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Fig. 6. The combination of sox17α and derriere phenocopies the VegT effect on the D1.1 lineage. (A) The retinas (r) in sox17α and Xnr1 mRNA injected embryos are normal in size, whereas derriere dramatically reduced both retinas. However, D1.1 progeny (green) were not located within the retina in sox17α, derriere or Xnr1 injected embryos. Co-expression of notch1 reversed the derriere phenotype, resulting in large retinas populated by abundant D1.1 progeny (green), but did not alter the Xnr1 phenotype. The expression of a dominant-negative derriere construct (Cm-derriere) resulted in a larger retina populated by large numbers of D1.1 progeny. (B) The mean volume of retinas in embryos injected with mRNAs for sox17α, derriere (Der), derriere plus Notch1 (Der/N), dominant-negative derriere (Cm-D), Xnr1 and Xnr1 plus notch1 (Xnr1/N). sox17α- and Xnr1-injected embryos do not significantly differ from GFP controls (Fig. 2B), whereas those from derriere embryos are significantly reduced on both sides. Co-expression of Notch1 partially rescues the derriere effect but has no effect on Xnr1 retinas (*, p < 0.01 compared to GFP controls; **, p < 0.05 compared to derriere). Cm-D significantly increased the size of both retinas (*, p < 0.01 compared to GFP controls). (C) Percentage of embryos in which D1.1 progeny populate the retina is dramatically reduced in embryos injected with sox17α, derriere (Der) or Xnr1 mRNAs. The derriere phenotype is partially rescued by co-expression of notch1 (Der/N), whereas the Xnr1 phenotype is not (Xnr1/N). All Cm-D expressing embryos have D1.1 progeny in the retinas, identical to GFP controls (Fig. 2C). (D) Domains of pan-neural plate (sox3, notch1), retinal (rx1), mesodermal (Xbra), and endodermal (sox17α, edd) genes after injection of one D1.1 blastomere with constructs listed on the left. The injected side of the embryo is on the right and the uninjected side is on the left. The sites of effects are indicated either by arrows, or in the neural plate by white bars indicating the width of the expression domains. Expression of sox17α does not affect neural/retinal genes, but inhibits Xbra and expands edd, consistent with its role in converting cells to an endodermal fate. Expression of derriere represses neural and retinal genes, and these effects are rescued by notch1. Cm-Derriere expanded neural and retinal markers and repressed Xbra. Xnr1 either represses (sox3, top rx1 panel) or expands (notch1; bottom rx1 panel) neural/retinal genes, and endo-mesoderm markers are dorsally expanded. No Xnr1 phenotype is rescued by notch1. (E) The expansion of neural (sox3, notch1) and retinal (rx1) expression domains by injection of VegTMO (Fig. 2C) is reversed by co-injection of derriere. Reduction of these markers by VegT (Fig. 2B) is reversed by cm-derriere (see also Table 1). |
