EVOLUTION OF EYE REGRESSION IN THE CAVEFISH ASTYANAX - APOPTOSIS AND THE PAX-6 GENE

The eye is an extraordinary organ in terms of its development and evol ution. In cave animals, the eye is sometimes reduced or eliminated as a consequence of adaptation to life in perpetual darkness. We have use d the characid teleost Astyanax mexicanus as a model system to investi gate the mechanisms of eye degeneration during the evolution of a cave vertebrate. Eyed surface populations of Astyanax entered caves during the Pleistocene, and their descendants lost their eyes and pigmentati on. Astyanax populations exhibiting various degrees of eye regression have been reported in 29 Mexican caves, Surface populations with chara cteristics of the ancestral stock still exist in the vicinity of these caves. Thus, Astyanax represents one of the few instances in which th e ancestral (surface fish) and the derived (cavefish) developmental mo des are extant and available far comparative studies. The cavefish emb ryo develops an optic primordium consisting of a lens vesicle and opti c cup but the rudimentary eye arrests in development and degenerates, Here we report that eye degeneration is accompanied by extensive apopt osis and downregulation of the Pax-6 gene in the developing lens. The results suggest that alterations in lens development are important fac tors in eye regression during cavefish evolution.