JENNINGS, D.H.*; WEISS, S.L.; MOORE, M.C.: Ontogenetic changes in embryonic yolk steroid content in tree lizards: Transfer of hormones from the developing embryo to the yolk?

In oviparous vertebrates, hormones that accumulate in the yolk during gametogenesis regulate development and can affect a variety of morphological, physiological, and behavioral traits. We examined yolk steroid hormone content during embryonic development in male and female tree lizards, Urosaurus ornatus. Progesterone (P), testosterone (T), and estradiol (E2) were present in most newly laid eggs, whereas corticosterone (CORT) was detectable in fewer eggs (34%). Later in development, at day 25 of the 36 day incubation, yolk P levels were higher in males than females. Levels of T, E2, and CORT at this stage were similar between the sexes. Between day 25 and 30, P declines in males but not in females. Other hormones change little during this period. Unlike the other steroids measured, yolk CORT levels increased during embryogenesis. We have also shown (1) that embryonic adrenal glands secrete CORT prior to hatching and, therefore, may be a potential source for the increase in yolk CORT and (2) that CORT may be necessary for hatching: only 10% of eggs treated with RU486 (a P and CORT inhibitor) hatched compared with 90% hatching success in eggs treated with vehicle or P. Although preliminary, these results suggest that CORT may mediate hatching in tree lizards as many of the RU486-treated eggs contained living embryos. Our results suggest that ontogenetic changes in yolk steroids are more complex than previously thought. Yolk hormone content is not simply a function of maternal provisioning, and hormones produced by the developing embryo during late stages of development also may contribute to yolk hormone content.