Genetic analysis of developmental mechanisms in Hydra. XXII. Two types of female germ stem cells are present in a male strain of Hydra magnipapillata.

Three types of interstitial stem cell subpopulation were isolated from Hydra magnipapillata, and their roles in sex determination were examined. A subpopulation of interstitial stem cells restricted to the sperm differentiation pathway was isolated previously from strain nem-1 (male). Another subpopulation restricted to the egg differentiation pathway was also isolated from the same strain. Hydroxyurea treatment was used for isolation in both cases. "Pseudoepithelial hydra" containing only sperm- or egg-restricted stem cell but no other interstitial stem cell types were maintained by force-feeding for 2 years. Sex reversal from egg- to sperm-restricted stem cells occurred three times during this period. Both of these two stem cell types are numerous in the central gastric region of the pseudoepithelial hydra, but absent in the foot region below the budding zone. Foot tissue was cut out from normal nem-1 polyps (male) and allowed to regenerate. The regenerates produced eggs but no sperm upon sex induction. These and other results suggest that the foot tissue contains multipotent stem cells capable of differentiating into eggs during sexual differentiation. These observations suggest that strain nem-1 (male) contains three types of interstitial stem cell subpopulations: (1) sperm-restricted stem cells, (2) egg-restricted stem cells, and (3) multipotent stem cells capable of differentiating into nerve cells, nematocytes, and eggs. Upon sex induction, however, differentiation of eggs by the latter two types is suppressed, and only sperm are produced by the sperm-restricted stem cells. Evidence is presented which suggests that similar "phenotypic males," which normally only produce sperm but contain the stem cell types capable of differentiating into eggs, occur widely in Hydra magnipapillata. A possible relationship between phenotypic male hydra and hermaphroditic hydra is discussed.