"Clocks are adaptive; they contribute to the fitness of an organism in any particular environment," says McClung. "We found that in species like Arabidopsis, which cover a huge environmental range, there are underlying variations in their circadian clocks to subtly optimize their ability to function in a particular environment."
Published in the November 7 issue of Science, the study examines the clock rhythms in Arabidopsis from different parts of the earth. The researchers analyzed leaf movements and measured the period of time it took the leaves to complete one circadian cycle, noted the time of day when the leaves were pointing straight upward, and calculated the distance the leaves moved during a cycle. All three measurements showed considerable differences in the plants from different areas.
"When we determined day length in each of the latitudes for the plants, we found the correlation was highly significant between the circadian variations in period length and the latitudes of origin of the different plants," says McClung.
A second component of the study provides evidence suggesting that a large number of genes contribute to fine-tuning the Arabidopsis clock.
By crossing Arabidopsis from two different geographic areas, and allowing them to segregate for a number of generations, the researchers identified five chromosomal regions, called quantitative trait loci (QTL), which significantly contribute to either period length or phase or amplitude of the rhythms. Each of these QTL regions includes many genes, at least one of which contributes to the plant's circadian rhythm. McClung's group chose one candidate gene, called APRR7, which was in the one of the QTL regions. When it was knocked it out, it affected period length. The researchers then examined the additional members of APRR7's gene family. They knocked out each of them, which all impacted normal clock function. The results indicate that this gene family works together to control different parts of the clock mechanism.
"This illustrates the point of the QTL analysis that some genes can contribute incrementally to clock behaviors," says McClung. "When we saw huge clock variations in the offspring, more than in the parents, it's commonly interpreted to mean that the behaviors in question are the products of interaction of multiple genes. Our study doesn't negate that there are a few major genes that work the clock, but it does demonstrate that there are a lot of genes that contribute incrementally to that. It argues for a much more complicated clock structure."
Other authors on the paper include: Todd Michael, a recent Dartmouth Ph.D. graduate; Patrice Salomé a Dartmouth Ph.D. candidate; Hannah Yu '02, Taylor Spencer '03, and Emily Sharp '05, all Dartmouth undergraduate students; Mark McPeek, Professor of Biological Sciences at Dartmouth; José Alonso, Assistant Professor at North Carolina State University; and Joseph Ecker, Professor at the Salk Institute for Biological Studies.
This work was funded by grants from the National Science Foundation to Ecker, McPeek and McClung. Yu and Spencer were supported by Richter Undergraduate Research Fellowships, and Sharp was supported by a Women in Science Project Internship, all administered through Dartmouth College.