Figure 1.

Competition between non-germ and germline stem cells can lead to three classes of outcomes. If non-germ cells dominate and suppress the proliferation of germ cells, animals can be expected to be vegetative reproducers with obligate WBR to replace missing structures following fission or damage (left branch of diagram). If germline cells dominate and suppress the proliferation or totipotency of non-germ cells, animals can be expected to be obligate gametic reproducers with at least one sex, the female, if parthenogenic (right branch of diagram). If neither dominates, or if dominance is only partial, animals can be expected to be facultatively sexual, and capable of vegetative reproduction with WBR, parthenogenesis, or both. Similar competitive mechanisms may operate in plants, though we do not consider these here