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 differ ent forms of asexual and sexual reproduction in the Turbellaria, three sele cted species were studied with respect to the cellular correlates underlyin g the diverse reproductive strategies. The exclusively asexual freshwater t riclad Dugesia tahitiensis holds the top position in regenerative and fissi oning capacities within the Tricladida and, in addition, has: the must neob lasts (stem or regenerative cells) known within this taxon. A reproductive rate of roughly one new specimen originating from a parent animal every thi rd 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 f ission (architomy) was observed and its dependence on population density an d melatonin content was studied. In two new marine representatives of the M acrostomorpha from the Adriatic Sea, the distribution of neoblasts in the b ody and their fate during regeneration were traced by a recently developed labelling method with 5-bromo-2 ' -deoxyuridine. In Macrostomum, a sexual s pecies with poor regenerative potential, the proliferating cells were local ized predominantly along the nerve cords and migrated to their final Locati on after mitosis. Microstomum n. sp., with asexual reproduction, showed a l ess concentrated pattern of proliferating stern cells, being localized in t he future areas of organ primordia forming before fission in the process of paratomy. The findings are discussed with respect to cell biological, ecol ogical and evolutionary implications.