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Background  Turbellarian species can post-embryonically produce germ line cells from pluripotent stem cells called neoblasts, which enables some of them to switch between an asexual and a sexual state in response to environmental changes. Certain low-molecular-weight compounds contained in sexually mature animals act as sex-inducing substances that trigger post-embryonic germ cell development in asexual worms of the freshwater planarian  Dugesia ryukyuensis  (Tricladida). These sex-inducing substances may provide clues to the molecular mechanism of this reproductive switch. However, limited information about these sex-inducing substances is available.    Results  Our assay system based on feeding sex-inducing substances to asexual worms of  D. ryukyuensis  is useful for evaluating sex-inducing activity. We used the freshwater planarians  D. ryukyuensis  and  Bdellocephala brunnea  (Tricladida), land planarian  Bipalium nobile  (Tricladida), and marine flatworm  Thysanozoon brocchii  (Polycladida) as sources of the sex-inducing substances. Using an assay system, we showed that the three Tricladida species had sufficient sex-inducing activity to fully induce hermaphroditic reproductive organs in asexual worms of  D. ryukyuensis . However, the sex-inducing activity of  T. brocchii  was sufficient only to induce a pair of ovaries. We found that yolk glands, which are found in Tricladida but not Polycladida, may contain the sex-inducing substance that can fully sexualize asexual worms of  D. ryukyuensis .    Conclusions  Our results suggest that within Tricladida, there are one or more common compounds or functional analogs capable of fully sexualizing asexual worms of  D. ryukyuensis ; namely, the crucial sex-inducing substance (hydrophilic and heat-stable, but not a peptide) produced in yolk glands.    Electronic supplementary material  The online version of this article (10.1186/s40851-018-0096-9) contains supplementary material, which is available to authorized users.

zoological letters

A comprehensive comparison of sex-inducing activity in asexual worms of the planarian Dugesia ryukyuensis: the crucial sex-inducing substance appears to be present in yolk glands in Tricladida