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Evolutionary endocrinology of juvenile hormone esterase, an insect regulatory enzyme. For several decades, wing polymorphism has served as one of the premier postulated examples of morphological evolution (i.e. evolution of wing size and flight muscle mass) that has occurred via modification of hormone levels (juvenile hormone and ecdysone) that regulate the expression of morphology. Detailed research in my laboratory over the past decade has provided the first direct evidence on variation in several endocrine mechanisms that potentially regulate aspects of morph development. Most importantly, my laboratory has documented dramatic variation in the activity of a regulatory enzyme, juvenile hormone esterase (JHE), that degrades and regulates the JH titer (level). Numerous pieces of evidence are consistent with JHE being an important regulator of genetically-determined wing polymorphism. For example, variation in JHE activity strongly co-segregates with wing morph and is correlated with in vivo variation in JH degradation. Although it is still uncertain whether variation in JHE activity causes variation in the juvenile hormone titer, our studies of JHE in wing polymorphic crickets remains the most intensively investigated case of juvenile hormone-mediated morphological polymorphisms in natural populations.
Molecular studies of JHE. Enzymological studies indicate that variation in JHE activity most likely results from regulatory variation that affects the concentration and tissue specificity of the JHE protein. Recently, we have purified the JHE protein, raised polyclonal antibodies, and screened a cDNA library using probes derived from the JHE amino acid sequence. This ongoing project is being undertaken in collaboration with John Oakeshott, CSIRO Entomology, Canberra, Australia and Larry Harshman, Biological Sciences, University of Nebraska . WE have recently obtained NSF funding (1/03-12/05) to continue this work. We plan to compare DNA sequences of JHE alleles from morphs differing in JHE activity to identify the specific molecular causes of variation in the activity and tissue distribution of this potentially important regulator of morphological/dispersal polymorphism.
Artificial selection on JHE activity. As a complimentary approach to studies of naturally-occurring co-variation between JHE activity and morphological polymorphism, we have undertaken direct selection on hemolymph JHE activity in a wing monomorphic insect. To my knowledge, this is the first, (and still the only) study in which an endocrine regulator has been directly subjected to artificial selection. Results of this extensive study (involving 20,000 individually-reared crickets) have important implications for the endocrine-genetic integration of juvenile development and adult reproduction. Here I mention only one important result. Bidirectional selection on JHE activity in juveniles led to a 10-fold difference in enzyme activity between lines in juveniles but no difference in adults. This indicates that the endocrine regulation of juvenile development can evolve independently of the endocrine regulation of adult reproduction. The endocrine-genetic integration of juvenile development and adult reproduction is a fascinating but poorly studied research problem. Future studies will focus on neurohormonal integration of juvenile development and adult reproduction.