Examining sources of variation in HPG axis function among individuals and populations of the dark-eyed junco

Abstract

Gonadal steroids are important mediators of traits relevant to fitness, and thus may be targets of selection. However, more knowledge is needed about sources of variation along the endocrine axes that may contribute to functional variation in steroid levels. In a controlled captive environment, we studied males of two closely related subspecies of the dark-eyed junco (Junco hyemalis) that differ in testosterone-related phenotype, asking whether they also differ in testosterone (T), and assessing the contribution of the sequential links of the hypothalamo-pituitary-gonadal axis. When males of both subspecies were challenged with gonadotropin-releasing hormone (GnRH), they were similar in circulating luteinizing hormone (LH) and T responses. When challenged with exogenous LH, they again produced levels of T similar to one another, and to the levels produced in response to GnRH. However, the smaller, less ornamented, and less aggressive subspecies had greater abundance of mRNA for LH receptor in the testes and for androgen receptor in the rostral hypothalamus, suggesting potential differences in regulatory feedback. We suggest that circulating hormone levels may be less prone to evolutionary change than the responsiveness of individual hormone targets. Among individuals, T titers were highly repeatable whether males were challenged with GnRH or with LH, but LH produced in response to GnRH did not covary with T produced in response to LH. Testis mass, but not LH receptor transcript abundance, predicted individual variation in T responses. These data implicate the gonad, but not the pituitary, as an important source of individual variation in T production.

Mechanisms Associated with an Advance in the Timing of Seasonal Reproduction in an Urban Songbird

“…Five minutes following the first GnRH injection, a second 50 µl blood sample was taken from the wing vein for LH. Previous work in this species and white-crowned sparrows (Zonotrichia leucophrys gambelii) indicated LH levels reach a peak 5 min following injection and then significantly decline by 20 min and return to baseline at 30 min (Wingfield et al, 1979;Bergeon Burns et al, 2014). The birds then received a second injection of GnRH (same dose as above) 30 min after the first injection, followed by a third injection of GnRH (same dose as above) 30 min following the second injection.…”

“…The intra-assay coefficient of variation was 2.9% and the minimum detectable dose was 0.2 ng/ml. This LH radioimmunoassay has been used extensively in many avian species including dark-eyed juncos (Deviche et al, 2000;Meddle et al, 2002;Wingfield et al, 2012;Bergeon Burns et al, 2014;Greives et al, 2016).…”

Divergence along the gonadal steroidogenic pathway: Implications for hormone-mediated phenotypic evolution

“…White-winged juncos ( J. h. aikeni ), which breed only in the Black Hills of South Dakota, are significantly larger, more ornamented, and more aggressive than the Carolina subspecies ( J. h. carolinensis ) (Bergeon Burns et al, 2013; Nolan et al, 2002), which breed in the Appalachian mountains of Virginia. The subspecies do not differ in average T levels when sampled in the field (Bergeon Burns et al, 2013) or when sampled in a common aviary environment 30 min after standardized HPG axis activation (Bergeon Burns et al, 2014). Despite these similar T levels measured in prior research, captive VA males appear to be more sensitive to LH stimulation of the gonad (more LH receptor mRNA), and they also may be more sensitive to negative feedback at the top of the HPG axis (more hypothalamic AR mRNA), compared to SD males (Bergeon Burns et al, 2014).…”

“…The subspecies do not differ in average T levels when sampled in the field (Bergeon Burns et al, 2013) or when sampled in a common aviary environment 30 min after standardized HPG axis activation (Bergeon Burns et al, 2014). Despite these similar T levels measured in prior research, captive VA males appear to be more sensitive to LH stimulation of the gonad (more LH receptor mRNA), and they also may be more sensitive to negative feedback at the top of the HPG axis (more hypothalamic AR mRNA), compared to SD males (Bergeon Burns et al, 2014). These findings led us to hypothesize that the populations may differ in temporal regulation of T secretion, despite not differing significantly in the magnitude of circulating T at the sampling points used in earlier research.…”

“…These findings led us to hypothesize that the populations may differ in temporal regulation of T secretion, despite not differing significantly in the magnitude of circulating T at the sampling points used in earlier research. Critically, individual male juncos are also repeatable in how much T they produce in response to exogenous injection of GnRH (Jawor et al, 2006), and this individual variation is remarkably similar to the amount of T produced in response to a standardized LH injection (Bergeon Burns et al, 2014). Thus, our past work suggested that individual differences in T production may lie primarily downstream of this LH signal (i.e.…”