Figure 6

Ectopic overexpression of NvSmad2/3 does not induce a secondary axis in Xenopus embryos. Injection of 0.5 ng XSmad2 mRNA into the marginal zone of one ventral vegetal blastomere at the 8-cell stage can produce a secondary body axis in Xenopus embryos. (A) Uninjected, wild type tadpoles. (B) Tadpoles that were injected with XSmad2 show a classic secondary body axis phenotype (marked with white arrowheads in this photo only). (C) Injection of XSmad3 shows a clear secondary axis. (D) dSmad2 is able to induce the formation of a second body axis. (E) NvSmad2/3 is not able to generate a second body axis, but can perturb the original axis. (F) The MH1 chimera acutely perturbs the original axis, but generates a complete second axis in only a few cases. Embryos were scored for axial phenotypes at neurula stage. Examples: (G) wild type, (H) double axis that would result in a second body axis at tadpole stage (result of XSmad2 mRNA, in this case), (I) another double axis (caused by dSmad2 mRNA, in this case), (J) ‘incipient’ axis that will eventually get subsumed into the primary axis and result in the ‘perturbed axis’ phenotype (result of XSmad3 mRNA, in this case, though it could be caused by any of the treatments), (K) phenotype that would be scored as ‘wild type’ (result of NvSmad2/3 mRNA, in this case). (L) Bar graph illustrating the range of phenotypes from each treatment. See Additional file6 for more photos illustrating the ‘perturbed axis’ phenotype.