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 comprising Metazoa and Choanoflagellata, which together form the Filozoa  (See Figure 1).
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 multicellularity, such as integrins, metazoan transcription factors, and protein tyrosine kinases. Moreover, it has relevance to human health because its host, the snail Biomphalaria glabrata, 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.
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