237oocytes (Kofron et al. 2007 ), providing the possibility that Wnt activation may
wholly or partially occur before fertilization. Possible mechanisms for Wnt activa-
tion in Xenopus are shown Fig. 6.8 (top panels).
Although there is good evidence for secreted Wnt11b activity, it is unclear when
Wnt11b is required. Also, this role for Wnt11b has not been reconciled with the
cytoplasmic activation model. One potential unifying model has been proposed;
that ongoing Wnt signaling in the oocyte generates activated Lrp6 signaling endo-
somes that are transported dorsally (Dobrowolski and De Robertis 2012 ). However,
it is not known to what extent these Lrp6-containing endosomes are formed in the
oocyte. Lrp6 is phosphorylated in eggs but becomes dephosphorylated following
egg activation (Davidson et al. 2009 ), suggesting that stable signaling complexes
may actually be inactivated prior to cortical rotation. Furthermore, analyses of the
relative activity of vegetal cortical cytoplasm suggests it acts at the level of the
destruction complex and does not mimic the activity of activated receptors
(Marikawa and Elinson 1999 ).
6.3.3.2 Zebrafish
The regulation of Wnt signaling in zebrafish also has intriguing parallels and differ-
ences to the situation in Xenopus (Fig. 6.8, bottom panels). Maternal/zygotic
mutants of wnt11 form the axis normally in zebrafish (Heisenberg et al. 2000 ), sug-
gesting that fish use other mechanisms or other Wnts. Maternal wnt8a has been
proposed to act as a dorsal determinant in zebrafish (Kelly et al. 1995a; Lu et al.
2011 ). Wnt8a is the only vegetally localized wnt transcript in the yolk cell, and is
shifted asymmetrically following cortical rotation. Vegetal localization also occurs
through a mitochondrial cloud dependent mechanism, similar to frog wnt11b (Lu
et al. 2011 ). In the fish however, injection of a dominant-negative Wnt8a construct
was able to reduce the expression of chrd and dharma, the latter being a direct Wnt
target gene. Full-length Wnt8a also rescues to some extent embryos ventralized by
nocodazole (Lu et al. 2011 ). Also, depletion of Sfrp1 or Frzb hyperdorsalizes
embryos, indicating dorsal enrichment of a maternal Wnt, Wnt8a in the case of fish,
may be a trigger to overcome generalized Wnt antagonism in the embryo. While this
and the similar mechanism proposed to act in frog are intriguing (Cha et al. 2008 ),
it remains to seen whether relatively small changes in Wnt levels involved are
responsible for dorsalization.
Evidence in fish is also suggests that other mechanisms of Wnt antagonism are
critical in suppressing beta-catenin activation. Notably, maternal/zygotic mutants for
wnt5b are hyperdorsalized, owing to defective Wnt/calcium signaling and failure to
repress Wnt/beta-catenin signaling (Westfall et al. 2003 ). A similar but not well-
characterized pathway may exist in frogs (Saneyoshi et al. 2002 ). In Xenopus, the
maternal role of Wnt5b has not been assessed, however Wnt5a is proposed to activate
instead of repress Wnt/beta-catenin signaling in frogs (Cha et al. 2008 ). Axis forma-
tion is largely normal however in wnt5a, wnt5b double mutant mice (Agalliu et al.
2009 ; spinal cord formation), suggesting that the roles of Wnt5 paralogues may be
6 Vertebrate Axial Patterning: From Egg to Asymmetry