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

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 <italic>Dugesia ryukyuensis</italic> (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 <italic>D. ryukyuensis</italic> is useful for evaluating sex-inducing activity. We used the freshwater planarians <italic>D. ryukyuensis</italic> and <italic>Bdellocephala brunnea</italic> (Tricladida), land planarian <italic>Bipalium nobile</italic> (Tricladida), and marine flatworm <italic>Thysanozoon brocchii</italic> (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 <italic>D. ryukyuensis</italic>. However, the sex-inducing activity of <italic>T. brocchii</italic> 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 <italic>D. ryukyuensis</italic>. Conclusions: Our results suggest that within Tricladida, there are one or more common compounds or functional analogs capable of fully sexualizing asexual worms of <italic>D. ryukyuensis</italic>; namely, the crucial sex-inducing substance (hydrophilic and heat-stable, but not a peptide) produced in yolk glands. [ABSTRACT FROM AUTHOR]