Isolation of the choanocyte in the fresh water sponge, Ephydatia fluviatilis and its lineage marker, Ef annexin

In order to investigate the cellular system of the freshwater sponge, Ephydatia fluviatilis, we isolated a molecular marker for the most prominent cell type, the choanocyte. After feeding sponge with fluorescent beads, fluorescent-labeled choanocytes were collected by fluorescence activated cell sorting (FACS). By protein profiling choanocyte and archeocyte (stem cell)-rich fractions, proteins characteristic of choanocyte were identified. The partial amino-acid sequence of one of the proteins characteristic of choanocyte matches the deduced amino-acid sequence of sponge expression tag (EST) clones and mouse annexin VII. These EST clones overlap and encode a protein, designated Ef annexin, which includes four annexin domains. Whole mount in situ hybridization shows Ef annexin expression in chamber-forming choanocytes in 7-day-old sponge, leading us to conclude that Ef annexin can be used as a choanocyte marker. In the early development stage, Ef annexin expression can be detected in both large single cells, characteristic of archeocytes, and cells forming 2-, 4- and multiple-cell clusters. These results indicate that Ef annexin is initially expressed in the choanocyte-committed archeocyte which then undergoes several mitotic cell divisions to form a choanocyte chamber. This suggests that the single choanocyte chamber essentially originates from a single archeocyte.