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Evolved patterns and rates of water loss and ion regulation in laboratory-selected populations of Drosophila melanogaster
Department of Ecology and Evolutionary Biology, University of California, Irvine, Irvine, CA 92697, USA
* Author for correspondence (e-mail: dfolk{at}uci.edu)
Accepted 15 May 2003
We have investigated water loss from, and ion regulation within, the
hemolymph and tissues of five replicate populations of Drosophila
melanogaster that have undergone laboratory selection for enhanced
desiccation resistance (i.e. the D populations). We compared the patterns and
rates of water loss and the ion content of the D populations prior to and
during desiccation with those of five replicate control (C) populations. The
net rate of water loss in the C flies was 
3-fold greater than that of the
D flies during the initial hours of desiccation. After 8 h, both C and D flies
had considerable reductions in water loss rate. During 24 h of desiccation,
the tissue water content of the D flies was conserved, while the C flies were
faced with significant loss of tissue water during the initial 8 h of
desiccation. We propose that the increased hemolymph volume of the D flies
plays a role in buffering water loss from the tissues. One consequence of this
large hemolymph pool is that the hydrated D flies contained approximately
seven times more sodium within the hemolymph than did the hydrated C flies.
Despite a continual loss of hemolymph volume in the D flies during lengthy
periods of desiccation, the sodium content of the hemolymph was significantly
reduced only during a single event. We provide evidence that the regulation of
extracellular sodium, as well as chloride, occurred via excretory
processes during desiccation. In addition, whole-body potassium was not
significantly decreased in the D flies during desiccation but was reduced
(i.e. excreted) in the C flies; hence, we suggest that the potassium content
paralleled tissue water level.
Key words: water loss, ion regulation, hemolymph, Drosophila melanogaster, desiccation, Na+, K+, Cl-, excretion, osmoregulation
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