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Date of Award

9-2010

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

Supervisor

Michael J. O'Donnell

Language

English

Abstract

Fruit flies (Drosophila melanogaster) are known for their ability to survive v,Tell on diets containing high concentrations (0.4 mol 1-1 or more) of NaCI and KCl. Drosophila has been used in recent years as a model species for identification of genes involved in salt tolerance in insects and other animals. This thesis examined the extent of haemolymph volume regulation and ionoregulation in larval or adult Drosophila reared on control diet or on salt-rich diets that contained an additional 0.4 mol 1-1 NaCI or KCl. K+ concentrations in the haemolymph of adults reared on KCl-rich (0.4 mol 1-1) diet did not differ from the values for insects reared on the control diet, whereas Na+ concentrations in the haemolymph of adults reared for Id - 7d on NaCI-rich (0.4 mol 1-1) diet increased ~50 % relative to values for insects reared on the control diet. K+ concentrations in the haemolymph of larvae reared on the KCl-rich diet increased transiently then returned to the control value within 48 hours whereas Na+ concentrations in the haemolymph of larvae reared on the NaCI-rich diet increased ~50 % relative to values for insects reared on the control diet.

This thesis also examined the role of the Malpighian tubules and gut in haemolymph ionoregulation during salt stress. It was hypothesized that increases in secretion of excess ions (Na+ or K+) by the Malpighian tubules and hindgut as well as decreased absorption across the midgut would contribute to haemolymph ionoregulation. Changes in fluid and ion secretion by Malpighian tubules isolated from the larvae were measured using the Ramsay assay and ionselective microelectrodes. The results showed that haemolymph ionoregulation in larvae reared on salt-rich diets involved both alterations in the basal secretion rates of Na+ or K+ by the Malpighian tubules as well as stimulatory effects produced by diuretic factors present in the haemolymph. Stimulation of tubule fluid and ion secretion may involve increases in intracellular Ca2+ in response to salt stress. Na+ and K+ fluxes across the isolated gut of larval Drosophila melanogaster reared on control or salt-rich diets were measured using the scanning ion-selective microelectrode technique (SIET). In larvae reared on KCl-rich diet, K+ absorption across the anterior portion of the middle midgut was reduced relative to the same gut segment of larvae reared on the control diet and there was there was also an increase in the magnitude and extent of K+ secretion across the posterior half of the middle midgut. In larvae reared on the NaCI-rich diet, there was a small reduction in Na + absorption by the middle midgut relative to larvae reared on the control diet and Na+ was secreted rather than absorbed across the hindgut. Overall, the results of the thesis suggest that haemolymph ionoregulation during salt stress involves a reconfiguration of the ion transport mechanisms of the gut and Malpighian tubules so that there are reductions in K+ and Na+ absorption and increases in K+ and Na+ secretion. These results indicate considerable phenotypic plasticity with respect to K+ and Na+ transport by the gut epithelia of larval Drosophila.

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