Figure 1

The classical model of germline determination and its controversies. (A) Germline determination by preformation. The germ plasm present in the zygote is inherited by the primordial germ cells (PGCs) and not by the rest of the somatic cells derived from it. The PGCs give rise to germ cells (GCs) and these in turn to sperm and oocytes. Somatic cells cannot affect the germline, and, therefore, the Weismann barrier can be easily imagined in this model. Both germline continuity and germ plasm continuity are observed. (B) Germline determination by epigenesis. The zygote gives rise only to somatic cells, from which a subpopulation is specified by epigenetic signals to become the PGCs. The Weismann barrier is, therefore, broken by these somatic cells, and neither the germline nor the germ plasm is continuous. (C) In animals classically thought to follow the epigenesis model as diverse as annelids and sea urchins germ plasm components are found in the zygote and inherited by cells with both somatic and germ potential. These cells give rise to the PGCs but also to somatic tissues, and often have stem cell-like properties. The Weismann barrier is broken by these cells, since they are classically considered to be somatic. However, even though the germline is considered to be discontinuous, germ plasm continuity can be observed flowing from the zygote to these cells and forth to the PGCs. (A-C) Germ plasm component expression is depicted in red-magenta colors and green dots. z, zygote; pgc, primordial germ cells; gc, germ cells; oc, oocyte; sc, somatic cell.