In animals with an early mechanism of germ line specification, the material determining the cell fate is sequestered early on in specific blastomeres, and for this reason the segregation of the most active mitochondria in gonadic presumptive blastomeres has to take place contextually (e.g. C. elegans and bivalves; figure 1 a,b).
Posterior patterning and germ line specification depend upon the posterior localization of the oskar transcript.
The fibroblast growth factor (FGF) signaling pathway also may contribute to mammary line specification.
If pluripotency is indeed unaffected, while germ line specification is presumably impaired, can the authors show normal alkaline phosphatase staining in G9a -/- embryos and not in germ cells?
11) Since the authors state that a significant proportion of genes affected by G9a deletion are involved in germ line specification, the authors need to show whether germ cells are affected.
When germ line specification happens at a later stage of development, owing to inductive signals from surrounding tissues (as in mammals), at least two modes of elimination of sperm mitochondria are possible.
Of course, genomic background and timing of action (in the presence of either early or late mechanisms of germ line specification) can influence the outcome of the segregation mechanism.
In order to be generalizable, such a hypothesis needs to take into account the different mechanisms of germ line specification, and the moment in which mitochondria and other material are set aside to be inherited through the germ line must be considered.
The only other functional genetic analysis of germ line specification in a hemimetabolous insect to date (Ewen-Campen et al., 2013) has also provided evidence that maternally supplied posterior germ plasm is absent, and that vasa is dispensable maternally and zygotically for germ cell formation.
In support of this notion, molecular analysis of murine BM-derived VSELs has revealed that these cells express several genes that are characteristic of epiblast SCs (Gbx2, Fgf5, and Nodal) and germ line specification (Stella, Prdm14, Fragilis, Blimp1, Nanos3, and Dnd1) [ 65, 66, 72].
