Divergence of Hypothalamic-pituitary-gonadal (HPG) Axis Gene Expression and Testosterone in Migrant and Resident Female White-crowned Sparrows

Photoinduction of the hypothalamic-pituitary-gonadal (HPG) axis in seasonally breeding animals activates the reproductive system but precise timing of breeding and sex steroid production are controlled by local environmental information. We aimed to understand functional regulation of the HPG axis by changes in gene expression and synthesis of testosterone in female migrant and resident subspecies of white-crowned sparrow. We hypothesized that regulation of the HPG axis would differ between residents and migrants during breeding and molt, but not during winter. Plasma testosterone was higher in migrants compared to residents during egg lay and incubation. Hypothalamic expression of estrogen receptor α was down-regulated, while androgen receptor, gonadotropin inhibitory hormone (GnIH), aromatase and 5α-reductase were up-regulated in migrants compared to residents. No differences were observed between subspecies in gene expression for luteinizing hormone receptor, follicle stimulating hormone receptor, side chain cleavage enzyme, steroidogenic acute regulatory protein, 3β-hydroxysteroid dehydrogenase, or aromatase. Ovarian gene expression associated with inhibiting the reproductive axis. GnIH, mineralocorticoid receptor, glucocorticoid receptor and 11β-hydroxysteroid dehydrogenase 2 were higher in migrants but not residents during breeding. These data suggest nesting onset may result in increased plasma testosterone regulated by differential gene expression in the hypothalamus and increased sensitivity to stress at the gonad level.