Juvenile Hormone and the Evolution of Insect Metamorphosis

During the evolution of insect life histories, the role of juvenile hormone (JH) to sustain different developmental events shifted as insect lifestyles became more complex. In a basal, non-metamorphic species, such as Thermobia domestica, the developmental functions of JH are restricted to embryogenesis where JH mediates the transition from morphogenesis to differentiation. With the shift to incomplete metamorphosis, JH acquired the postembryonic function of maintaining the form of the juvenile (the nymph) and preventing metamorphosis. This movement of JH into the postembryonic realm likely followed the new morphogenesis supporting wing formation in the nymph. The postembryonic use of JH during wing morphogenesis resulted in growing wings as wing pads rather than as winglets. A decline in JH, though, was required for wing differentiation at the adult molt. The shift to complete metamorphosis, with its larval, pupal, and adult stages, was accompanied by a second, brief JH peak during the formation of the pupa. Analogous to its role in nymphs, JH maintains the larval form during growth but also acts on persisting embryonic primordia to prevent their morphogenesis while allowing isometric growth. JH decline late in larval growth then allows these primordia to undergo morphogenesis and cell determination. The evolving role of JH is also discussed in the context of the major genes that control complex insect life histories: chinmo, broad, and E93.