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Fig. 8. foxa gene interactions in the endomesoderm regulatory
network. Relevant network subcircuits are depicted as `views from the
nuclei' (Bolouri and Davidson,
2002) up to beginning of gastrulation in veg2 SMC (left boxes) and
veg2 endodermal cells (right boxes). Architecture is as presented previously
(Davidson, 2006;
Erwin and Davidson, 2002;
Levine and Davidson, 2005;
Oliveri and Davidson, 2004b)
(for a current version see
http://sugp.caltech.edu/endomes/),
with the addition of data from this paper. (A) Blastula; (B)
mesenchyme blastula to early gastrula. Solid connecting lines are predicted to
be direct interactions between the product of a given regulatory gene and the
target cis-regulatory module of other gene(s). The dashed line is an implied
and possibly indirect interaction. Arrows indicate activating interactions and
bars indicate repressive inputs. Bold colors are genes expressed or
interactions operating in that cell type at that time; faded colors indicate
genes that are inactive or interactions not then taking place. Yellow stars
symbolize the Delta ligand and turquoise ovals the Notch receptor embedded in
the diagrammatic membrane. White stars and white ovals indicate that
Delta/Notch signaling is not active at a particular stage in a particular cell
type (because the delta gene is not expressed there). At blastula
stage (A), the genes gatae (blue) and otx (green) enter into
a feedback-stabilizing loop (Yuh et al.,
2004), and are responsible for initiating expression of the
foxa gene (red) in all endomesodermal cells. In SMCs only, the
gcm gene (orange) is activated in response to the Delta signal
arriving from the adjacent micromeres
(Sherwood and McClay, 1997;
Sherwood and McClay, 1999).
Gcm is then responsible for activating the pigment cell program represented by
the pks gene (olive green). By mesenchyme blastula stage (B),
gatae is expressed in veg2 endoderm as well as in mesoderm
(Lee and Davidson, 2004).
foxa expression becomes very strong and is restricted to endodermal
cells. High levels of foxa are responsible for autorepression of
foxa, and for repression of gcm. In the gcm
expressing SMCs, foxa expression fades out; an unknown function
downstream of gcm is inferred. The mesodermal cells now begin to
express the Delta ligand that will signal to the adjacent veg2 endodermal
cells, where Notch signal is used to drive the expression of gatae
(P. Y. Lee and E.H.D., unpublished). At this time, gcm expression in
response to Delta/Notch signaling is prevented in veg2 endoderm by the
repressive action of Foxa. (C) Timeline of expression of the components
in the model based on QPCR and WMISH data.
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