Melanotic mutants in Drosophila: Pathways and phenotypes

Mutations in > 30 genes that regulate different pathways and developmental processes are reported to cause a melanotic phenotype in larvae. The observed melanotic masses were generally linked to the hemocyte-mediated immune response. To investigate whether all black masses are associated with the cellular immune response, we characterized melanotic masses from mutants in 14 genes. We found that the melanotic masses can be subdivided into melanotic nodules engaging the hemocyte-mediated encapsulation and into melanizations that are not encapsulated by hemocytes. With rare exception, the encapsulation is carried out by lamellocytes. Encapsulated nodules are found in the hemocoel or in association with the lymph gland, while melanizations are located in the gut, salivary gland, and tracheae. In cactus mutants we found an additional kind of melanized mass containing various tissues. The development of these tissue agglomerates is dependent on the function of the dorsal gene. Our results show that the phenotype of each mutant not only reflects its connection to a particular genetic pathway but also points to the tissue-specific role of the individual gene.