FOLK,D.F.*; BRADLEY,T.J.: How Does Selection for Desiccation Resistance Affect the Distribution of Water in Drosophila melanogaster?

We are conducting experiments with five populations of Drosophila melanogaster, which have undergone laboratory selection for increased desiccation resistance for over 200 generations. For each population of desiccation-resistant flies (D flies), a control population (C flies) was maintained under identical conditions, except that C flies had ad libitum access to water. Previous studies have demonstrated that increase in desiccation resistance in D flies is associated with increases in bulk water content and reduction in cuticular permeability to water. We tested two hypotheses concerning acquisition and storage of bulk water in the desiccation-tolerant populations: (1) that reduction in post-eclosion diuresis in the D flies allows flies to accumulate more water early in the adult phase and (2) that the greatest portion of surplus water is stored in the hemolymph. Our results indicate that rate of urine elimination up to 4 h post-eclosion was not differentiated between the D flies and their controls. Differences in post-eclosion diuresis have, therefore, not evolved during selection for increased desiccation resistance. Analysis of water distribution in the body of the flies demonstratedthat the D flies have a statistically significant, 3.4-fold increase in hemolymph volume. On average 89% of the extra bulk water in the D flies is stored in the hemolymph. We are currently examining the significance of the extracellular location of this water on mechanisms of water use during desiccation in fruit flies. Funded by NSF Grant IBN9723404.