It is well-known that exposure to juvenile hormone mimics induces a variety of aberrant developmental and physiological effects in insects. One such effect in Drosophila melanogaster is the appearance of melanized tumor-like structures in larval stages. To understand the nature of these tumors and identify the constituent cell-types, we examined the effects of two juvenile hormone mimics, methoprene and pyriproxyfen, and found that both mimics induce hematopoietic tumors in flies in a consistent manner. These effects are not observed in flies lacking functional receptors for the juvenile hormone signaling pathway. Using cell-type-specific markers, we found that the juvenile hormone mimic-induced tumors are composed mainly of lamellocytes, a specialized blood cell type that normally differentiates in response to parasitoid wasp infection. Surprisingly however, the larvae without functional juvenile hormone receptors are able to mount a robust encapsulation response when exposed to parasitoid wasps. These results suggest that juvenile hormone mimics require juvenile hormone receptor function to activate cellular immunity and raise new questions about the effect of juvenile hormone mimics on hematopoietic development.
