Juvenile Hormone and the endocrine regulation of wing polymorphism in insects: new insights from circadian and functional-genomic studies in Gryllus crickets

For decades, Juvenile Hormone (JH) has been a major focus of studies investigating the endocrine regulation of wing-polymorphism. The most general model postulates a single threshold, above which JH causes the expression of traits that define the short-winged morph (SW), and below which JH causes the expression of traits that define the long-winged morph (LW). Early studies in aphids and crickets reported ambiguous results as a result of the small size of aphids or the very low JH titre in nymphal crickets. Detailed studies in wing morphs of adult Gryllus firmus Scudder uncovered an unexpected and novel morph-specific JH titre circadian cycle (cycling in LW but not in SW) in both the laboratory and field. This finding clearly contradicts the classic model. Morph-specific daily rhythms in global gene expression are strongly associated with (and are possibly caused by) the morph-specific JH titre rhythm. Daily rhythms for hormonal traits and gene expression, which are largely ignored in studies of life-history evolution, may be common and play an important role in adaptation. Juvenile Hormone has likely evolved a specialized within-morph function in G. firmus, regulating aspects of daily flight in the LW morph, which exhibits circadian flight. Other hormones, such as insulin-like peptides and ecdysteroids, possibly regulate the expression of chronic (long-term, noncircadian) differences between LW and SW morphs. Future studies should aim to investigate JH titres in more detail, as well as other hormones, most notably peptides and biogenic amines, which are largely ignored in endocrine studies of wing polymorphism.