Germ plasm liberates constraints on somatic development. ~ American biologist Andrew Johnson et al
Germline cells are the special cells of sexual reproduction. In contrast, the bodily cells of a multicellular eukaryote are somatic.
Passed from one generation to the next, germline cells are immortal. Conversely, somatic cells can live only as long as the organism in which they are produced.
Early in animal development a small number of embryonic cells produce primordial germ cells (PGCs). These develop into sperm or eggs. All other cells develop as the soma.
There are 2 ways to produce a PGC: epigenesis or preformation. In epigenesis, PGCs are induced from pluripotent cells via extracellular signals.
In preformation, material in the egg – germ plasm – is inherited directly by a few cells in the embryo. The germ plasm instructs these cells to become PGCs.
Germ plasm functions to segregate PGCs from somatic cells at the inception of development. This relaxes genetic constraints on the mechanisms that govern early somatic development. ~ Andrew Johnson
Some invertebrates, including roundworms and vinegar flies, employ germ plasm for germline cell differentiation. Preformation also repeatedly evolved in certain vertebrates, including frogs and teleosts.
This convergent evolution had a singular purpose: to enhance evolvability. Clades that practice preformation can speciate much more rapidly than those that rely upon epigenesis.
Animals can evolve more rapidly with germ plasm. ~ Andrew Johnson
Epigenesis is a conserved process. Mammals produce PGCs via epigenesis. That is because enhanced evolvability via preformation has a price. Germ plasm leads to an evolutionary dead end.
Salamander-like amphibians, lacking germ plasm, evolved into reptiles, which begat birds and mammals.
Epigenesis is open-ended phenotypically. Preformation is not. Frogs with germ plasm can only evolve into other frogs – many different frogs, but still just frogs.