We have isolated from the hydrozoan Podocoryne carnea a genomic clone and the corresponding cDNA of a homeobox gene. This gene, called cnox1-Pc, is most closely related to the lab class of homeobox genes. In the life cycle of P. carnea, which involves an asexually reproducing, sedentary polyp and a sexual planktonic medusa stage, we found cnox1-Pc transcripts specifically expressed in the medusa stage. Expression studies on isolated medusa organs and tissues indicate that the message is absent in the tentacles and the feeding and sexual organ of the medusa and present in the striated muscle cells. During medusa bud development, expression of cnox1-Pc can be correlated with the development of the striated muscle tissue, a cell type only found in the medusa. The results suggest that cnox1-Pc is involved in pattern formation of the medusa of P. carnea and might have a regulatory role in the differentiation of striated muscle.
An antiserum to transdifferentiated striated muscle cells from the medusa of Podocoryne carnea was prepared and used to screen a λ gt11-expression library prepared from gonozoids of P. carnea. We isolated a cDNA clone termed Pod-EPPT with at least 63 tandem repeats of the tetrapeptide-motive glu-pro-pro-thr, named Pod-EPPT. Using Pod-EPPT as a molecular marker for head quality the morphological relationship between the two metagenic life stages of this hydroid, the polyp and the medusa, was studied. In situ hybridization demonstrated that expression of the gene corresponding is restricted to secretory cells in the endoderm of the oral hypostome region of polyps and medusae and, presumably, to progenitor cells of this type. Cells expressing Pod-EPPT could not be observed in the larval stage. During head regeneration in polyps, Pod-EPPT expression is upregulated soon after head removal in previously non-expressing cells and in newly differentiating secretory cells. This activation of a head-specific gene precedes the morphologically obvious events of head regeneration. Pod-EPPT is one of the genes that are activated during manubrium (mouth) regeneration from experimentally combined subumbrellar plate endoderm and striated muscle of the medusa.
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