Insect juvenile hormone: from "status quo" to high society

Juvenile hormone (JH) exerts pleiotropic functions during insect life cycle s. The regulation of JH biosynthesis by neuropeptides and biogenic amines, as well as the transport of JH by specific binding proteins is now well und erstood. In contrast, comprehending its mode of action on target organs is still hampered by the difficulties in isolating specific receptors. In conc ert with ecdysteroids, JH orchestrates molting and metamorphosis, and its m odulatory function in molting processes has gained it the attribute "status quo" hormone. Whereas the metamorphic role of JH appears to have been wide ly conserved, its role in reproduction has been subject to many modificatio ns. In many species, JH stimulates vitellogenin synthesis and uptake. Ln mo squitoes, however, this function has been transferred to ecdysteroids, and JH primes the ecdysteroid response of developing follicles. As reproduction includes a variety of specific behaviors, including migration and diapause , JH has come to function as a master regulator in insect reproduction. The peak of pleiotropy was definitely reached in insects exhibiting facultativ e polymorphisms. In wing dimorphic crickets, differential activation of JH esterase determines wing length. The evolution of sociality in Isoptera and Hymenoptera has also extensively relied on JH. In primitively social wasps and bumble bees, JH integrates dominance position with reproductive status . In highly social insects, such as the honey bee, JH has lost its gonadotr opic role and now regulates division of labor in the worker caste. Its meta morphic role has been extensively explored in the morphological differentia tion of queens and workers, and in the generation of worker polymorphism, s uch as observed in ants.