Meeting Abstract

S2-1.4  Jan. 4  Why cavefish lost their eyes? “Natural Selection or Neutral Theory" YAMAMOTO, Yoshiyuki*; JEFFERY, William; University College London, London, UK; University of Maryland, College Park, Maryland, USA yoshiyuki.yamamoto@ucl.ac.uk

The question of why cave animals have lost their eyes is currently unresolved. There are two possible hypotheses: Neutral Theory and Selection hypothesis. Neutral Theory explains that eye degeneration is caused by accumulation of mutations in eye forming genes that has no advantageous or disadvantageous effects for living in completely darkness cave. Selection hypothesis suggests that eye degeneration could have some advantageous effects for surviving in a cave based on Darwin's “Natural Selection Hypothesis” However, Darwin, himself, could not explain how blindness would get any benefit living in a cave. We study this problem in the blind cavefish, Astyanax mexicanus. Cavefish have evolved regressive characters, such as a degenerate eye and less pigment, and constructive characters, such as a large jaw and additional teeth, cranial neuromasts, and taste buds. Although functional eyes are lacking in adults, cavefish embryos form a small eye primordium, which later arrests in development and degenerates. Lens apoptosis plays a major role in eye degeneration. Signaling molecule, Shh controls eye separation from a single embryonic optic field and jaw development in vertebrates. In Astyanax, shh has an expanded expression pattern in the presumptive head region of the cavefish embryo. The expansion correlates with programmed cell death in the lens, increased jaw size, and more numerous taste buds in the cavefish compared to surface fish. Our hypothesis is that Shh controls all three traits, and that a developmental trade-off occurred whereby functional eyes were sacrificed to allow the development of extra taste buds and a larger jaw that would help to survive in a complete darkness cave. This could only happen, of course, in the cave environment, where selection for sight is relaxed.