Optic Nerve and Tectum Development in the Blind Cavefi sh Astyanax

We used the teleost Astyanax mexicanus to examine the role of the lens in optic nerve and tectum development. This species is unusually suited for studies of nervous system development and evolution because of its two extant forms: an eyed surface dwelling (surface fi sh) and several blind cave dwelling (cavefi sh) forms. Cavefi sh embryos initially form eye primordia, but the lens eventually dies by apoptosis, then the retina ceases to grow, and fi nally the degenerating eyes sink into the orbits. Transplantation of an embryonic surface fi sh lens into a cavefi sh optic cup restores eye development. We show here that retinal nerve fi bers are formed and project to the optic tectum in cavefi sh embryos. In adult cavefi sh that have completed lens degeneration, however, the number of retinal axons in the optic nerve is substantially reduced compared to surface fi sh. The presumptive brain domains of embryonic cavefi sh are not altered relative to surface fi sh based on expression of the regional marker genes Pax6 , Pax2.1 , and engrailed2 . In contrast, the adult cavefi sh brain is elongated, the optic tectum is diminished in volume, and the number of tectal neurons is reduced relative to surface fi sh. Unilateral transplantation of an embryonic surface fi sh lens into a cavefi sh optic cup increases the size of the optic nerve, the numReceived: March 4, 2004 Accepted: June 23, 2004