Hormonal circadian rhythm in the wing-polymorphic cricket gryllus firmus: Integrating chronobiology, endocrinology, and evolution

Abstract

Hormonal circadian rhythms, an important aspect of endocrinology, have been extensively studied in vertebrates, but much less so in insects. Moreover, adaptive evolutionary change of these rhythms has been barely investigated in any species. In the wing-polymorphic cricket, Gryllus firmus, the key insect hormone, juvenile hormone (JH), exhibits a large-amplitude (ca. tenfold change) circadian rhythm in the flight-capable (long-winged, LW(f)) morph that delays reproduction, but a much shallower (twofold change) rhythm in the flightless, reproductive morph (SW, short-winged). The morph-specific cycle is a genetic polymorphism and occurs in both the laboratory and field in G. firmus, as well as in several other cricket species. The cycle is primarily driven by a circadian rhythm in the rate of JH biosynthesis. The function of the morph-specific cycle is unknown at present, but possibly regulates some aspect of flight, which is also circadian in crickets (only occurs at night), and which first occurs when the morph-specific JH titer cycle is also first manifest. Endocrine circadian cycles are likely common and functionally important in insects but have largely been unstudied. Recent transcriptome profiling has identified numerous morph-specific diel changes in transcript abundance that are more common in the LW(f) than in the SW morph and thus are correlated with the greater circadian change in the JH titer in the LW(f) morph. Thus, morph-specific daily change in the JH titer and global transcript abundance appear to be key features of wing polymorphism in Gryllus. These studies in Gryllus are contributing significantly to the synthesis of the previously independent subdisciplines of chronobiology, endocrinology, and life history evolution.