The role of stem cell strategies in coping with environmental stress and choosing between alternative reproductive modes: Turbellaria rely on a single cell type to maintain individual life and propagate species

To contribute to a better understanding of the interrelationships of different forms of asexual and sexual reproduction in the Turbellaria, three selected species were studied with respect to the cellular correlates underlying the diverse reproductive strategies. The exclusively asexual freshwater triclad Dugesia tahitiensis holds the top position in regenerative and fissioning capacities within the Tricladida and, in addition, has: the must neoblasts (stem or regenerative cells) known within this taxon. A reproductive rate of roughly one new specimen originating from a parent animal every third day at optimal experimental conditions correlated well with the speed of head regeneration ranging from 43 to 59 h at 23 degreesC. The process of fission (architomy) was observed and its dependence on population density and melatonin content was studied. In two new marine representatives of the Macrostomorpha from the Adriatic Sea, the distribution of neoblasts in the body and their fate during regeneration were traced by a recently developed labelling method with 5-bromo-2 ' -deoxyuridine. In Macrostomum, a sexual species with poor regenerative potential, the proliferating cells were localized predominantly along the nerve cords and migrated to their final Location after mitosis. Microstomum n. sp., with asexual reproduction, showed a less concentrated pattern of proliferating stern cells, being localized in the future areas of organ primordia forming before fission in the process of paratomy. The findings are discussed with respect to cell biological, ecological and evolutionary implications.