409cell cycle (Mendez and Richter 2001 ; Kotani et al. 2013 ). By repressing the cyclin
B1 mRNA, the levels of Cyclin B1 protein would be expected to decline in PGCs.
Consistent with this observation, whereas endodermal cells can divide every
40–75 min, PGCs undergo cell divisions less frequently, dividing during three dis-
crete time periods. A second requirement for correct initiation depends on de novo
translation and accumulation of key maternal transcription factors, factors that are
in limited supply. In Xenopus, as mentioned above, VegT is one of these.
Translational repression by Nanos/Pumilio of VegT, cyclinB1 and other maternal
factors in PGCs may contribute to the observed delay in transcription because suf-
ficient levels of core factors have not been reached. In PGCs, VegT is degraded by
gastrulation (Lai et al. 2012 ).
Xenopus Oct60 and Oct25 are maternally expressed and play key roles in main-
taining pluripotency in the early cleaving embryo by blocking signaling pathways
that promote differentiation including Activin/Nodal, BMP, and WNT (Whitfield
et al. 1993 ; Cao et al. 2004 , 2006 , 2008 ). While Oct25 is not detected in PGCs, Oct60
is and expression persists until gastrulation. Oct60 expression and function have
primarily been investigated in somatic cells and its role in PGCs remains unknown.
The maternally expressed transcription factor, Sox7, localizes to the vegetal cortex
as well as the germ plasm (Zhang and Klymkowsky 2007 ; Owens and King, unpub-
lished observations). Sox7 may also have a role in PGCs in addition to its known role
in somatic cells (Zhang et al. 2005 ). Interestingly, maternal expression of Oct60
depends on an Octamer-Sox binding motif in its promoter region that Oct60 has been
shown to bind (Morichika et al. 2014 ). Thus Oct60 and Sox7 may form a positive
feedback loop that promotes pluripotency in the germ line. PGCs are transcription-
ally active at gastrulation/early neurula. An early transcript is Oct91 (Venkatarama
et al. 2010 ). A TRANSFAC promoter analysis identifies putative oct and sox binding
motifs within the Oct91 promoter region. Although the maternal factors that activate
the zygotic program in PGCs are unknown, it seems reasonable to suggest that
Oct60, with Sox7, are those factors, and that together they activate Oct91. In this
way, totipotency in the germ line could be preserved (Hinkley et al. 1992 ; Venkatarama
et al. 2010 ; Owens and King, unpublished observations). Further research is required
to determine exactly what maternal factors are required to initiate the zygotic program
in PGCs and what the downstream targets are of Oct91.
While nanos and Pumilio repress translation in the germ line, the RNA-binding
protein Xdazl promotes translation (Collier et al. 2005 ; reviewed in Brook et al.
2009 ). Almost certainly, Xdazl does this by interacting with PABP (Poly A Binding
Protein), a translation initiation factor that binds poly A in the 3′ end, thereby facili-
tating efficient translation. Xdazl may recruit PABP to specific germ line mRNAs,
but the identity of those RNAs in PGCs remains unknown. Whatever mRNAs Xdazl
regulates, its function is essential in PGCs. Depletion of Xdazl from PGCs results in
their failure to migrate properly within the endoderm and their eventual loss
(Houston et al. 1998 ; Houston and King 2000a). In oocytes, Dazl has a critical func-
tion in progression of meiosis and gamete development perhaps providing hints as
to what RNAs it regulates in PGCs (Padmanabhan and Richter 2006 ). In oocytes,
Xdazl and Pumilio both bind cyclin B1 mRNA, suggesting the interesting paradigm
8 Mechanisms of Vertebrate Germ Cell Determination