71.5 Saturday, Jan. 5 Tracing cell identity through metamorphosis in a freshwater sponge larva WINDSOR, P.J.*; LEYS, S.P.; Univ. Alberta; Univ. Alberta firstname.lastname@example.org
Sponges, like other animals, show anterior-posterior polarity especially in their larval stages. However whether larval polarity is carried over to the adult is not yet known, nor is the fate of any of the cells from the larva. We used diI injection coupled with cell labelling using CMFDA and bisection experiments to determine the fates of different regions of the larva of the freshwater sponge, Eunapius fragilis. This larva has a distinct anterior end with a large larval cavity, and a posterior end that contains amoeboid cells, feeding chambers and spicules; the entire outer surface consists of columnar ciliated cells. We cultured anterior and posterior halves separately to determine the fates of each half. The anterior half of the larva settles and flattens into a thin sheet, but lacks adult structures such as canals and an osculum. Without the anterior half, the posterior half is not competent to settle. We tattooed the anterior and posterior poles with diI to define specific cells that arose from these regions. The anterior pole largely gives rise to basopinacocytes that adhere to the substrate during settling, while sclerocytes derive from the posterior pole. Interestingly, the osculum, the terminus of the aquiferous system axis in the adult, is also derived from posterior cells. We dyed the outer layer of columnar ciliated cells in swimming larvae with CMFDA. At metamorphosis these cells appear to be resorbed. Choanocyte chambers of the juvenile sponge arise from multinucleated cells very soon after settlement. Importantly, we show larval polarity is reversed in the adult – the posterior pole gives rise distinctly to cells that form the osculum – confirming a long held concept that sponge polarity is inverted relative to other metazoa.