ABSTRACT
In metazoans, the separation of somatic and germ cells, referred to as the determination of primordial germ cells, occurs via three mechanisms: preformation, epigenesis, and postembryonic germ cell development. Some metazoans occasionally switch between asexual and sexual reproduction based on environmental changes, life cycle phases, or both. In general, asexual animals possess pluripotent stem cells and can regenerate lost body parts by asexual reproduction. Planarians can undergo “degrowth” under starvation, since their body size is homeostatically regulated by “cell turnover” from neoblasts. The regional distribution of d-amino acids is dependent on the expression of amino acid racemases and d-amino acid degrading enzymes. The crucial sex-inducing substances needed to overcome the point-ofno-return in asexual worms of D. ryukyuensis may be contained in worms of Tricladida, but not in those of Polycladida. The crucial sex-inducing substances we have targeted so far are able to activate all necessary endocrine systems to induce postembryonic reproductive development.
