Gonads and the evolution of hormonal phenotypes.

Area of Science:

• Endocrinology
• Evolutionary Biology
• Animal Behavior

Background:

• Hormones, like testosterone (T), are crucial signaling molecules influencing gene activity and phenotypes.
• Testosterone mediates many fitness-related traits in vertebrates, but the evolutionary mechanisms of T level variation remain unclear.
• Understanding T evolution is key to explaining hormone-mediated phenotypic evolution.

Purpose of the Study:

• To investigate the mechanisms driving differences in testosterone production between two subspecies of dark-eyed juncos.
• To identify the roles of the gonad and hormonal axes in regulating T levels and phenotypic divergence.

Main Methods:

• Comparative study of two dark-eyed junco subspecies differing in aggression, size, and ornamentation.
• Common garden experiments to control for environmental influences.
• Analysis of gene expression for steroidogenic enzymes and hormone receptors (luteinizing hormone, gonadotropin-inhibitory hormone, glucocorticoid receptor, mineralocorticoid receptor) in the pituitary and gonads.

Main Results:

• Subspecies differ in the timing of T production, with the gonad being a primary source of variation.
• No difference in pituitary luteinizing hormone output, but increased gonadal steroidogenic enzyme expression in the more androgenized subspecies.
• Altered cross-talk between the hypothalamo-pituitary-gonadal and -adrenal axes, indicated by reduced glucocorticoid and mineralocorticoid receptor expression in the more androgenized subspecies.

Conclusions:

• Divergence in T production mechanisms, including gonadal function and hormonal axis sensitivity, contributes to phenotypic evolution.
• Multiple pathways can generate functional variation in T, impacting hormone-mediated trait evolution.
• Findings illuminate the complex interplay between hormones and evolutionary adaptation.