Holozoa
The clade that includes animals and their closest single-celled relatives, excluding fungi.
Holozoa is a clade of organisms that includes animals and their closest single-celled relatives, but excludes fungi and all other organisms. Together they amount to more than 1.5 million species of purely heterotrophic organisms, including around 300 unicellular species. It consists of various subgroups, namely Metazoa (or animals) and the protists Choanoflagellata, Filasterea, Pluriformea and Ichthyosporea. Along with fungi and some other groups, Holozoa is part of the Opisthokonta, a supergroup of eukaryotes.
- 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
The holozoan protists play a crucial role in understanding the evolutionary steps leading to the emergence of multicellular animals from single-celled ancestors. Recent genomic studies have shed light on the evolutionary relationships between the various holozoan lineages, revealing insights into the origins of multicellularity. Some fossils of possible metazoans have been reinterpreted as holozoan protists.
Reader's Guide
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. The choanoflagellates, animals and filastereans group together as the clade Filozoa. Within Filozoa, the choanoflagellates and animals group together as the clade Choanozoa. Uncertainty remains around the relationship of the two most basal groups, Ichthyosporea and Pluriformea. They may be sister to each other, forming the putative clade Teretosporea. Alternatively, Ichthyosporea may be the earliest-branching of the two, while Pluriformea is sister to the Filozoa clade. The position of Tunicaraptor is still unresolved.
Did You Know?
- Choanofila was previously used as a name for a group similar in composition to Holozoa, but its usage is discouraged now because it excludes animals and is therefore paraphyletic.
- The first sequenced unicellular holozoan genome was that of Monosiga brevicollis, a choanoflagellate, measuring around 41.6 Mbp.
- A billion-year-old freshwater microscopic fossil named Bicellum brasieri is possibly the earliest known holozoan.
- Environmental DNA surveys have revealed a potential new holozoan lineage tentatively named MASHOL (for 'marine small Holozoa').
More in Eukaryote Groups And Taxonomy 1-24
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