Interspecific variation in photo-induced GnRH plasticity among nomadic cardueline finches

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.