Sphaeroforma arctica

Sphaeroforma arctica
Sphaeroforma arctica stained with phalloidin and DAPI
Scientific classification
Domain:	Eukaryota
(unranked):	Holozoa
Class:	Ichthyosporea
Order:	Ichthyophonida
Family:	Creolimacidae
Genus:	Sphaeroforma
Species:	S. arctica
Binomial name
Sphaeroforma arctica Jøstensen, Sperstad, Johansen & Landfald, 2002[1]

Sphaeroforma arctica, is a unicellular eukaryote with a pivotal position in the tree of life. It was first isolated from the arctic marine amphipod Gammarus setosus.^[2][1] Like other Ichthyosporeans such as Creolimax and Abeoforma, Sphaeroforma arctica are spherical cells characterized with their capacity to grow into multi-nucleated coenocytes (multi-nucleates cell). However, a unique feature of S. arctica, is that no obvious budding, hyphal, amoeboid, sporal or flagellated growth stages have been observed in laboratory growth conditions.

Sphaeroforma is a member of the Ichthyosporea clade, which is the earliest branching holozoan lineage. It is a key organism to understand the origin of animals.

Sphaeroforma arctica grows easily in marine broth, with the particularity of growing synchronously.^[3][4] From a cell with 1 single nucleus (although newborn cells have been shown to contain 2 or even 4 nuclei, too^[4]), it can grow to reach a 128 nuclei cell before undergoing cellularization (the process by which the coenocytes split up to give new-born cells). Cellularization involves coordinated inward plasma membrane invaginations dependent on an actomyosin network that leads to the formation of a polarized layer of cells resembling an epithelium. This process is associated with tightly regulated transcriptional activation of genes involved in cell adhesion.^[5]

Moreover, it has been recently shown that S. arctica contains miRNA as well as the complex miRNA processing machinery. Taken together, Sphaeroforma arctica is a great model organism from both evolutionary and cell biology perspectives.^[6]