Drosophila wing melanin patterns form by vein-dependent elaboration of enzymatic prepatterns

Background: Animal melanin patterns are involved in diverse aspects of thei r ecology, from thermoregulation to mimicry. Many theoretical models have s imulated pigment patterning, but little is known about the developmental me chanisms of color pattern formation. in Drosophila melanogaster, several ge nes are known to be necessary for cuticular melanization, but the involveme nt of these genes in melanin pattern evolution is unknown. We have taken a genetic approach to elucidate the developmental mechanisms underlying melan in pattern formation in various drosophilids. Results:We show that, in D. melanogaster, tyrosine hydroxylase (TH) and dop a decarboxylase (DDC) are required for melanin synthesis. Ectopic expressio n of TH, but not DDC, alone was sufficient to cause ectopic melanin pattern s in the wing. Thus, changes in the level of expression of a single gene ca n result in a new level;of melanization. The ontogeny of this ectopic melan ization resembled that found in Drosophila species bearing wing melanin pat terns and in D. melanogaster ebony mutants. Importantly, we discovered that in D. melanogaster and three other Drosophila species these wing melanin p atterns are dependent upon and shaped by the circulation patterns of hemoly mph in the wing veins. Conclusions: Complex wing melanin patterns are determined by two distinct d evelopmental mechanisms. Spatial prepatterns of enzymatic activity are esta blished late in wing development. Then, in newly eclosed adults, melanin pr ecursors gradually diffuse out from wing veins and are oxidized into dark b rown or black melanin. Both the prepatterning and hemolymph-supplied compon ents of this system can change during evolution to produce color pattern di versity.