Amphibian populations have been declining for over two decades. One hypothesis for this decline is that a commonly used herbicide, atrazine, is causing abnormalities in gonadal development. The Environmental Protection Agency has set the drinking water standard level of atrazine at three parts per billion (ppb). This experiment studied four groups: juveniles exposed to two ppb atrazine, juviles exposed to 95% ethanol (control), tadpoles exposed to two ppb atrazine, and tadpoles exposed to 95% ethanol. Juveniles were exposed for 107 days, while the tadpoles were exposed for 22 days. There were no gross morphological differences resulting from atrazine exposure. However, histological analysis of the tadpole groups showed that the cortex widths of atrazine-exposed females were significantly larger than control females (p=0.0111). Cortex width per body weight of the tadpoles showed no significant difference (p=0.0776), but previous research has shown that gonad size is not dependent on body weight. Further research is needed to determine if these findings are repeatable at this low dosage. If these findings are further supported, the EPA standard level of atrazine of three ppb will need to be reevaluated to ensure safe environmental toxicant levels
Hometown: Murfreesboro, TN
Senior Study Title: “Effects of Atrazine on Xenopus laevis Ovarian Development”
Advisor: Dr. Drew Crain, associate professor of biology
Every semester new Senior Studies are completed, presenting original, undergraduate research from every academic division on campus. Students get to experience the tangible joy of conducting research guided by their own interests, synthesizing ideas and concepts from their respective fields of academic study. But throughout the course of each study, there comes at least one time of unexpected challenge.
“The reality of research is that you always face problems to overcome,” commented Dr. Drew Crain, associate professor of biology. But the problems facing a biological study are quite different than those confronted by an English or math major.
“We had to do our experiments many times. The first time around everything was going very well and then—mass mortality,” recalled Austin Mackens. The “mass mortality” referenced by Mackens was the result of a slime-mold outbreak in the water of the frogs. “But we learned from the error and in the end, it improved the study.”
Setting the stage for Mackens’ study was his advisor’s experience (Crain studied the effects of pesticides on alligator reproductive organs as part of his Ph.D research) and a former student’s research on the effects of the estrogen ethinylestradiol on the gonadal development of Xenopus laevis (Jenny Wilson ’05). It was an article published by Dr. Tyrone B. Hayes in BioScience (December 2004/Vol. 54, No. 12) entitled, “There Is No Denying this: Defusing the Confusion about Atrazine” that actually triggered the idea to connect Crain’s doctoral research and Wilson’s previous Senior Study.
“Jenny had used frogs, Hayes’ article outlined the confusion surrounding the effects of atrazine, and my work had previously focused on ovary and herbicide interaction,” commented Crain. “But Austin is the one who put it all together.”
Effective and consistent animal husbandry played a critical role in the four-month long study. From feeding and water changes, to atrazine dosing, to the measuring and behavioral observations, Mackens and a team of four other students (Heather Hedden ‘06, Lauren Wood ‘06, Travis Groth ‘06, and Ginger Loveingood ‘06) utilized the frogs in a comprehensive research project examining the influence of atrazine on the frogs’ ovaries (Mackens), livers (Hedden and Wood), testes (Groth) and larynx (Lovingood).
“The EPA lists 3ppb (parts per billion) as the safe standard for atrazine levels,” said Mackens. “For our study, we used 2ppb of atrazine to show that even a lower standard than that approved by the EPA could cause issues with frogs.” At these levels, mammals and birds do not exhibit direct negative effects, but they do experience a ripple effect. With frog ovaries compromised or destroyed, frog populations -- a food source for birds and mammals -- are decreasing. Less available food results in higher competition among birds and mammals for the same resources. Mackens’ study confirmed that atrazine is contributing to a known amphibian decline.
“Having the experience of this research and the tangible work of my study was incredibly helpful as I interviewed for graduate school. It boosted my confidence and sparked the interest of my interviewers for medical school. They weren’t aware this kind of research was being conducted at the undergraduate level,” said Mackens.
“I was able to educate some of my interviewers about some of the toxicants studied. They weren’t aware of them because they’d been out of active research for five or six years.” Interviewers were also impressed with Mackens’ undergraduate experience with comprehensive exams.
But it is not only the student who reaps benefits from the process and experience of Senior Study. Dr. Crain quickly pointed to his observances and clarified notion of what makes a great study truly great.
“What this reinforced for me was that with any good scientific study there are more questions at the end than there are answers,” concluded the advisor. “We didn’t find as dramatic results as we expected, such as ovotestes [i.e., frogs exhibiting both male and female reproductive organs].” Crain also recognized that successful studies are often not so much about the experiment but about how one responds to the unanticipated problems.
“The mark of a good scientist is how he or she responds to problems, and Austin’s responses were both positive and productive during the entire process.”
Mackens applied to medical school at Wake Forest, East Tennessee State University and the University of Tennessee-Memphis, and is attending medical school at UT-Memphis.