Capsaspora is a monotypic genus containg the single species Capsaspora owczarzaki, a single-celled eukaryote which is a symbiont in the haemolymph of the tropical freshwater snail Biomphalaria glabrata. Nowadays, the knowledge about Capsaspora has hugely increased, reaching to the point that Broad Institute of MIT and Harvard (Cambridge, USA) have sequenced its genome (http://www.broadinstitute.org/annotation/genome/multicellularity_project/MultiHome.html)
C. owczarzaki cells were described as 3 to 5 μm amoebas with a nucleus ⅓ - ½ of thediameter of the cell (containing a central nucleolus), long unbranched pseudopodia, mitochondria with flattened cristae, numerous phagosomes, lipid vacuoles, glycogen granules and a Golgi apparatus  . Cells could encyst in response to crowding, generating 4 to 5 μm cysts with a double wall: the outer thin, irregular and loosely attached; and the inner thicker, smooth.
C. owczarzaki is together with Ministeria vibrans a member of the Filasterea clade  . This group is probably the sister group to a clade comprised of Metazoa and Choanoflagellata, which together form the Filozoa    (See Figure 1).
Because of being one of the closest unicellular relatives of multicellular animals, C. owczarzaki is having huge scientific interest. The genome of Capsaspora has recently been sequenced. It has been shown that Capsaspora has several genes involved in metazoan multicellularity, such as integrins, several transcription factors , or protein tyrosine kinases . Moreover, Capsaspora has relevance to human health, because its host, the snail Biomphalaria glabrata, is also the intermediate host of thedigenean flatworm Schistosoma mansoni,the causative agent of widespread schistosomiasis in humans. C. owczarzaki not only parasitizes theintermediate host of S. mansoni but also attacks and kills the sporocysts of the flatworm living inside the snail.
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