Inter- and intraspecific differences in Drosophila cold tolerance are linked to hindgut reabsorption capacity

Abstract

Maintaining extracellular osmotic and ionic homeostasis is crucial to maintain organismal function. In insects, hemolymph volume and ion content is regulated by the combined actions of the secretory Malpighian tubules and reabsorptive hindgut. When exposed to stressful cold, homeostasis is gradually disrupted, characterized by a debilitating increase in extracellular K+ concentration (hyperkalemia). In accordance with this paradigm, studies have found a strong link between the cold tolerance of insect species and their ability to maintain ion and water homeostasis at low temperature. This is also the case for drosophilids where studies have already established how inter- and intra-specific differences in cold tolerance are linked to the secretory capacity of Malpighian tubules. However, presently there is little information on the effects of temperature on the reabsorptive capacity of the hindgut in Drosophila. To address this question we developed a novel method that allows for continued measurements of hindgut ion and fluid reabsorption in Drosophila. Firstly we demonstrate that this assay is temporally stable (> 3 hours) and that the preparation is responsive to humoral stimulation and pharmacological intervention of active and passive transport in accordance with the current insect hindgut reabsorption model. Using this method at benign (24{degrees}C) and low temperature (3{degrees}C) we investigated how cold acclimation or cold adaptation affected the thermal sensitivity of osmoregulatory function. We found that cold tolerant Drosophila species and cold-acclimated D. melanogaster are innately better at maintaining rates of fluid and Na+ reabsorption at low temperature. Furthermore, cold adaptation and acclimation causes a relative reduction in K+ reabsorption at low temperature. These characteristic responses of cold adapted/acclimated Drosophila will act to promote maintenance of ion and water homeostasis at low temperature and therefore provide further links between adaptations in osmoregulatory capacity of insects and their ability to tolerate cold exposure.