Maintenance of hindgut reabsorption during cold exposure is a key adaptation for Drosophila cold tolerance

Abstract

Maintaining extracellular osmotic and ionic homeostasis is crucial for organismal function. In insects, hemolymph volume and ion content is regulated by 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). Accordingly, studies have found a strong link between species-specific cold tolerance and the ability to maintain ion and water homeostasis at low temperature. This is also true for drosophilids where inter-and intra-specific differences in cold tolerance are linked to the secretory capacity of Malpighian tubules. There is, however, little information on the reabsorptive capacity of the hindgut in Drosophila. To address this, we developed a novel method that permits continuous measurements of hindgut ion and fluid reabsorption in Drosophila. We demonstrate that this assay is temporally stable (∼2 h) and responsive to cAMP stimulation and pharmacological intervention in accordance with the current insect hindgut reabsorption model. We then investigated how cold acclimation or cold adaptation affected hindgut reabsorption at benign (24°C) and low temperature (3°C). Cold-tolerant Drosophila species and cold-acclimated D. melanogaster maintain superior fluid and Na + reabsorption at low temperature. Furthermore, cold adaptation and acclimation caused a relative reduction in K + reabsorption at low temperature. These characteristic responses of cold adaptation/ acclimation will promote maintenance of ion and water homeostasis at low temperature. Our study of hindgut function therefore provides evidence that adaptations in the osmoregulatory capacity of insects are critical for their ability to tolerate cold.

Biogeographic parallels in thermal tolerance and gene expression variation under temperature stress in a widespread bumble bee

“…For example, studies have revealed that many terrestrial ectotherms show little variation in CT MAX but pronounced differences in CT MIN with geographic temperature gradients 14 – 16 . The mechanisms underlying this spatial variation in thermal limits will inform species responses to changing temperatures but have rarely been studied (but see 17 ). Shifts in gene expression likely contribute to physiological responses that underly range-wide variation in thermal tolerances, responses that may also be shaped by natural selection and ultimately limit species distributions 5 , 10 , 18 .…”

Hyperkalaemia, not apoptosis, accurately predicts insect chilling injury

“…To gain further insight into the relationship between survival, ion balance disruption and caspase-3-like activity, we took advantage of the wide inter-individual variation noted in these variables in the first set of experiments. Here, we scored survival and measured haemolymph K + concentration and flight muscle caspase-3-like activity in the same individuals, using unexposed locusts and locusts exposed to 24 royalsocietypublishing.org/journal/rspb Proc. R. Soc.…”

“…Changes to cold tolerance within an insect appear to arise from adjustments that attenuate the physiological cascade of failure [1]. For example, cold-acclimated and adapted insects rely less on Na + as an extracellular osmolyte [21,22], better maintain paracellular barrier function in the cold [11,12], have renal systems more efficient at clearing excess K + from the haemolymph [12,[23][24][25], and defend against muscle depolarization [17,26,27]. All of these adjustments serve to protect against injury by targeting upstream causes of failure, but chill tolerance may also be intimately tied to the ability to prevent cell death in the face of homeostatic collapse [20,27], or even the ability to clear damaged tissue following rewarming [28].…”

The evolution of cold nociception in drosophilid larvae and identification of a neural basis for cold acclimation

“…A substantial and growing body of literature details how insect RCH and cold acclimation are mediated by a variety of physiological, genetic, and biochemical changes, many of which are tied to ionoregulatory balance (8)(9)(10)(11)(12)(13)(14)(15)(16)(17). There is also evidence that absolute temperature information encoded by cold sensors drives changes in sleep and wakefulness, and that shutdown in the central nervous system (via spreading depolarization) is coincident with cold acclimation (18,19).…”