Testing the "rate of living" model: further evidence that longevity and metabolic rate are not inversely correlated in Drosophila melanogaster.

In a recent study examining the relationship between longevity and metabolism in a large number of recombinant inbred Drosophila melanogaster lines, we found no indication of the inverse relationship between longevity and metabolic rate that one would expect under the classical "rate of living" model. A potential limitation in generalizing from that study is that it was conducted on experimental material derived from a single set of parental strains originally developed over 20 years ago. To determine whether the observations made with those lines are characteristic of the species, we studied metabolic rates and longevities in a second, independently derived set of recombinant inbred lines. We found no correlation in these lines between metabolic rate and longevity, indicating that the ability to both maintain a normal metabolic rate and have extended longevity may apply to D. melanogaster in general. To determine how closely our measurements reflect metabolic rates of flies maintained under conditions of life span assays, we used long-term, flow-through metabolic rate measurements and closed system respirometry to examine the effects of variables such as time of day, feeding state, fly density, mobility of the flies, and nitrogen knockout on D. melanogaster metabolic rate. We found that CO2 production estimated in individual flies accurately reflects metabolic rates of flies under the conditions used for longevity assays.