engleski

Convergent evolution of albinism in cave adapted plant hoppers

Cave adapted animals are often characterized by the reduction or absence of melanin pigment, a condition known as albinism. Here we ask whether albinism has evolved by the same or different changes in the melanin biosynthesis pathway in two albino cave cixiid planthoppers, one from limestone caves in Mljet and Biokovo, Croatia (undescribed species) and the other from lava tubes in Hawaii (Oliarus polyphemus). We have developed a functional assay for detecting the position of a block in the melanin biosynthesis pathway by supplying exogenous substrates, such as L-tyrosine or L-DOPA, to lightly fixed specimens, and subsequently detecting melanin as deposits of black pigment. Supplying exogenous L-DOPA but not L-tyrosine produced black pigment in both species of cixiids, implying that a defect occurs in the first step of the melanin biosynthesis pathway in two independently evolved planthopper cave lineages. The deposition of melanin pigment is blocked by prior treatment of the specimens with high temperature (which denatures proteins) or co-assay with phenylthiourea (an inhibitor of tyrosinase) indicating that the reaction is enzyme-catalyzed. The only case in which the cause of albinism is already known in a cave organism is in Astyanax mexicanus cavefish. In this species the oca2 gene, which acts during the initial step of melanin biosynthesis, the conversion of L-tyrosine to L-DOPA, is mutated. Therefore, albinism has evolved via convergent evolution by interfering with the same initial step of the melanin biosynthesis pathway in both cave-adapted insects and fishes. We are currently investigating the possibility that this convergence is even broader than described here by conducting the melanogenesis substrate assays in many different albino invertebrate species.

cave planthoppers; convergent evolution; melanin; albinism; pigmentation

nije evidentirano