PAX-6 GENE-EXPRESSION IN NEWT EYE DEVELOPMENT

pax-6 is thought to be a master control gene of eye development in spe cies ranging from insects to mammals. We have isolated a pax-6 cDNA ho molog of the newt, Cynops pyrrhogaster. RT-PCR and sequence analyses p redicted four alternatively spliced forms derived from inclusion or ex clusion of the region corresponding to exons 5a and 12 in the human pa x-6 ortho-log. This gene shared extensive sequence identitiy and simil ar expression patterns with those of mouse and zebrafish. pax-6 signal was first detected at the anterior ridge of the neural plate, and lat er at the eye and nasal primordium and in the central nervous system - except for the midbrain. The injection of sonic hedgehog (shh) RNA in hibited the expression of pax-6 within the optic vesicle and disturbed eye cup formation. A similar suppressive effect of shh was also obser ved 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 pa x-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 possib ility that the ectoderm is competent to the lens-inducing signal at a stage as early as mid gastrula.