Switching from asexual to sexual reproduction in the planarian dugesia ryukyuensis

We have established an assay system for sexual induction in an exclusively fissiparous strain (OH strain) of Dugesia ryukyuensis by feeding them with matured worms of Bdellocephala brunnea, an exclusively oviparous species. In this assay system, the asexual worms gradually differentiate the ovaries, testes, genital pore (copulatory apparatus) and yolk glands in this order, and eventually mate and lay cocoons. Almost all worms hatching from the cocoons become sexual. It was found that sexual induction has a point-of-no-return; the worms before reaching this point return to the asexual condition unless feeding on B. brunnea is maintained (reversible phase), while those beyond the point develop into fully sexual worms even if feeding on B. brunnea is discontinued (irreversible phase). Most worms just beyond the point-of-no-return still undergo fission for a few weeks, even though they have already been committed to become sexual. However, once they reach a certain stage, fission stops and they reproduce exclusively sexually. Therefore, it is likely that the process of switching from asexual to sexual reproduction in planarians consists of a series of crucial events.