2 41(nodal3.1; Smith et al. 1995 ), likely functions in controlling morphogenesis during
organizer formation. Nodal 3.1 is an atypical member of the Nodal protein family
present only in anuran amphibians (Smith et al. 1995 ). This protein has BMP antag-
onist activity (Hansen et al. 1997 ) and, during gastrulation, is restricted to the super-
ficial epithelial layer of the organizer (Glinka et al. 1996 ), a region with discrete
morphogenetic regulatory ability (Shih and Keller 1992a). Loss-of-function experi-
ments suggest that Nodal3.1 regulates convergent extension, acting as a Fibroblast
growth factor (FGF) receptor ligand (Yokota et al. 2003 ), although this mechanism
is not well understood.
In summary, beta-catenin function is critical at many different regulatory levels to
specify dorsal fate across all prospective germ layers in the early blastula in Xenopus
and zebrafish. These general functions would include generating dorsal- inducing sig-
nals, mediating competence to respond to those signals, reducing the ability of dorsal
cells to respond to ventralizing BMP signals, and regulating dorsal morphogenesis. In
this context, the strong expression of nodals in the dorsovegetal blastomeres is likely
responsible for the Nieuwkoop center phenomenon, although beta-catenin activity is
required in prospective organizer mesoderm and ectoderm as well. In amniotes, how-
ever, axis formation is also governed by the dynamic regulation of Nodal signaling in
concert with Wnt/beta-catenin activity. In these cases, the initial establishment of
restricted Nodal signaling occurs without early maternal Wnt signaling and the role
of a Nieuwkoop center analog in inducing the organizer is less clear.
6.3.5 Wnt/Beta-Catenin and Nodal Signaling During Axis
Formation in Amniotes
6.3.5.1 Chick
The role of concerted Wnt and Tgfb signaling in the dynamic regulation of Nodal
expression and activity is conserved during amniote axis formation. In contrast how-
ever to the case in fish and frogs, axis formation in birds and mammals is driven by
spatially restricted Nodal activity overlapping with more generalized Wnt/beta- catenin
signaling in the marginal zone of the epiblast (Fig. 6.9a). In the chicken, there is no
evidence for localized Wnt expression in the egg or early cleavage stages. Wnt8a
(alias, CWnt8C) is the predominant Wnt expressed prior to gastrulation and is enriched
in the PMZ epiblast, but is also found in a decreasing gradient around the marginal
zone from posterior-to-anterior (Hume and Dodd 1993 ; Skromne and Stern 2001 ). By
contrast, the Nodal-related gene Gdf1 is specific to the PMZ at the same stage (Shah
et al. 1997 ; Skromne and Stern 2001 ) and represents one of the earliest asymmetrically
expressed genes in the chick blastoderm (and the first to be discovered).
Grafting of Gdf1-expressing cells induces an ectopic axis anywhere in the mar-
ginal zone (Shah et al. 1997 ; Skromne and Stern 2001 ). Interestingly, ectopic
expression of Wnt alone is not a robust axis inducer in chicken (Joubin and Stern
1999 ). Gdf1 and Wnt8a can synergize in axis induction, and the axis-inducing abil-
6 Vertebrate Axial Patterning: From Egg to Asymmetry