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.[1] [2] 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), 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]