Figure 6

The germ-to-soma boundary according to the classical view and the PriSC hypothesis. (A-C) The dotted blue and red lines depict the classical and the proposed germ-to-soma boundary respectively. The black line depicts the Weismann barrier. Green arrows depict the origin of new generations. Green boxes indicate the source of contributions to forthcoming generations. (A) In animals with unlimited PriSCs these are classically considered to be somatic, breaking the germline continuity. The proposed germ-to-soma boundary makes both germline and germ plasm continuous and consistent with the Weismann barrier. New generations originate by sexual reproduction (large green arrow) or by asexual reproduction (small green arrow). The PriSCs contribute to forthcoming generations by enabling regeneration after asexual division, by embryonically generating the GCs or by regenerating GCs if these are lost. (B) In animals with restricted PriSCs the classical germ-to-soma boundary has to separate the PriSCs according to their germ or somatic potential and similarly breaks the germline continuity. The proposed germ-to-soma boundary again makes both germline and germ plasm continuous and consistent with the Weismann barrier. New generations originate only by sexual reproduction and the PriSCs only contribute to forthcoming generations by generating GCs. It is still unclear if PriSCs can regenerate GCs if lost (question mark). (C) In animals with rudimentary PriSCs the classical germ-to-soma boundary coincides with the proposed germ-to-soma boundary and the Weismann barrier, with both a continuous germline and germ plasm. New generations originate only by sexual reproduction and the PriSCs only contribute to forthcoming generations by generating PGCs. GCs, germ cells; PGCs, primordial germ cells; PriSCs, primordial stem cells.