Mechanism and evolutionary significance of the loss of melanin pigmentation in the cave fish Astyanax mexicanus.

Abstract

The regressive evolution of traits such as eyes and pigmentation is common among cave organisms. As a model to study regressive evolution, I have used the teleost Astyanax mexicanus, which consists of eyed and pigmented epigean forms and many populations of cave-dwelling forms that have lost those traits. This study investigates the mechanism for the loss of melanin production, from the origin of chromatophores from the neural crest to the synthesis of melanin within the melanocyte. I show that cavefish retain a migratory population of neural crest derived cells that are tyrosinase positive and respond to exogenous signals as expected of a melanocyte. I then propose that the regressive evolution of melanin pigmentation is a selectively evolved trait that provides for an excess of dopamine, supported by the near two-fold increase in dopamine in cavefish brains, quantified via HPLC analysis. This study suggests that regressive evolution sometimes occurs via selection.