engleski

Convergent Evolution of Albinism in Diverse Cave Adapted Animals

Albinism, the regression or loss of melanin pigmentation, is an omnipresent feature that has evolved by convergence in all phyla with cave dwelling representatives. We are interested in the molecular mechanisms that are underlying the evolution of albinism in diverse cave animals. The molecular basis of albinism is currently known in only one cave adapted animal: Astyanax mexicanus cavefish. In this species, different loss-of-function mutations in the oca2 gene cause albinism in at least three independently evolved cavefish lineages. The function of OCA2 is not completely understood, but it is known to act during the first step of melanin biosynthesis, the conversion of L-tyrosine to L-DOPA. Since the melanin synthesis pathway is generally conserved among animals, we have used a melanogenic substrate assay to survey for defects in this pathway in albino cave animals belonging to many different phyla. The assay involves supplying exogenous substrates, such as L-tyrosine or L-DOPA, to lightly fixed specimens, and subsequently detecting the presence of melanin as deposits of black pigment. The addition of L-DOPA, but not L-tyrosine, produced black pigment in diverse albino cave animals, including a sponge, a planarian, annelids, mollusks, arthropods, and several vertebrates other than Asytanax, indicating that the initial step of the pathway is defective in all these animals. In some of the cases, L-DOPA treatment restored melanin pigmentation in patterns resembling those of closely related surface-dwelling relatives. Therefore, albinism has evolved by a defect at the first step of melanin biosynthesis in all cave-adapted animals tested thus far, and L-DOPA can restore pigmentation, indicating that all downstream steps of the pathway are present and potentially functional in these animals. Our results show that albinism has evolved by a defect in the first step of melanin biosynthesis in cave animals belonging to many different phyla, suggesting that there is an adaptive advantage for a block at the beginning of the pathway.

melanin synthesis pathway; albinism; cave animals; convergent evolution

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