environmental DNA surveys have revealed a potential new holozoan lineage tentatively named MASHOL (marine small Holozoa).
- The choanoflagellate Salpingoeca rosetta exhibits sexual reproduction and gametogenesis.
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