| Eggs of many vertebrates contain yolk steroids which can profoundly influence offspring survival, development, growth, and behavior. Thus, yolk steroids may serve as a non-genetic mechanism for altering offspring phenotype. While a great deal of attention has focused on phenotypic effects, the timing and mechanism of yolk steroid deposition remain unclear. This dissertation research involved a series of laboratory experiments using the tree lizard ( Urosaurus ornatus) to examine the transfer and effects of yolk steroids, particularly when maternal plasma corticosterone (CORT) was elevated. CORT is an important regulator of homeostatic energy balance, and is also the primary hormone involved in the stress response. In particular, this research examined the effects of increased maternal CORT at different times during reproduction on yolk CORT levels and egg composition, how increased yolk CORT at the time of egg laying changes both yolk and embryonic steroids during incubation, and the phenotypic effects due to increased yolk CORT. Results demonstrate that the relationship between maternal plasma and yolk steroids is more dynamic than previously thought, and that increased yolk CORT can significantly affect offspring phenotype even though yolk CORT levels decrease to control levels during the first third of incubation.;Specifically, the results found that (1) CORT can be transferred to both preovulatory and postovulatory eggs; (2) the amount of yolk CORT depends on maternal CORT concentration in a non-linear fashion; (3) yolk CORT concentrations can decrease prior to egg laying; (4) both yolk and embryo steroid concentrations change dramatically during incubation; (5) increased yolk CORT has numerous detrimental effects on offspring phenotype; and (6) offspring phenotypic effects are likely due to yolk steroids and not alterations of other egg constituents. Together, these results demonstrate the dynamic nature of yolk steroids, in particular that yolk steroid concentrations are not static and do not change consistently during egg formation or incubation. This research is the first to describe the deposition of CORT into yolk and its effects in an oviparous reptile. Furthermore, this research highlights the need for more detailed studies of hormone transfer, metabolic pathways, and enzyme reactions that may occur within the developing egg. |
