Phenoloxidase, melanin and amyloid fibrils in the compound ascidian Botryllus schlosseri

In invertebrates, the recognition of non-self molecules frequently leads to the release of phenoloxidase (PO) by circulating granulocytes, in the form of its precursor, pro-PO, which is quickly activated by soluble serine proteases. This response represents a powerful weapon against potentially pathogenic microorganisms and, in several taxa, independently of their phylogenetic position, the pro-PO activation system is responsible of a massive production of melanin. The latter is supported by the formation of amyloid fibrils as well as other concurrent events, such as ACTH production, NEP overexpression and α-MSH formation. In the colonial ascidian Botryllus schlosseri, allorecognition between contacting, genetically incompatible colonies, leads to the recruitment of haemocytes in the contact region and their degranulation with the consequent release of PO and the production of melanin which is deposited in the necrotic spots along the colony contacting border.