Molecular Evolution of Juvenile Hormone Signaling

Abstract

Insect development proceeds through a series of discrete developmental stages called instars. During hexapod evolution, the development of complete metamorphosis introduced a novel mechanism for separating feeding and reproductive stages (Truman & Riddiford, 2002), facilitating the tremendous evolutionary success of holometabolous insects. In contrast to hemimetabolous insects, which progress through a series of instars that appear as smaller iterations of the adult form, holometabolous insects proceed from egg to adult through a progression of isomorphic larval instars and a pupal transitory stage. In each case, the physical boundary for growth during an instar is established by a chitinous exoskeleton, which must be periodically shed. This molting process is under the control of two counteracting hormones. Toward the end of an instar, a pulse of the insect molting hormone, 20-hydroxyecdysone (20E) initiates a transcriptional cascade that carries the molt to a subsequent instar. However, it is the interaction of 20E and the sesquiterpenoid juvenile hormone (JH) that governs the developmental outcome of each molt. During larval development, an elevated JH titer and 20E directs the sequential progression through larval development until the final larval instar, when the JH titer substantially declines. The removal of circulating JH facilitates a 20E-directed developmental switch that initiates the metamorphic molt. Thus, it was proposed that JH can modulate 20E activity, maintaining the status quo during preadult development.