Egg yolk layers vary in the concentration of steroid hormones in two avianspecies

Maternally derived steroid hormones are known to be present in the yolks of avian eggs; however, the physiological mechanisms involved in their deposi tion remain largely unexplored. Investigations of steroid production by avi an follicles have demonstrated temporal differences in the concentrations o f progesterone, 17 beta-estradiol, and testosterone during yolk formation. Because yolk is deposited peripherally in concentric spheres as the oocyte develops, differences in the production of follicular hormones during yolk formation should be manifested in differences in the localization of steroi ds within layers of the yolk. To investigate this hypothesis we analyzed st eroid hormone concentrations in layers of individual eggs of the dark-eyed junco (Junco hyemalis) and the red-winged blackbird (Agelaius phoeniceus). We found that in the dark-eyed junco the concentration of progesterone is s ignificantly greater at the periphery of the yolk, while the concentration of 17 beta-estradiol is significantly greater near the center of the yolk. We also found in bath the dark-eyed junco and the red-winged blackbird that the concentration of testosterone remains constant from the interior to th e intermediate layers of the yolk and then drops sharply between the interm ediate and exterior layers. The patterns of hormone localization that we fo und agree with those predicted by studies of temporal changes in steroidoge nesis in the maturing follicle of the chicken, thus suggesting that within- yolk variation in yolk steroid concentrations in the dark-eyed junco and th e red-winged blackbird reflects temporal differences in the pattern of foll icular steroidogenesis. Variation in the concentration of hormones among yo lk layers presents a methodological concern for studies that involve the re moval of yolk samples from viable eggs for subsequent hormonal analysis. Th is variation also has implications for the timing of embryonic exposure to steroid hormones. (C) 1999 Academic Press.