Dissertation Defenses

Doctoral Dissertation Announcement

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Candidate: Renee M. Zaya

Degree of: Doctor of Philosophy

Department: Biological Sciences

Title: Molecular, Cellular and Systematic Effects of Atrazine on Xenopus laevis Tadpoles

Committee:
Dr. Charles F. Ide, Chair
Dr. Bruce E. Bejcek
Dr. Robert R. Eversole
Dr. Susan R. Stapleton

Date: Tuesday, May 4, 2010 1:30 p.m. - 3:30 p.m.
2902 Wood Hall

Abstract:
Atrazine is one of the two most commonly used herbicides in the US and it is the most commonly found herbicide in ground water. As a result, a great deal of attention has been placed on its use and safety. It has also been implicated to play a role in the decline of frog populations worldwide.
This dissertation reports results from studies into the effects of atrazine on developing tadpoles of the amphibian laboratory model, the African Clawed frog (Xenopus laevis). The levels of atrazine tested include levels found in field runoff (200 and 400 μg/L) and a low dose (25 μg/L) studied by others. During all exposures, growth, metamorphosis, fat body (a lipid storage organ) size and liver weights were measured. In selected studies, feeding behavior was recorded, microscopic evaluations were conducted, qRT-PCR was performed on selected genes, and ATP and ADP levels were measured.
The low-level exposure had no effect on tadpoles. Tadpoles exposed to runoff levels were smaller than unexposed controls by three days into the exposure and were still smaller at the end of the studies. Some groups also experienced slowed metamorphosis. Interestingly, appetite was not decreased in these tadpoles. Livers and fat bodies were also smaller at the end of exposure. Liver ADP:ATP ratios indicated these animals had some difficulty maintaining adequate energy levels. Using qRT-PCR, changes were shown in gene expression 24 hours into exposure. The genes affected were indicative of an early stress response to exposure. Clearly, acutely and chronically exposed tadpoles were compromised potentially decreasing their ability to survive the stresses of metamorphosis and reducing their reproductive fitness.
It is significant that changes were noted within 72 hours of exposure since, in the field, tadpoles could encounter similar exposure rates. Therefore, these data are relevant to field conditions. These studies are among the first to link physiological effects of atrazine to changes in tissue and gene expression in Xenopus laevis tadpoles and provide the groundwork for future studies into the mechanisms behind responses to chemical stressors generating data resulting in a better understanding of how these chemicals affect us all.