engleski

Evolution of melanin pigment regression in cave animals

Majority of cave adapted animals share a suite of traits that are called troglomorphies. One of the most notable one is the loss of pigmentation, however not much is known about the molecular and evolutionary basis of this trait. Traditionally, regressive phenotypes were assumed to occur due to accumulation of mutations in genes underlying a trait that was not selected out from the population. Using melanogenic substrate assay we showed that melanin pigmentation is lost due to a disruption of the first step of the biosynthetic pathway in cave animals from various phyla. These results suggests that there could be an advantage to targeting the first step of melanin synthesis. A study in the fish model system Astyanax mexicanus where deletion in oca2 gene underlies albinism showed that oca2 gene may have a pleiotrophic effect on both pigmentation and levels of catecholamines. Knock down of oca2 in Astyanax surface fish caused the abolition of melanin synthesis as well as a rise in L-tyrosine and dopamine levels. Albino cave fish have increased levels of catecholamines compared to surface fish, and so do albino cave bivalves, insects and polychaetes when compared to their most closely related surface relatives. In addition, we are testing an alternative hypothesis that the advantage of the loss of melanin pigmentation is energy conservation, an important feature for organisms living in a food depleted environment such as caves. Using gene silencing techniques and membrane-inlet mass spectrometry we have obtained evidence that the interruption of melanin synthesis in Astyanax embryos results in lower oxygen consumption. So, it seems that multiple advantages drive the loss of melanin in cave animals. Interestingly, all cave animals we tested still retain melanocytes, suggesting melanocytes have roles other than just in pigment production. It is known that Arthropods use melanin in their innate immune response, and several albino cave arthropods we tested retain the ability to synthesize melanin after an immune challenge. In Astyanax we have observed the accumulation of L-DOPA positive cells around the wound site and a treatment with chemical agent that kills melanocytes abolishes this reaction. Therefore, it is possible that melanocytes are never lost due to their important roles in innate immune response.

cave animals ; loss of pigmentation ; pleiotrophic effects, catecholamines ; innate immune response ; melanocytes

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