pax-6 gene expression in newt eye development

pax-6 is thought to be a master control gene of eye development in species ranging from insects to mammals. We have isolated a pax-6 cDNA homolog of the newt, Cynops pyrrhogaster. RT-PCR and sequence analyses predicted four alternatively spliced forms derived from inclusion or exclusion of the region corresponding to exons 5a and 12 in the human pax-6 ortholog. This gene shared extensive sequence identitiy and similar expression patterns with those of mouse and zebrafish. pax-6 signal was first detected at the anterior ridge of the neural plate, and later at the eye and nasal primordium and in the central nervous system - except for the midbrain. The injection of sonic hedgehog (shh) RNA inhibited the expression of pax-6 within the optic vesicle and disturbed eye cup formation. A similar suppressive effect of shh was also observed in the conjugation of the animal caps preloaded with exogenous shh and noggin mRNA, which was used as an inducer of pax-6. In contrast, shh injection had no effect on the expression of pax-6 in the surface ectoderm overlying the optic cup, suggesting that the expression of pax-6 in the surface ectoderm is not regulated by shh in vivo. Moreover, we found transient activation of pax-6 in animal cap explants at the sibling stage of mid-late gastrula. This observation raises the possibility that the ectoderm is competent to the lens-inducing signal at a stage as early as mid gastrula.