Higher-Level Taxonomy & Evolution Codexery

Holozoa

Clade including animals and their closest single-celled relatives.

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Holozoa primarily comprises unicellular protist lineages such as choanoflagellates, filastereans, ichthyosporeans, and the genera Corallochytrium, Syssomonas, and Tunicaraptor.[1] Animals, their most recent common ancestor, and all lineages more closely related to them than to fungi constitute the clade Holozoa.[1] Using a branch-based definition, Holozoa includes every descendant of the lineage leading to Homo sapiens while excluding those leading to Neurospora crassa.[1]

Quick Facts

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)

Facts from the source article.

Key facts and history

A 2002 paper in Current Biology by Franz Bernd Lang, Charles J. O'Kelly, and co-authors first identified Holozoa as a clade through phylogenomic analysis.[2] To determine where Ichthyosporea fits in the tree of life, the researchers analyzed the complete mitochondrial genomes of Monosiga brevicollis (a choanoflagellate) and Amoebidium parasiticum (an ichthyosporean).[2] The name Holozoa, translating to 'whole animal', was chosen to reflect the broad animal lineage encompassed by this group.[2] Opisthokonta, a major eukaryotic supergroup, contains both Holozoa and Holomycota (the clade uniting fungi and their protist relatives).[3]

Approximately 1070 million years ago, the Holozoa lineage split from its opisthokont ancestor.[3] The choanoflagellate Monosiga brevicollis provided the first genome sequence from a unicellular holozoan.[4] This genome is roughly 41.6 Mbp in size and encodes about 9200 protein-coding genes.[4] Bicellum brasieri*, a microscopic freshwater fossil dated to one billion years old, may represent the oldest known holozoan.[5]

What the research sources say

This suggests that the origin of multicellular animals involved pre-existing genes that were adapted or utilized in new ways, not solely the appearance of new genes.[6] Analyses of single-celled holozoan genomes show that many traits once considered animal-specific are also present in these unicellular organisms.[6] Unicellular holozoan relatives already exhibit animal-like processes, including sexual reproduction (e.g., in Salpingoeca rosetta) and various forms of multicellular differentiation.[7] Oceanic environmental DNA surveys have uncovered multiple new holozoan lineages; one unnamed clade, provisionally called MASHOL (marine small Holozoa), may represent an entirely new branch.[8]

Fossil "embryos" from the Ediacaran Doushantuo Formation, once thought to be early animals, are now considered non-animal protists belonging to Holozoa.[9] Some researchers argue that these fossils lack key metazoan features like tissue differentiation and evidence of nearby juveniles or adults.[9]

References

  1. Wikipedia, "Holozoa" (CC BY-SA 4.0), which cites: "Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes", Journal of Eukaryotic Microbiology, 2019, p. 4–119; Hehenberger, "Novel Predators Reshape Holozoan Phylogeny and Reveal the Presence of a Two-Component Signaling System in the Ancestor of Animals", Current Biology, 2017, p. 2043–2050.e6; "New Lineage of Microbial Predators Adds Complexity to Reconstructing the Evolutionary Origin of Animals", Current Biology, 2020, p. 4500–4509 (original not read by us) en.wikipedia.org
  2. Wikipedia, "Holozoa" (CC BY-SA 4.0), which cites: "The Closest Unicellular Relatives of Animals", Current Biology, 2002, p. 1773–1778 (original not read by us) en.wikipedia.org
  3. Wikipedia, "Holozoa" (CC BY-SA 4.0), which cites: "Cold climate adaptation is a plausible cause for evolution of multicellular sporulation in Dictyostelia", Scientific Reports, 2020 (original not read by us) en.wikipedia.org
  4. Wikipedia, "Holozoa" (CC BY-SA 4.0), which cites: "The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans", Nature, 2008, p. 783–788 (original not read by us) en.wikipedia.org
  5. Wikipedia, "Holozoa" (CC BY-SA 4.0), which cites: Strother, "A possible billion-year-old holozoan with differentiated multicellularity", Current Biology, April 2021, p. 2658–2665.e2 (original not read by us) en.wikipedia.org
  6. Wikipedia, "Holozoa" (CC BY-SA 4.0), which cites: "The origin of Metazoa: a unicellular perspective", Nature Reviews Genetics, 2017, p. 498–512; Hehenberger, "Novel Predators Reshape Holozoan Phylogeny and Reveal the Presence of a Two-Component Signaling System in the Ancestor of Animals", Current Biology, 2017, p. 2043–2050.e6 (original not read by us) en.wikipedia.org
  7. Wikipedia, "Holozoa" (CC BY-SA 4.0), which cites: "The origin of Metazoa: a unicellular perspective", Nature Reviews Genetics, 2017, p. 498–512 (original not read by us) en.wikipedia.org
  8. Wikipedia, "Holozoa" (CC BY-SA 4.0), which cites: Arroyo, "Gene Similarity Networks Unveil a Potential Novel Unicellular Group Closely Related to Animals from the Tara Oceans Expedition", Genome Biology and Evolution, September 2020, p. 1664–1678 (original not read by us) en.wikipedia.org
  9. Wikipedia, "Holozoa" (CC BY-SA 4.0), which cites: "Cell differentiation and germ–soma separation in Ediacaran animal embryo-like fossils", Nature, September 2014, p. 238–241; "Fossilized Nuclei and Germination Structures Identify Ediacaran "Animal Embryos" as Encysting Protists", Science, 2011, p. 1696–1699 (original not read by us) en.wikipedia.org

Each numbered sentence above was checked against the source it cites before publication. Sentences citing Wikipedia adapt text from the English Wikipedia article Holozoa (contributors), used under CC BY-SA 4.0; this entry is available under the same licence. Other sources were used for facts only; no text or images are copied from them.

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Sources

Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.

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