The effects of pesticide exposure on gonadal development, phonotaxis, and calling in the African clawed frog, Xenopus laevis.

摘要

Amphibian populations have been declining in recent decades, and pesticide exposure has been implicated as a contributing factor. Pesticides and other man-made chemicals have been shown to cause a multitude of effects in amphibians, including decreased growth, altered gonadal development, suppressed immune function, and diminished reproductive ability. This dissertation specifically addresses the sub-lethal long-term exposure effects of pesticides on growth and gonadal development, as well as effects on phonotaxis and calling behavior, in the African Clawed Frog, Xenopus laevis.;Chapter 1 discusses amphibian endocrine disruption, and reviews the developmental and activational effects caused by pesticide exposure in X. laevis. Chapter 2 examines the effect of atrazine (0.1 ppb), atrazine and metalochlor (0.1 ppb each), and a nine-pesticide mixture (0.1 ppb each) on larval development. Animals were euthanized at metamorphosis, and snout-vent length, body weight, mortality, and time to metamorphosis were recorded. Gonads were examined histologically for abnormalities. Animals in the nine-pesticide group experienced delayed development, and were smaller than controls at metamorphosis. Animals from the estradiol treatment group took longer than controls to undergo metamorphosis, and were larger at metamorphosis than control animals. Estradiol treatment successfully sex-reversed 100% of the males. Gross gonadal analysis revealed normal gonadal morphology among males and females across all treatments. Histological analysis revealed immature male gonads in some males across all treatments. Several males from each treatment possessed slightly feminized gross morphology, but histology revealed them as normal males.;The experiment outlined in Chapter 3 uses a novel experimental design to record phonotaxis of pesticide-exposed males in response to a pre-recorded sexually stimulating female call. Pesticide treatments included atrazine, atrazine and metolachlor, and a nine-pesticide mix. The effects of human chorionic gonadotropin (hCG) on phonotaxis were also recorded. Treatment and hCG had no effect on time to first movement and total active time. Time (total active and total overall) spent closer or farther away from the speaker was not affected by treatment or hCG. Time spent swimming towards or away from the speaker was not altered by treatment or hCG. HCG had a significant effect on whether or not males attempted to amplex the speaker, but no interaction variables (interaction duration, number of interaction bouts, time spent interacting) were found to significantly differ between treatments. Total call duration correlated significantly with total active time, except for males from the atrazine and metalochlor treatment group.;Chapter 4 focuses on the effect of pesticide exposure on advertisement call characteristics. Pesticide exposure had no effect on inter-click interval (ICI) or click number. Also unaffected by treatment were dominant frequency, time to first call, and total call duration. A significant seasonality was discovered, with time to first call increasing for males tested later in the year. Dominant frequency significantly differed between the fast and slow trills of the advertisement call.;The novel experimental design shows promise in being developed to study reproductive behavior in X. laevis. Chapter 5 concludes the dissertation, and outlines recommendations for future studies using X. laevis as a model system to study behavioral endocrine disruption.