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LETTER TO THE EDITOR |
1 Biology Department
2 Biomedical and Health Sciences Department Grand Valley State University Allendale, Michigan
Address correspondence to Alexey G. Nitikin, Biology Department, Grand Valley State University, 1 Campus Dr., Allendale, MI 49401. E-mail: nikitin{at}gvsu.edu
To the Editor:
Despite a considerable interest in the effect caffeine has on metazoan life span, the extent, as well as the direction, of this effect is still unclear. One of the most extensively used metazoan models in longevity studies is the fruit fly Drosophila melanogaster (1). There are a number of studies on the effect of caffeine on Drosophila, most of them conducted using natural populations. However, flies derived from natural populations for longevity studies display substantial variations in longevity (2), most likely brought about by heterogeneous population-specific genetic background. In addition, the genetic instability brought about by an unbalanced genetic background can significantly affect the outcome of a longevity analysis.
A good scientific experiment requires fixing of all variables except for the one under investigation. A control-experiment comparison analysis cannot be adequately achieved in longevity analyses involving natural populations due to their inherent genetic variability and, thus, varying genetic background of organisms involved in the study.
In the present study, we used a co-isogenic inbred line of D. melanogaster derived from the Canton-S (CS) stock to assess the effect of varying dietary doses of caffeine on life span in an equalized genetic background. Being a laboratory strain of D. melanogaster, CS has reduced genetic variation within the strain (1). Another feature of CS is that it is reported by the Drosophila Stock Center to have no potentially transpositionally active transposable elements (TEs), such as P or hobo, in its genome (3). Transposable elements can introduce additional genetic heterogeneity and, thus, additional instability in response to environmental stress (4). Previous studies with fruit fly longevity did not take the TE factor into account.
In our experiment, we observed a significant reduction in life span of male flies reared on food containing 2.5 mg/ml and 1.25 mg/ml of caffeine (Table 1, Figure 1). There was a significant decrease in life span in the experimental group in one out of three replicas with 0.625 mg/ml and 0.3125 mg/ml caffeine concentrations. In two out of three replicas with 0.625 mg/ml and one out of three replicas with 0.3125 mg/ml of caffeine, flies lived longer on caffeine than on control food, although the difference in life span was not significant. The overall relationship between caffeine concentration and Drosophila life span in our experiment is presented in Figure 1.
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Transposable elements are major contributors to genetic instability in Drosophila, which contributes greatly to variations in life span (4). Somatic movement of TEs found in the D. prosultans genome has been documented to reduce life span in other species of Drosophila (10,11). Transposable element activity could have modified the effect of caffeine on life span in the Itoyama and colleagues study (5).
Data presented here show that high concentrations of caffeine reduce the longevity of an inbred strain D. melanogaster with a co-isogenic background lacking TEs. Whether the observed relationship between caffeine and life span will hold in other genetic backgrounds, whether or not they contain TEs capable of transposition, remains the subject of further investigation.
References
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