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Fig. 9. Model summarizing the data presented. p38 is asymmetrically active
on the future oral side of the embryo at late blastula stage, and initiates
the expression of Nodal through an unknown intermediate transcription factor
(T) that is a direct target of p38. The relationships between Nodal, Gsc, Dri
and Tbx2/3 observed in L. variegatus are consistent with those
observed in other species (Amore et al.,
2003; Croce et al.,
2003; Duboc et al.,
2004). The induction of Dri is positioned downstream from Gsc via
an intermediate repressor R, in keeping with the relationship observed in
S. purpuratus embryos (Amore et
al., 2003). Both Dri and Gsc are upstream from the oral
differentiation genes, first because SpDri is necessary but not sufficient to
induce oral-specific genes (Amore et al.,
2003), and second because Gsc is sufficient for oralization
downstream from p38 (Fig. 8C).
Gsc is depicted as relieving the repression of the oral differentiation genes.
A single repressor (R) is shown, although multiple repressors might exist. A
single ubiquitous activator of aboral genes is shown in the aboral compartment
for simplicity; there may of course be several. This activator may also induce
the repressor/s of oral differentiation genes that is/are in turn repressed by
Gsc. In the aboral ectoderm, the transcription factor Tbx2/3 might contribute
to the induction of the aboral differentiation genes
(Croce et al., 2003;
Gross et al., 2003); the 1c10
antigen is expressed later in development and is thus likely to be an aboral
differentiation gene.
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