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

Abstract

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 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 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.

Functional Plasticity of the gut and the Malpighian tubules underlies cold acclimation and mitigates cold-induced hyperkalemia inDrosophila melanogaster

“…Each sample 632 contained the same number of tubules, so we measured Malpighian tubule size and found that 633 cold-acclimated tubules are significantly wider than warm-acclimated tubules ( Figure 6D-E). 634 Similar measurements of the Malpighian tubules revealed no such size differences in chill 635 tolerant Drosophila species when corrected for total body mass (Andersen et al, 2017b). 636…”

“…The chill tolerance of insects (including species of the genus Drosophila) can vary as a 110 result of evolutionary adaptation, thermal acclimation, or even acute exposure to low 111 temperatures (rapid cold-hardening) (Andersen et al, 2017b;Chown and Terblanche, 2006; 112 passing posteriorly to the hindgut where the reabsorption of water, ions, and metabolites takes 136 place prior to the excretion of wastes (Phillips et al, 1987;Wigglesworth, 1932)…”

“…Maintenance of 148 this ratio may assist in preventing hyperkalemia by (1) maintaining K + excretion in the cold or 149 (2) minimizing excretion of hemolymph Na + , an osmolyte important to the maintenance of 150 hemolymph volume. Further, the rectal pads of chill tolerant Drosophila species reabsorb less K + 151 in the cold than those of chill susceptible species, which would also facilitate the maintenance of 152 low hemolymph [K + ] (Andersen et al, 2017b). 153…”

Anti-diuretic activity of a CAPA neuropeptide can compromiseDrosophilachill tolerance

“…There is a 54 growing interest in understanding the biochemical and physiological mechanisms underlying 55 chill susceptibility in ectothermic animals, and several studies have demonstrated that the ability 56 of terrestrial insects to maintain ion and water homeostasis in the cold is closely associated with In particular, the ability to maintain low extracellular [K + ] appears critical to chill tolerance 60 (Overgaard and MacMillan, 2017). Chilling slows the activity of membrane bound ATPases 61 (such as Na + /K + -ATPase), leading to rapid membrane depolarization through a reduction in the 62 electrogenic contribution of these primary active transporters to membrane potential (Andersen 63 acclimated Drosophila better defend rates of fluid and K + secretion at low temperatures, a 97 modification that would help prevent against hyperkalemia (Andersen et al, 2017c; MacMillan 98 et al, 2015d; Yerushalmi et al, 2018). Chill tolerant drosophilids also reduce rectal K + 99 reabsorption during cold stress (preventing hyperkalemia) while those that are chill susceptible 100 have higher rates of K + reabsorption in the cold (which would contribute to hyperkalemia) 101 (Andersen et al, 2017c).…”

“…Chilling slows the activity of membrane bound ATPases 61 (such as Na + /K + -ATPase), leading to rapid membrane depolarization through a reduction in the 62 electrogenic contribution of these primary active transporters to membrane potential (Andersen 63 acclimated Drosophila better defend rates of fluid and K + secretion at low temperatures, a 97 modification that would help prevent against hyperkalemia (Andersen et al, 2017c; MacMillan 98 et al, 2015d; Yerushalmi et al, 2018). Chill tolerant drosophilids also reduce rectal K + 99 reabsorption during cold stress (preventing hyperkalemia) while those that are chill susceptible 100 have higher rates of K + reabsorption in the cold (which would contribute to hyperkalemia) 101 (Andersen et al, 2017c). Similarly, cold acclimated D. melanogaster have reduced rates of K + 102 reabsorption at the rectum relative to warm acclimated flies (Yerushalmi et al, 2018).…”