engleski

Evolution of Albinism in an Extreme Environment: Dark Caves

Animals adapted to dark caves are characterized by albinism, the regression of melanin pigmentation. We will address three questions concerning albinism in cave animals. First, what are the biochemical and genetic mechanisms responsible for albinism in the cavefish Astyanax mexicanus? Second, are the evolutionary processes underlying albinism in Astyanax the same or different in other albino cave animals? Third, are there adaptive benefits of losing pigmentation in cave animals? The mechanisms of albinism are best known in Astyanax cavefish. In this system, a block at the first step of the melanin biosynthesis pathway, the conversion of L-tyrosine to L-DOPA, causes albinism. The single gene underling the block is oca2, which acts at the first step of the pathway and is subject to different loss-of-function mutations in independently evolved cavefish lineages. A block in the first step of melanin biosynthesis has also been discovered in a diverse group of albino cave animals, including planarians, mollusks, annelids, arthropods, and vertebrates. In some of these species, exogenously applied L-DOPA can restore melanin pigmentation patterns resembling those of pigmented surface relatives. Therefore, albinism has evolved via convergent evolution by interfering with the first step of the melanin biosynthesis pathway in a diverse assemblage of cave animals. Studies with Astyanax cavefish suggest an explanation for the convergent evolution of albinism: blockage at the first step maybe be advantageous because it results in shunting excess L-tyrosine from the melanin pathway to a branch pathway leading to catecholamine synthesis, which could be beneficial in cave-adapted animals.

melanin synthesis pathway; albinism; cave animals; convergent evolution

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