Geographic Variation in Freeze Tolerance: Alaskan vs. Ohioan Populations
Temperate amphibians exhibit strong clinal variation in thermal tolerance and are thus useful subjects in the study of environmental adaptation. Owing to its high dispersal ability, broad geographic range, and extreme northern distribution (into the Arctic Circle), the wood frog, Rana sylvatica, is a singularly valuable species in such studies. Our research follows from preliminary findings that the lower thermal limit of freezing survival in a R. sylvatica population near the northern limit of the species’ distribution is at least 12°C lower than that of conspecifics from several more temperate locales. University of Alaska-Fairbanks zoophysiologist Dr. Brian M. Barnes and his students have discovered that local R. sylvatica tolerate extensive freezing, with many surviving exposure to –17°C (Middle LB, Barnes BM. Am Zool 41:1526, 2001). Working in collaboration with the Barnes' lab, our ultimate goal in this project is to confirm and extend these preliminary findings and to test hypotheses concerning the mechanistic basis for extreme freeze tolerance in these frogs. Funding for the project is provided by the National Science Foundation (IOS-1022788).
Organized into three major aims, our work promises to provide new insights into the evolutionary development of freeze tolerance as a cold-hardiness strategy, and to enhance understanding of the molecular and biochemical basis of freeze tolerance in vertebrate ectotherms.
We are characterizing freeze tolerance in R. sylvatica indigenous to Interior Alaska, comparing their limits of freezing survival and physiological responses to experimental freezing with those of conspecifics from a well-studied population in Ohio, near the species’ southern limit of distribution.
Because cryoprotection is a central adaptation in natural freeze tolerance, several studies address adaptations of the glycemic cryoprotectant system that underlie enhanced freeze tolerance in northern R. sylvatica, particularly as they relate to the regulation of glucose biosynthesis and distribution in the freezing response. Specifically, this work focuses on quantitative and qualitative enzymatic adaptation in the hepatic glucose production pathway, as well as the membrane transporter system governing efflux of glucose from the liver and, hence, its delivery to other tissues.
In collaboration with Dr. Gary Lorigan (Department of Chemistry & Biochemistry, Miami University), solid-state NMR spectroscopy is used to directly measure membrane fluidity and the chemical composition of membrane phospholipids, testing the idea that homeoviscous adaptation contributes to enhanced freeze tolerance in northern R. sylvatica.
This project contributes to the professional development of Miami University graduate and undergraduate students by engaging them in all phases of the research process and by encouraging them to develop grantsmanship skills, publish their work in peer-reviewed journals, and communicate research findings at professional meetings.
Our research findings contribute to scientific and technological development in fields other than comparative physiology. As examples, studies of natural osmolyte systems have contributed to the formulation of clinical and commercial media for preserving biologicals in anhydrous and frozen states; elucidating mechanisms of glucose transporter regulation is germane to disease states associated with anomalous transporter expression (e.g., chronic pancreatitis, diabetes/obesity); and understanding the winter biology of amphibians may help predict consequences of climate change for their survival.
Our work enhances scientific and technological understanding by creating a traveling exhibit, “Nature’s Frozen Marvels” that educates the general public about the little-known yet fascinating adaptation of natural freeze tolerance, drawing special attention to the underlying principles of biology, chemistry, and physics. The exhibit is being designed and fabricated in collaboration with Miami University’s Center for Environmental Education and Hefner Zoology Museum and will be made available to various local and national not-for-profit institutions for a term of at least 5 yr. In conjunction with this project, we will implement a special-topic course that engages undergraduate students representing various academic disciplines (e.g., communication, engineering, psychology, as well as the natural sciences) in the task of designing the exhibit.