Adaptive evolution of eye degeneration in the Mexican blind cavefish.

The evolutionary mechanisms responsible for eye degeneration in cave-adapted animals have not been resolved. Opposing hypotheses invoking neural mutation or natural selection, each with certain genetic and developmental expectations, have been advanced to explain eye regression, although little or no experimental evidence has been presented to support or reject either theory. Here we review recent developmental and molecular studies in the teleost Astyanax mexicanus, a single species consisting of a sighted surface-dwelling form (surface fish) and many blind cave-dwelling forms (cavefish), which shed new light on this problem. The manner of eye development and degeneration, the ability to experimentally restore eyes, gene expression patterns, and comparisons between different cavefish populations all provide important clues for understanding the evolutionary forces responsible for eye degeneration. A key discovery is that Hedgehog midline signaling is expanded and inhibits eye formation by inducing lens apoptosis in cavefish embryos. Accordingly, eyes could have been lost by default as a consequence of natural selection for constructive traits, such as feeding structures, which are positively regulated by Hh signaling. We conclude from these studies that eye degeneration in cavefish may be caused by adaptive evolution and pleiotropy.