Cavefish as a model system in evolutionary developmental biology

The Mexican tetra Astyanax mexicanus has many of the favorable attributes t hat have made the zebrafish a model system in developmental biology. The ex istence of eyed surface (surface fish) and blind cave (cavefish) dwelling f orms in Asryanax also provides an attractive system for studying the evolut ion of developmental mechanisms. The polarity of evolutionary changes and t he environmental conditions leading to the cavefish phenotype are known wit h certainty, and several different cavefish populations have evolved constr uctive and regressive changes independently. The constructive changes inclu de enhancement of the feeding apparatus (jaws, taste buds, and teeth) and t he mechanosensory system of cranial neuromasts. The homeobox gene Prox 1, w hich is expressed in the expanded taste buds and cranial neuromasts, is one of the genes involved in the constructive changes in sensory organ develop ment. The regressive changes include loss of pigmentation and eye degenerat ion. Although adult cavefish lack functional eyes, small eye primordia are formed during embryogenesis, which later arrest in development, degenerate, and sink into the orbit. Apoptosis and lens signaling to other eye parts, such as the cornea, iris, and retina, result in the arrest of eye developme nt and ultimate optic degeneration. Accordingly, an eye with restored corne a, iris, and retinal photoreceptor cells is formed when a surface fish lens is transplanted into a cavefish optic cup, indicating that cavefish optic tissues have conserved the ability to respond to lens signaling. Genetic an alysis indicates that multiple genes regulate eye degeneration, and molecul ar studies suggest that Pax6 may be one of the genes controlling cavefish e ye degeneration. Further studies of the Astyanax system will contribute to our understanding of the evolution of developmental mechanisms in vertebrat es. (C) 2001 Academic Press.