Effects of several preservation methods on the isotopic content of Drosophila samples.

Researchers working with stable isotopes are faced with the problem of preserving animal samples without altering their isotope ratios. We evaluated the effects of several preservation treatments on the isotopic content of Drosophila samples. Results show that, when there is a danger of rotting, preservation treatment is indispensable to preserve intact isotope ratios, but that not all treatments are equally appropriate. The best is freezing without liquid nitrogen. Ethanol or ethylene glycol preserve the delta 15N, but change the delta 13C. Formalin should be avoided. Storage in a NaCl solution may be a good short-term alternative to freezing for both elements. In the control males and females, the trophic isotopic shift was, respectively, +2.9 and +2.6/1000 for delta 15N, and +1.1/1000 for delta 13C (no difference between sexes). Despite a standardised rearing protocol, there were small but significant inter-tube differences, caused by microenvironmental rather than by genetic factors.