Pattern specific melanin synthesis and DOPA decarboxylase activity in a butterfly wing of Precis coenia Hübner

Abstract

Black pigment extracted from wings of Precis coenia was identified as melanin by solubility characteristics and incorporation of radiolabeled melanin precursors. Different colours in wing scales appeared successively, starting with white followed by red, black and grey pigments. At corresponding times [ 14C]-tyrosine and [ 14C]-3,4-dihydroxyphenylalanine injected into pupae were incorporated most intensively into black and grey scales. By contrast, red scales were labeled by 3,4-dihydroxyphenylalanine (DOPA) and, to a lesser extent, by tyrosine when injected at the time of red pigment synthesis. Labeled β-alanine, a component of the sclerotizing agent N-β-alanyldopamine, was incorporated into all scales with the exception of intensely black scales. It is discussed whether the mechanical stability of black scales may be due to melanin itself whereas the stability of all other scales may be due to N-β-alanyldopamine. Amounts of extractable melanin increased during visible pigment formation in black and grey scales. When [ 14C]-tyrosine was offered to isolated wings in Grace's medium before melanization, 93% of incorporated activity was found in the hydrolysable fraction and only 7% in the melanin fraction. However, when supplied during intensive melanin synthesis up to 70% was incorporated into melanin. The same incorporation pattern resulted after injection into intact developing pupae. Incorporation of [ 14C]-tyrosine into melanin began earlier, and was higher in presumptive black scales than in grey scales. Incorporation into white scales was always low. DOPA decarboxylase (DDC) activity in whole wings was low during the pupal stage but increased at the time when scale melanization started. DDC activity in different coloured wing pieces increased first in presumptive black pattern elements, the eyespots. DDC activity in the eyespots was 3.5 times higher than in presumptive white parts of the wing; it was intermediate in grey areas. This demonstrates that wing colour pattern as well as colouration intensity are related to selective activity of DDC in time and different locations resulting in different amounts of melanin in grey and black scales.