Genetic analysis of developmental mechanisms in hydra. XX. Cloning of interstitial stem cells restricted to the sperm differentiation pathway in Hydra magnipapillata.

Hydra magnipapillata polyps containing a subpopulation of interstitial stem cells restricted to the germline differentiation pathway were obtained. Chim-C1 is a chimeric strain produced by combining wild-type epithelial cell lineages with a temperature-sensitive interstitial cell lineage. It grows normally at 18 degrees C. When cultured at 25 degrees C, many polyps lost interstitial cells and their differentiation products (nerve cells and nematocytes), and subsequently turned into epithelial hydra unable to move or feed. Some polyps, however, turned into an unexpected type, termed "pseudo-epithelial hydra." These polyps resembled epithelial hydra in the absence of nerve cells or nematocytes in the tissue and in their inability to move or feed. In contrast to epithelial hydra, however, their tissue contained proliferating interstitial cells. Similar pseudo-epithelial hydra were also produced from another strain, nem-1, by means of hydroxyurea treatment. Clones of pseudo-epithelial hydra were maintained through force-feeding over 130 days for chim-C1 and over 2 years for nem-1. In both cases, interstitial cells proliferated throughout the period without producing any nerve cells or nematocytes. These interstitial cells, however, differentiated into sperm. Thus, the interstitial cells present in pseudo-epithelial hydra were able to differentiate into gametic cells but not into somatic cells (nerve cells and nematocytes). These observations suggest that, as Littlefield (1985, Dev. Biol. 112, 185-193) has shown for H. oligactis, the interstitial stem cell population in H. magnipapillata includes a subpopulation which can differentiate only into gametic cells.