Hydractinia is a colonial hydrozoan that forms a stolon network covered with
polyps. The colonies consist primarily of sexual polyps and feeding polyps. Colonies
arise from a single polyp formed by metamorphosis of a planula larva. In response to
appropriate environmental signals, neuropeptides of the GLWamide family are
secreted into larval tissue to induce metamorphosis (Schmich et al., 1998). Recent
experiments by Seipp et al. (2001) have demonstrated that metamorphosis is
accompanied by degradation of large parts of the planula larva by apoptosis. Figure 2
shows large numbers of TUNEL-positive cells in a planula larva undergoing
metamorphosis. Apoptotic cells were not only removed by phagocytosis but also shed
into the environment. Metamorphosis was accompanied by a dramatic increase in
caspase activity measured with the fluorogenic caspase substrate DEVD-AMC (Seipp,
Böttger, unpublished observations).
Figure 2. Apoptosis during metamorphosis of Hydractinia planula larva to primary polyp.
The schematic diagram illustrates the process of metamorphosis, which takes about 24 h to
complete. The micrographs show, respectively, a TUNEL-stained larva before metamorphosis
and a 3-h metamorphosing larva containing large numbers of apoptotic cells. A planula larva
contains approximately 10^4 cells. Scale bar: 20 μm.
EVOLUTION OF CELL DEATH 155