Interspecific Variation in Photo-Induced GnRH Plasticity among Nomadic Cardueline Finches

Abstract

Changes in response to photoperiod are of fundamental importance to seasonal cycles in avian reproductive development. In this study we present data on photo-induced changes in gonadotropin-releasing hormone (GnRH) immunoreactivity associated with the development of photorefractoriness in males of 3 closely related species of cardueline finch: common redpoll (Carduelis flammea), pine siskin (C. pinus) and white-winged crossbill (Loxia leucoptera). All are nomadic, mid- to high-latitude breeders with varying levels of reproductive flexibility: redpolls are typically seasonal, pine siskins more flexibly seasonal and white-winged crossbills temporally opportunistic. Males were exposed to either long (20L:4D) or short days (5L:19D), beginning in January, and GnRH was assessed via immunocytochemistry 4.5 months later. Plasma LH, testis size, and molt score were also measured. Significant reductions in the number of irGnRH cells detected, cross-sectional cell area and optical density of these cells occurred in both redpolls and siskins on long, as compared with short, days. These decreases in irGnRH, coupled with spontaneous testicular regression and onset of molt were consistent with the development of absolute photorefractoriness on constant long days. In contrast, the tendency for numbers of GnRH immunopositive cells and mean measures of GnRH immunoreactivity (cell area, optical density and axonal fiber density) to decrease were much more moderate in white-winged crossbills held on long versus short days. Although none of the four measures of GnRH immunoreactivity differed significantly between treatments in crossbills, significant shifts in the frequency distribution of irGnRH neurons to include greater numbers of less immunoreactive cells suggested that antigenic GnRH content was reduced in many cells. These photo-induced shifts in optical densities of GnRH cells in long-day, as compared with short-day crossbills followed, to a lesser extent, the pattern in pine siskins and common redpolls. For all three species, patterns of decrease in GnRH immunoreactivity, gonadal regression and molt suggest that some form of refractoriness to photostimulation develops on constant long days. However, more moderate tendencies for GnRH immunoreactivity to decrease in white-winged crossbills, coupled with limited gonadal regression and molt on long days suggest that absolute photorefractoriness might develop more slowly in this taxon than in others, or alternatively, a condition arises that is more similar to relative photorefractoriness. These data indicate that relatively small differences in the response of the neuroendocrine system to stimulatory cues such as day length may lead to remarkably different capabilities for reproductive flexibility even within a closely related family of birds.

Photoperiodic induced changes in reproductive state of border canaries (Serinus canaria) are associated with marked variation in hypothalamic gonadotropin-releasing hormone immunoreactivity and the volume of song control regions

“…This form of the decapeptide has been clearly linked to the control of gonadotropin secretion in birds and has been found to exhibit a marked variation in several other species collected in different physiological states (e.g., Dawson et al, 1985, Ball and for a review). Not all species exhibit such variation in GnRH content in relation to reproductive state (see Ball and Hahn, 1997, Dawson et al, 2001, Pereyra et al, 2005. Initially it appeared as if the degree of seasonal GnRH plasticity was associated with variation in the onset of photorefractoriness (Ball andHahn, 1997, Hahn et al, 1997), but recent work has defined a set of general rules influencing plasticity (Pereyra et al 2005).…”

“…Initially it appeared as if the degree of seasonal GnRH plasticity was associated with variation in the onset of photorefractoriness (Ball andHahn, 1997, Hahn et al, 1997), but recent work has defined a set of general rules influencing plasticity (Pereyra et al 2005). In particular, comparative studies of cardueline finches with diverse photoperiod response patterns suggest that GnRH plasticity tends to be observed in species that do exhibit gonadal regression while still experiencing long daylengths that were previously able to stimulate gonadal growth (Pereyra et al 2005, Macdougall-Shackleton et al, 2006. As demonstrated here, Border canaries fit this pattern.…”

“…Within the cardueline subfamily, that includes canaries, species variation in the degree of breeding flexibility is associated with species variation in the neuroendocrine plasticity in the hypothalamo-pituitary-gonadal axis (MacDougall-Shackleton et al, 2001, Hahn et al, 2004, Pereyra et al, 2005. Species in which photostimulation solely can instate vernal breeding activity tend to exhibit marked declines in preoptic-hypothalamus area during the non-breeding season of the key releasing peptide that controls gonadotropin function, gonadtrophin-releasing hormone-I (GnRH; e.g.…”

“…Species in which photostimulation solely can instate vernal breeding activity tend to exhibit marked declines in preoptic-hypothalamus area during the non-breeding season of the key releasing peptide that controls gonadotropin function, gonadtrophin-releasing hormone-I (GnRH; e.g. Pereyra et al, 2005). Those species that can respond more opportunistically to a variety of cues tend to maintain appreciable GnRH throughout the year (e.g.…”

“…Those species that can respond more opportunistically to a variety of cues tend to maintain appreciable GnRH throughout the year (e.g. Pereyra et al, 2005). Despite many years of study, there is disagreement as to whether seasonal changes in reproductive activity are associated with such plasticity in the neuroendocrine system of canaries, perhaps because of the failure to recognize the flexibility of canary breeding strategies.…”