Our knowledge of the D lineage in Crepidula suggests a fuzzy-logical mode of lineage specification in which phenotypic divergence precedes a definitive cell fate decision.
A similar sev-independent mode of R7 specification has been observed for other negative regulators of the EGFR pathway such as gap1 and sprouty [16]-[18] [16]-[18]
Here, we propose that this 'stochastic' mode of PGC specification is conserved in vertebrates, including non-rodent mammals.
In the preformistic mode of germline specification, the cells in the embryo that inherit this maternal 'germplasm' passively inherit germline identity.
Comparisons across vertebrates support the conclusion that endoderm is specified in two distinct temporal phases in the catshark as in all major osteichthyan lineages, in line with an ancient origin of a biphasic mode of endoderm specification in gnathostomes.
From an evolutionary standpoint, the reiteration of a biphasic mode of endoderm specification now found in chondrichthyans as in all major osteichthyan lineages supports the hypothesis that it may be an ancestral characteristic of jawed vertebrates (Godard and Mazan, 2013).
The conserved mode of PGC specification, however, is called epigenesis, which describes a process wherein PGCs are induced from pluripotent cells by extracellular signals (Johnson et al., 2003a,b, 2011).
This mode of NB specification extends to the ~100 identified procephalic NBs derived from the procephalic neuroectoderm (pNE), which form most of the brain (Urbach and Technau, 2003; Urbach et al., 2006; Urbach and Technau, 2008).
Conversely, we also proposed that the stochastic mode of PGC specification, found in axolotls, inhibits the potential of somatic innovations to evolve, resulting in conservation of the molecular mechanisms governing development of the basal vertebrate body plan (Johnson et al., 2003b).
Immunohistochemical analyses, using specific antibodies against CVH protein, demonstrated that CVH-expressing cells were detectable during early embryogenesis of chicken embryos, starting from the first cleavage of fertilized eggs, (Tsunekawa et al., 2000), suggesting that a preformation mode of germline specification was adopted in chicken.
