Term of Award

Fall 2007

Degree Name

Master of Science in Biology (M.S.)

Document Type and Release Option

Thesis (open access)

Department

Department of Biology

Committee Chair

David C. Rostal

Committee Member 1

Bruce Schulte

Committee Member 2

Lance D. McBrayer

Abstract

In oviparous vertebrates, the maternal transfer of steroid hormones to egg yolk can vary among species, within and among clutches, seasonally, due to age, and in response to social and environmental cues. In previous studies, significant effects on hatchling phenotype, sex and survival have been attributed to a variation in egg yolk hormone levels. The differential allocation of yolk hormones in sea turtles, multiple clutching oviparous reptiles with a high reproductive output and temperature dependent sexdetermination (TSD), has yet to be investigated. This study examined the variation of maternally derived egg yolk hormones of the loggerhead sea turtle (Caretta caretta) within a clutch and with increasing nest events across the season. The quantification of the relationship of yolk hormones to maternal circulating plasma hormones also was examined. Estradiol (E2) and testosterone (T) are associated with vitellogenesis while progesterone (P) is associated with follicular ovulation. Therefore, E2 and T were hypothesized to increase with progressive season and nesting event while (P) was expected to remain constant. Radioimmunoassays (RIA) were used to examine levels of E2, T and P in maternal blood plasma and corresponding egg yolk. Temperature data loggers were placed within the egg chambers to estimate hatchling sex ratios based upon incubation temperatures. Egg yolk E2 and T within a clutch did not vary significantly with reference to position within the nest, while yolk progesterone was significantly higher in eggs collected from the bottom of the egg chamber. Each of the yolk hormones examined increased as the season progressed and with subsequent nesting events while maternal plasma hormones decreased. Thus, an inverse relationship existed between steroid hormone levels in the blood plasma and corresponding egg yolk. In addition, the egg yolk estradiol-testosterone ratio (E2:T) increased significantly as the season progressed and with subsequent nest events. This differential allocation of yolk hormones corresponded with increased nesting events, higher incubation temperatures and predicted female biased hatchling ratios. Sea turtles differentially allocate more resources in the form of yolk hormones to hatchlings of subsequent nest events in the late-season. These findings suggest that late-season hatchlings face different physiological or environmental challenges than their early-season counterparts, thus requiring more maternally derived resources in the form of increased yolk hormones.

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