Capsaspora
Scientific classification
Domain:	Eukaryota
Class:	Filasterea
Order:	Ministeriida
Family:	Capsasporidae Cavalier-Smith, 2008[2]
Genus:	Capsaspora Hertel, Bayne & Loker 2002[1]
Species:	C. owczarzaki
Binomial name
Capsaspora owczarzaki Hertel, Bayne & Loker 2002

Capsaspora is amonotypic genus ofprotists containing the single speciesCapsaspora owczarzaki. C. owczarzaki is a single-celled eukaryote that occupies a key phylogenetic position in our understanding of the origin of animal multicellularity, as one of the closest unicellular relatives to animals. It is, together withMinisteria vibrans, a member of theFilasterea clade. Thisamoeboid protist has been pivotal to unraveling the nature of the unicellular ancestor of animals, which has been proved to be much more complex than previously thought.^[3][4][5]

C. owczarzaki was originally described as an amoeba-like “symbiont” of the fresh-water snailBiomphalaria glabrata.^[6][7] The amoebae were obtained from thehaemolymph of snails originally sampled in Puerto Rico.

C. owczarzaki’s life cycle comprises 3 different stages with three different cell types, which was reported in 2017.^[8] Under culture conditions,C. owczarzaki’s filopodial cells crawl attached to the substrate, with active replication until the end of the exponential growth phase. Then, cells start to detach, retracting the branching filopodia and encysting. During this cystic phase, division is stopped. Alternatively, amoebae can actively aggregate to each other by unknown factors, forming a multicellular, aggregative structure and secreting an unstructured extracellular material that seems to prevent direct cell-to-cell contact.

C. owczarzaki cells, in the filopodial stage, were described as 3 to 5 μm amoebas with a nucleus ⅓ - ½ of the diameter of the cell (containing a central nucleolus), long branched filopodia, mitochondria with flattened cristae, numerous phagosomes, lipid vacuoles, glycogen granules and a Golgi apparatus.^[6][7] Cystic cells measure 4 to 5 μm with a double wall: the outer thin, irregular and loosely attached; and the inner thicker, smooth.^[9]

C. owczarzaki is together withMinisteria vibrans a member of theFilasterea clade.^[10][2] This group is the sister group to a clade comprisingMetazoa andChoanoflagellata, which together form theFilozoa.^[10][2][11]C. owczarzaki was originally described asnucleariids.^[7] However, later molecular ribosomal phylogenies placedC. owczarzaki somewhere closer to animals than the rest of nucleariids.^[12][13][14] Finally, a multi-gene phylogenetic analysis with several opisthokont taxa clearly showed thatC. owczarzaki is not a nucleariid, but part of theHolozoa.^[15][16] This was later on corroborated by phylogenomic analyses,^[17][18] one of which^[18] situated it as sister-group toMinisteria forming the Filasterea clade, which is the sister-group to Choanoflagellatea and Metazoa.^[19][20]

C. owczarzaki is of scientific interest because it is one of the closest unicellular relatives of multicellular animals. Its genome has recently been sequenced and shows several genes involved inmetazoan multicellularity, such asintegrins,^[21] metazoan transcription factors,^[22] and protein tyrosine kinases.^[23] Moreover, it has relevance to human health because its host, the snailBiomphalaria glabrata, is also the intermediate host of thedigenean flatwormSchistosoma mansoni, the causative agent of widespreadschistosomiasis in humans.C. owczarzaki not only parasitizes the intermediate host ofS. mansoni but also attacks and kills thesporocysts of the flatworm living inside the snail.^[24]

• Monotypic eukaryote genera
• Filasterea
• Protist genera

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• Articles with 'species' microformats
• Taxonbars with multiple manual Wikidata items
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