Morph-Dependent Cyclic JH Titer in a Wing-Polymorphic Insect: Adaptive Significance and Underlying Causes

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Some insects, such as the cricket (Gryllus firmus), are wing polymorphic (i.e., they contain both flight capable and flightless morphs). Wing polymorphism has been extensively used to investigate the hormonal control of dispersal and the evolution of endocrine regulation in insects. However, all previous studies have focused exclusively on the hormonal control of flight capability (e.g., growth of wings and flight muscles) in the laboratory. No study has investigated the endocrine regulation of flight itself, and no study has investigated the hormonal regulation of wing polymorphism in the field. Recent studies indicate that the endocrine regulation of morph-specific traits is more complex than previously expected. The key regulator of wing polymorphism is juvenile hormone (JH). In the cricket, Gryllus firmus, the mean concentration of JH is similar in the flight capable and flightless morphs, but the morphs differ in how the level of this hormone varies over time. The JH titer changes 10-50 fold during the day in the flight capable morph, but it is relatively constant in the flightless morph. This study focuses on the underlying causes and adaptive significance of daily changes in the JH titer. Prolonged elevation of the JH titer is associated with ovarian growth and with the breakdown of flight muscles--changes that are detrimental with respect to flight. This study will test the hypothesis that brief elevation of the JH titer is of sufficient duration to cause the expression of flight behaviors but of insufficent duration to cause ovarian growth or the breakdown of flight muscles. Studies will involve direct comparisons of hormone titers between morphs throughout the day and evening. The JH titer will be experimentally manipulated, and its effect on flight propensity will be monitored. Finally, physiological processes (hormone biosynthesis and degradation) that regulate morph-specific daily changes in the JH titer will be investigated. Importantly, these studies will be undertaken in both the laboratory and the field. This study will substantially enhance our understanding of the endocrine regulation of dispersal and evolution of dispersal in insects.