The malpighian tubules of Drosophila melanogaster: a novel phenotype for studies of fluid secretion and its control.

The insect renal (Malpighian) tubule has long been a model system for the study of fluid secretion and its neurohormonal control (Maddrell, 1981; Maddrell and O’Donnell, 1992). Classical physiology suggests a model for tubular secretion of iso-osmotic fluid in most insects, in which ions are thought to enter basally either through a series of ion channels (Na+, K+ and Cl2) or through a bumetanide-sensitive Na+/K+/2Cl2 cotransport. Apical fluxes are energised by a plasma-membrane H+-pumping V-ATPase, driving secretion of Na+ or K+ through one or more exchangers, at least one of which is amiloride-sensitive and appears to be closely similar to the Na+/H+ exchanger of vertebrates (Maddrell and O’Donnell, 1992). Cl2 follows passively, perhaps through apical Cl2 channels. Water follows the major ions, and haemolymph solutes diffuse across the tubule wall passively via a paracellular route. There are also transcellular active transport processes for certain metabolites or toxins, such as acylamides (Maddrell et al. 1974) and plant alkaloids (Maddrell, 1976; O’Donnell et al. 1983). To extend studies of ion transport beyond that revealed by the techniques of classical physiology, it is necessary to adopt a molecular genetic approach, in which the relevant genes are identified, characterised and mutated. However, the generalised molecular genetic dissection of the major genes responsible for a stimulus–secretion pathway in any vertebrate epithelium remains a daunting task, as the genetic tools available are relatively unsophisticated. Encouraged by previous physiological work on the larger, but less well genetically mapped, Drosophila hydei (Bertram et al. 1991; Wessing and Eichelberg, 1978; Wessing et al. 1987), we have investigated the feasibility of adapting the established tubule secretion assay to the study of tubules of larval and adult Drosophila melanogaster. The results (i) demonstrate that these Malpighian tubules, to