Cold tolerance of Drosophila species is tightly linked to the epithelial K+ transport capacity of the Malpighian tubules and rectal pads.

Insect chill tolerance is strongly associated with the ability to maintain ion and water homeostasis during cold exposure. Maintenance of K+ balance is particularly important due to its role in setting the cell membrane potential that is involved in many aspects of cellular function and viability. In most insects, K+ balance is maintained through secretion at the Malpighian tubules, which balances reabsorption from the hindgut and passive leak arising from the gut lumen. Here, we used the scanning ion-selective electrode technique (SIET) at benign (23°C) and low (6°C) temperatures to examine K+ flux across the Malpighian tubules and the rectal pads in the hindgut in five Drosophila species that differ in cold tolerance. We found that chill-tolerant species were better at maintaining K+ secretion and suppressing reabsorption during cold exposure. In contrast, chill-susceptible species exhibited large reductions in secretion with no change, or a paradoxical increase, in K+ reabsorption. Using an assay to measure paracellular leak, we found that chill-susceptible species experience a large increase in leak during cold exposure, which could explain the apparent increase in K+ reabsorption found in these species. Our data therefore strongly support the hypothesis that cold-tolerant Drosophila species are better at maintaining K+ homeostasis through an increased ability to maintain K+ secretion rates and through reduced movement of K+ towards the hemolymph. These adaptations are manifested both at the Malpighian tubule and at the rectal pads in the hindgut, and ensure that cold-tolerant species experience less perturbation of K+ homeostasis during cold stress.