This past Friday, Santa Clara University biology professor Elizabeth Dahlhoff lectured at Dartmouth as a part of the biology department’s Cramer Seminar Series. Her research focuses on temperature adaptation and climate variation, specifically in the High Sierra region of California.
Dahlhoff’s model organism for her study was the herbivorous Willow Leaf Beetle (Chrysomela aeneicollis).Throughout their life cycles, these beetles experience very stressful temperature conditions, from the pressure of laying eggs during scorching summers to overwintering in sub-zero temperatures.Even during the course of a day, they experience wide temperature ranges.Ectotherms such as beetles and other herbivorous insects have internal body temperatures proportional to the air temperature; as a result, both their performance and their genetic diversity are inherently linked to the thermodynamics of nature.
The main focus of Dahlhoff’s seminar was on two different allelic forms of the gene for the glycololytic enzyme, Phosophoglucose Isomerase (PGI).PGI converts glucose-6-phosphate to fructose-6-phosophate in glycolysis just before an important phosphorylation, and does the reverse in gluconeogenesis.Their performance in extreme climates is related to how well they cope, and the different forms of PGI are related to expression of heat shock proteins.
In extreme temperatures, polypeptides/proteins may unfold, which can expose hydrophobic regions.If proteins like PGI denature, then it is up to heat shock proteins to recover and refold them; otherwise, these unfolded polypeptides can aggregate and become toxic.Furthermore, without heat shock resistance, it is possible for these beetles to lose voluntary muscle control.
The “1” and the “4” alleles are present at different frequencies depending on the temperature of the area—“1” is cold adapted and “4″ is warm adapted. Dahlhoff’s lab studied not only the polymorphism of PGI in areas such as Big Pine Creek and Rock Creek, California, but also how the gene itself changes physiological performance.
Most of the energy expenditure by these beetles is running; Dahlhoff measured running speed using a small stick and a light source towards which beetles and larvae ran.When exposed to either -4 (cold extreme), 20 (normal), or 36 (hot extreme) degree temperatures, the 1-1 beetles were able to run faster than the 4-4 beetles, but upon a second extreme exposure (-4 or 36), the 4-4 beetles were able to up-regulate heat shock proteins and increase running speed.Speed is essential for reproductive fitness.In addition, beetles with the 1-1 genotype lay more eggs in colder areas, while beetles with the 4-4 genotype lay more eggs in warmer sites.
What does the future hold for Dahlhoff’s research?She plans to continue studying the relation between climate changes and genetic changes, and also patterns involving local populations going extinct when environmental conditions change.Dahlhoff concluded, “The ultimate fate of organisms relies on their ability to adapt to extremes throughout the year.”