Abstract
Melanin is a brown-black pigment that is distributed to variety of terrestrial organisms. In the case of insects, melanin is synthesized not only for the formation of body color pattern but also in the situation where immune reactions occur. In insect cuticle, two types of enzymes, laccase and prophenoloxidase, are involved in melanin synthesis. They both are copper-containing enzyme and have a role to oxidize phenolic compounds such as L-dopa and dopamine. Laccase and prophenoloxidase belong to distinct family of proteins, three-domain multi-copper oxidase and type-III copper protein, respectively. It has been reported that the RNAi experiments of multiple insect species have shown that laccase is involved in brown-black pigmentation and cuticle hardening during the ecdysis process. In contrast, the knockdown of prophenoloxidase gene does not affect the process of cuticle pigmentation. In contrast to laccase, prophenoloxidase is thought to be responsible for melanin formation at wounded part of the cuticle and at around the invading parasites. This indicates that the melanin formation by the protein is important for defense reactions in insects. Here, this review describes about the systems for melanin synthesis in insects by focusing mainly on the two phenol oxidizing enzymes. It also discusses about the possible history of the systems for melanin synthesis during the evolution of insects.
