''Capsaspora'' is a

monotypic genus In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

containing the single species ''Capsaspora 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 with ''Ministeria vibrans'', a member of the

Filasterea Filasterea is a proposed basal Filozoan clade that includes '' Ministeria'' and '' Capsaspora''. It is a sister clade to the Choanozoa in which the Choanoflagellatea and Animals appeared. Originally proposed by Shalchian-Tabrizi et al. in 2008 ...

clade (see “Taxonomy” below). This amoeboid protist has been pivotal to unravel the nature of the unicellular ancestor of animals, which has been proved to be much more complex than previously thought.

Description

''C. owczarzaki'' was originally described as an amoeba-like “symbiont” of the fresh-water snail ''

Biomphalaria glabrata ''Biomphalaria glabrata'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Planorbidae, the ram's horn snails. ''Biomphalaria glabrata'' is an intermediate snail host for the trematode ' ...

''. The amoebae were obtained from the

haemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...

of snails originally sampled in Puerto Rico. ''C. owczarzaki''’s life cycle comprises 3 different stages with three different cell types, which was reported only recently. 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-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. Cystic cells measure 4 to 5 μm with a double wall: the outer thin, irregular and loosely attached; and the inner thicker, smooth.

Taxonomy

''C. owczarzaki'' is together with ''

Ministeria vibrans ''Ministeria vibrans'' is a bacterivorous amoeba with filopodia that was originally described to be suspended by a flagellum-like stalk attached to the substrate. Molecular and experimental work later on demonstrated the stalk is indeed a flagella ...

'' a member of the

clade. This group is the sister group to a clade comprising

Metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

and

Choanoflagellata The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates, having a funnel shaped collar of interconn ...

, which together form the Filozoa (See Figure 1). ''C. owczarzaki'' was originally described as nucleariids. However, later molecular ribosomal phylogenies placed ''C. owczarzaki'' somewhere closer to animals than the rest of nucleariids. Finally, a multi-gene phylogenetic analysis with several opisthokont taxa clearly showed that ''C. owczarzaki'' is not a nucleariid, but part of the

Holozoa Holozoa is a group of organisms that includes animals and their closest single-celled relatives, but excludes fungi. ''Holozoa'' is also an old name for the tunicate genus ''Distaplia''.'' Because Holozoa is a clade including all organisms mor ...

. This was later on corroborated by phylogenomic analyses, one of which situated it as sister-group to ''Ministeria'' forming the Filasterea clade, which is the sister-group to Choanoflagellatea and Metazoa. Phylogenetic tree

Applications

''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 in

metazoan Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

multicellularity, such as

integrins Integrins are transmembrane receptors that facilitate cell-cell and cell-extracellular matrix (ECM) adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle, ...

, metazoan transcription factors, and protein tyrosine kinases. Moreover, it has relevance to human health because its host, the snail ''

'', is also the intermediate host of the digenean flatworm '' Schistosoma mansoni'', the causative agent of widespread schistosomiasis in humans. ''C. owczarzaki'' not only parasitizes the intermediate host of ''S. mansoni'' but also attacks and kills the sporocysts of the flatworm living inside the snail.

References

{{Taxonbar, from=Q864425 Eukaryote genera Monotypic eukaryote genera Filasterea