The hydrozoan life cycle: a small secreted protein is involved in specification of the polyp stage

In the life cycle of many Hydrozoa (Cnidaria), the pelagic medusa is produced by a budding process from the sessile polyp stage. In order to investigate the molecular mechanisms controlling medusa budding, we screened a cDNA library made from Podocoryne carnea (Cnidaria, Hydrozoa, Anthomedusa) for life-stage-specific genes. As a result of this screening, a gene called Pol-1 was isolated. The deduced amino acid sequence of Pol-1 suggests that the translation product is a small, very acidic secreted protein, with repeated domains. This novel gene is specifically expressed in epithelial muscle cells of polyps and early medusa buds of Podocoryne. In more advanced medusa bud stages and liberated medusae, Pol-1 expression is not observed. However, reexpression can be induced experimentally in medusa buds where Pol-1 transcription normally is switched off. When isolated and traumatized they will dedifferentiate and transform back into polyps. Reexpression of the Pol-1 message is observed 2 days after traumatization, when the medusa-typical structures have dedifferentiated but polyp-structures have not yet formed. These results indicate that the Pol-1 gene is involved in the specification of the polyp stage. The pattern of expression of Pol-1 further suggests that, during the medusa budding process, a tightly controlled switch from polyp- to medusa-specific gene expression occurs which is reversible in the case of regeneration.