HOSHI, M.; KOBAYASHI, K.; ARIOKA, S.; HASE, S.; MATSUMOTO, M.: Switch From Asexual To Sexual Reproduction

Many metazoans convert the reproductive modes presumably depending upon the environmental circumstances and/or the phase of life cycle, but the mechanisms underlying the switching from the asexual to the sexual reproduction, and vice versa, remain unknown in metazoans. We have recently established an experimental system to analyze the mechanism in the planarian, Dugesia ryukyuensis (Kobayashi et al., 1999). Worms of exclusively asexual clone (OH strain) of the species gradually develop ovaries, testes and other sexual organs, then copulate and eventually lay cocoons filled with fertilized eggs, if they are fed with sexually mature worms of Bdellocephala brunnea (an exclusively oviparous species) or with thus sexualized OH worms. This suggests the presence of a species-unspecific sexualizing substance in sexually mature worms. Pairs of experimentally sexualized worms (acquired sexuals) produce an F1 population of sexuals and asexuals, but pairs of F1 sexual worms (innate sexuals) produce only sexual offspring. All regenerants from various portions of innate sexuals become sexuals. In the case of acquired sexuals, head fragments without sexual organs regenerated into asexuals though regenerants from other portions became sexuals. Thus, we conclude that neoblasts, the totipotent stem cells in the planarians, of acquired sexuals remain �asexual� and requires a sexualizing substance for their differentiation into sexual organs and gametes. On the other hand, some, if not all, neoblasts in innate sexuals are �sexual� and can differentiate into sexual organs and gametes in the absence of the sexualizing substance. Besides these findings, the molecular nature of the sexualizing substance and changes in gene expression during the course of sexualization will be discussed.