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One candidate for an endogenous mesoderm-inducing factor in Xenopus is derrière, a member of the TGFbeta family closely related to Vg1. In this paper we first show that derrière is able to exert long-range effects in the early Xenopus embryo, reinforcing the view that it functions as a secreted factor required for proper formation of posterior structures. Analysis of the derrière promoter shows that expression of the gene is controlled through a complex inductive network involving VegT and TGFbeta-related molecules and also, perhaps, FGF family members. The work confirms that derrière plays an important role in mesoderm formation and it illustrates the complex regulation to which inducing factors are subject.
Fig. 1. Comparison of the expression patterns of derrière (A-C) and VegT (D-F) at stages 9 (A,D), 10.5 (B,E) and 12 (C,F). Embryos were fixed at the indicated stages and processed for in situ hybridisation. Note the similarities between the expression patterns of the two genes.
Fig. 11. Inhibition of activin-like or FGF signalling does not interfere with activation of derriã¨re but does prevent its maintenance. Embryos were injected at the two-cell stage with 800 pg RNA encoding the truncated activin receptor î XAR (A,D), the truncated FGF receptor XFD (B,E), or the control construct d50 (C,F). RNA encoding î²-galactosidase was co-injected as a cell lineage label in all cases. Embryos were allowed to develop to stage 9.5 (A-C) or 11 (D-F) and expression of derriã¨re was analysed by in situ hybridisation. The initial activation of derriã¨re is not affected by inhibition of activin-like (A) or FGF (B) signalling; note the overlap of î²-galactosidase staining (blue) with the in situ hybridisation reaction product (purple), which at this early stage is largely nuclear. Continued expression of derriã¨re, however, does require activin-like and FGF signalling; note down-regulation of expression in D and E. Vg, vegetal pole.
