Genetic stability under stresses expected in a space station environment:  Effect of hypergravity and vibration in Drosophila melanogaster.

 

Thompson, James N., jr. ¹, R.C. Woodruff ², Jenna J. Hellack ³, Tera L. Beaird ¹, Gerald P. Camren ¹, Wade P. Dressler ¹, Ann Gettys¹, Greg S. Hendrix ¹, D. Jeremy Madrid ¹, Matthew J. Potthoff ¹, H. Nathanial Scott ¹, Morsal R. Tahouni¹, and Brian T. Torgerson ¹.  ¹ Department of Zoology, University of Oklahoma, Norman, OK 73019;  ² Department of Biological Sciences, Bowling Green State University, Bowling Green, OH  43403;  ³ Department of Biology, University of Central Oklahoma, Edmond, OK  73034.

 

            Genetic and developmental systems will be challenged by new stresses when organisms begin to adapt to long-term habitation of a space environment, such as that on the International Space Station (ISS).  Previous assays of mutation and chromosome damage in response to hypergravity or stress (Pence, 1999) have yielded conflicting results or have used extremes that are unlikely to be experienced in actual space exposures.  Our initial ground-based studies in Drosophila melanogaster are designed to estimate mutation rates, aneuploidy, somatic mutation, and developmental stability under some of the stress exposures that organisms can experience in a space environment like that on the ISS.  In addition to providing valuable information about genetic and developmental stability and about the capacity of an organism to adapt to a space environment, these experiments yield initial ground control data for possible multi-generation mutation rate experiments on the ISS or other space environment. 

            Experimental cultures were exposed to hypergravity and vibration stresses at NASA/Ames Research Center, and genetic breeding programs were then completed at the University of Oklahoma and at Bowling Green State University.  The hypergravity conditions we have chosen to test first are near the high end of the range that organisms might experience on vehicle launch and travel.  Drosophila are exposed to hypergravity using the 1-Foot Diameter Centrifuge (2 - 5 g), which is designed to maintain carefully regulated low-level hypergravity conditions for extended periods.  Our treatments were typically 2-hour or 4-hour exposures, although pilot studies with other treatments not reported here have also been done. Vibration conditions are modeled on the mid-deck vibration of a shuttle launch (Figure 1).  Vibration exposures (typically 5 minutes in duration) were done on a computer-controlled Vibration Table (20 - 2000 Hz).  “Full Range” refers to a five-minute exposure to random vibration that has the cumulative frequency profile shown in Figure 1.  Five-minute exposures to just low range (20-150 Hz), mid-range (150-1000 Hz), and high range (1000-2000 Hz) are also reported here.

Figure 1.  Random vibration profile (Full Range) as established in Interface Definitions Document NSTS-21000-IDD-MDK for a shuttle mid-deck.  This representative profile printout is from an experimental treatment conducted in January 2001 (Log #2529). 

    Sex-linked lethal mutation rate estimates using the Basc balancer stock indicate about a two-fold increase at 5 g (2h:  0.28%, P < 0.05;  but marginally non-significant at 4h: 0.24%) compared to 1 g controls (0.14%).  The 2g, 4g, and vibration treatments may be slightly elevated, but are not significantly different form the controls at the present sample sizes.  Additional replicates will be completed soon. 

    There is no experimental evidence for chromosome breakage due to either hypergravity or vibration stresses (Table 3), although there is significant chromosome breakage caused by gamma radiation, as expected.

In conclusion, it appears that exposures to some of the stress conditions that can be experienced in a space environment might cause an increase in genetic damage, but the degree of that damage is not necessarily very large.  This might help account for some of the disagreement in results from earlier studies.  Additional ex-periments are now being done to explore other treat-ment levels and stresses, such as continuous expo-sure to low level radiation, and possible interac-tion effects among these stress condi-tions.