| This thesis reports the use of the turtle as a sentinel species to monitor the effects of exposure to contaminated groundwater at the Superfund site on Cape Cod, MA. Observations on Chrysemys picta field-trapped from ponds on Cape Cod extended and supported prior observations of reproductive deficits including lower levels of vitellogenin, estradiol-17beta, oviduct weights, and oocyte numbers in females and lower testicular weight and sperm count in males. Possible loci in the hypothalamic-pituitary-gonadal-liver axis at which xenobiotics may act were determined in turtles trapped from Moody Pond (a test site) and Washburn Pond (a reference site). Specifically, gonadotropin and estrogen responses were assessed using plasma steroids and vitellogenin as markers. Basal vitellogenin levels were significantly lower in Moody Pond females; however, vitellogenin responses to estradiol-17beta were the same in both groups, indicating a normal hepatic response to estrogen. In contrast, estradiol-17beta secretion was not stimulated by gonadotropin in Moody Pond females, compared to Washburn animals. Basal plasma testosterone and the response to gonadotropin in males were similar, although steroid levels in Moody Pond animals were slower to return to baseline after gonadotropin injection. The potential impact of xenobiotics on the gonad was investigated in adult, juvenile and embryonic turtles. Moody Pond adult males had a markedly reduced seminiferous tubule diameter and higher incidence of gonial apoptosis than Washburn males and Moody Pond females had more apoptotic oocytes and atretic follicles. In neonate Trachemys scripta exposed to Moody or Washburn Pond environments in the laboratory, oocyte proliferation and apoptosis were not different but spermatogonial proliferation was lower and apoptosis was higher in Moody Pond exposed males. In T. scripta embryos, in ovo exposure to an environmentally relevant dose of cadmium reduced the number of gonia in the gonadal anlagen. The effects of cadmium extended into neonatal life as seen by an increase in oocyte apoptosis in T. scripta and C. picta. The results suggest that a low-level mixture of xenobiotic contaminants, including cadmium, may interfere with gametogenesis and the steroid metabolic pathways in turtles exposed to the test site, but not the reference site, environment. |
