Early and late changes in Pax6 expression accompany eye degeneration during cavefish development

We have compared Pax6 expression during embryonic development in the eyed s urface form (surface fish) and several different eyeless cave forms (cavefi sh) of the teleost Astyanax mexicanus. Despite lacking functional eyes as a dults, cavefish embryos form small optic primordia, which later arrest in d evelopment and show various degrees of eye degeneration. The pattern of Pnx 6 mRNA expression was modified early and late during cavefish development. In early surface fish embryos, two bilateral Pnx6 expression domains are pr esent in the anterior neural plate, which extend across the midline and fus e to form the forebrain and optic primordia. In cavefish embryos, these Pax 6 domains are diminished in size and remain separated, resulting in an ante rior gap in Pax6 expression and presumably the formation of smaller optic p rimordia. The anterior gap in Pnx6 expression was confirmed by double stain ing for Pax6 and distalless-3 mRNA, which marks the anterior margin of the neural plate and is unaltered in cavefish. Similar anterior gaps in Pnx6 ex pression occurred in independently derived cavefish populations, suggesting that they are important in eye degeneration. Later during surface fish dev elopment, Pax6 protein is expressed in the cornea, lens, and ganglion and a macrine cells of the neural retina. Pax6 expression was gradually reduced d uring cavefish lens development, concomitant with lens arrest and degenerat ion, and was absent in the corneal epithelium, which does not differentiate in cavefish. In contrast, Pax6 expression in the retinal ganglion and amar cine cells is unmodified in cavefish, despite retarded retinal development. The results suggest that changes in Pax6 expression are involved in the ev olution of cavefish eye degeneration.