The hormone testosterone is classically viewed as a promoter of growth, but this generalization is based on model species in medicine, agriculture, and aquaculture. In most of these model species, males also happen to be larger than females. However, in several lizard species in which females are larger, testosterone inhibits growth. This raises the intriguing possibility that, far from being hard-wired, the effects of testosterone (and other hormones) on growth and other processes may change considerably during evolution. This project will test for such "evolutionary reversals" in the hormonal regulation of growth across lizard species. These species have been selected to span repeated evolutionary shifts between those in which males are larger and those in which females are larger. By simultaneously characterizing the effects of testosterone on the expression of thousands of genes, this project will also determine how such "evolutionary reversals" in the hormonal regulation of growth are accomplished at the genetic level. The results of this collaborative study will broaden understanding of the endocrine system. It may also challenge generalizations derived from traditional animal models by investigating how closely related species evolve different responses to the same hormone. The project will also contribute to filling national teaching needs and will involve high school teachers in research experiences in an intensive summer program. Research training opportunities are being provided for a post-doctoral fellow, a PhD and an MS graduate student, and two undergraduate students.This Collaborative project addresses an issue of current debate in comparative endocrinology: whether the highly conserved vertebrate endocrine system tends to constrain adaptation, or whether key evolutionary changes in the coupling of hormones to gene networks instead facilitate rapid evolution. This project will combine hormone manipulations and high-throughput RNA sequencing aimed at (1) characterizing how the expression of growth-regulatory gene networks differs with respect to age, tissue, and hormone treatment using detailed experiments on three focal lizard species, (2) testing for evolutionary reversals in the hormonal regulation of growth and gene expression by extending these methods to a broader, phylogenetically informed sampling of related lizard species with variable patterns of sexual dimorphism in body size (male-larger or female-larger), and (3) characterizing rates of sequence evolution for thousands of expressed genes to test whether the specificity of expression that is achieved by hormonal regulation also frees coding sequences themselves to evolve more quickly. This research will support development of the recently sequenced genome of the Eastern Fence Lizard as an emerging resource for the fields of comparative endocrinology and evolutionary genetics. This project enhances the professional development of science educators through Teacher Workshops and Teacher Fellowships administered through an ongoing education program, which fosters teacher-scientist collaboration and authentic scientific discovery in the K-12 classroom. In addition, research training opportunities and funding are provided for a post-doctoral fellow, a PhD student, a MS student, and for two undergraduates each year.This award is co-funded by the Physiological Mechanisms and Biomechanics Program in the Division of Integrative Organismal Systems, the Evolutionary Processes Program in the Division of Environmental Biology, and the Rules of Life Venture Fund of the Emerging Frontiers Program in the Directorate for Biological Sciences.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.