Generation of Morphological Diversity by Pleiotropy in Blind Cavefish

The evolution of eye degeneration and related morphological traits is studied in the teleost Astyanax mexicanus , a single species with eyed (surface fish) and eyeless (cavefish) forms. In addition to eye and pigment regression, cavefish exhibit increased numbers of taste buds and other feeding structures. A candidate gene survey showed that about 150 key eye regulatory and structural genes are expressed normally in cavefish. However, a small number of these genes show subtle changes in their embryonic expression patterns. The midline signaling gene Sonic hedgehog ( Shh ) and its downstream target genes are expanded in cavefish embryos. Shh overexpression in surface fish embryos resulted in a blind cavefish phenocopy. We have also shown that Shh expression is sufficient and necessary for increasing taste bud development in cavefish. The results support a developmental trade‐off in which adapative and regressive characters evolve in concert based on the pleiotropic effects of Shh .