Metabolic cold adaptation contributes little to the interspecific variation in metabolic rates of 65 species of Drosophilidae

Metabolic cold adaptation (MCA) is a controversial hypothesis suggesting that cold adapted species display an elevated metabolic rate (MR) compared to their warm climate relatives. Here we test for the presence of MCA in 65 species of drosophilid flies reared under common garden conditions. MR was measured at both 10 and 20°C for both sexes and data were analyzed in relation to the natural thermal environment of these species. We found considerable interspecific variation in MR ranging from 1.34 to 8.99µWmg^-1 at 10°C. As predicted by Bergmann's rule body mass of fly species correlated negatively with annual mean temperature (AMT), such that larger species were found in colder environments. Because larger flies have a higher total MR we found MR to vary with AMT, however, after inclusion of mass as a co-variate we found no significant effect of AMT. Furthermore, we did not find that thermal sensitivity of MR (Q₁₀) varied with AMT. Based on this broad collection of species we therefore conclude that there is no adaptive pattern of metabolic cold adaptation within drosophilid species ranging from sub-arctic to tropical environments.

Physiological correlates of chill susceptibility in Lepidoptera

The majority of insects enter a state of reversible coma if temperature is lowered sufficiently. If the cold treatment is not too severe these insects recover gradually when returned to benign temperatures in a time-dependent manner that often depends on the duration and intensity of the cold exposure. Previous studies have associated these phenotypes to changes in membrane potential (V_m) and ion balance, and especially hemolymph [K⁺] is known to be of importance for the recovery time. In the present study we examined this link in three species of Lepidoptera as insects from this order are known to possess resting hemolymph [K⁺] that would severely compromise V_m in other insects. Specifically, we exposed larval and adult Manduca sexta, larval Bombyx mori, and adult Heliconius cydno to stressful cold (0°C) for extended periods of time. Subsequently we measured chill coma recovery time (CCRT), ion- and water balance, and muscle V_m. As expected we find that resting hemolymph [K⁺] is high and that resting hemolymph [Na⁺] is low compared to most other insect species. Muscle V_m depolarised considerably during acute cold exposure, but did so in a manner that was not associated with changes in ion balance. However, prolonged cold exposure coincided with an increase of hemolymph [K⁺] and further depolarisation of V_m which correlated well with prolongation of CCRT. Combined this demonstrates how insects with different ionic compositions generally suffer from similar consequences of cold stress as other species, such that cold tolerance of chill-susceptible insects within Lepidoptera is also intimately linked to maintenance of ion balance and membrane polarisation.

Cold tolerance of Drosophila species is tightly linked to 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 balancing reabsorption from the hindgut and passive leak arising from the gut lumen. Here, we used a scanning ion-selective electrode technique (SIET) system at benign (23°C) and low (6°C) temperature 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 supressing 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 increased 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 leakage 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.

Evolutionary and ecological patterns of thermal acclimation capacity in Drosophila: is it important for keeping up with climate change?

Phenotypic plasticity of temperature tolerance (thermal acclimation) is often highlighted as an important component of the acute and evolutionary adaptation to temperatures in insects. For this reason, it is often suggested that thermal acclimation ability could be important for buffering the consequences of climate change. Based on data from Drosophila we discuss if and how phenotypic plasticity is likely to mitigate the effects of climate change. We conclude that plasticity of upper thermal limits is small in magnitude, evolves slowly and that acclimation ability is weakly correlated with latitude and environmental heterogeneity. Accordingly plasticity in upper thermal limits is unlikely to effectively buffer effects of global warming for species already close to their upper thermal boundaries.

Students' motivation toward laboratory work in physiology teaching

The laboratory has been given a central role in physiology education, and teachers report that it is motivating for students to undertake experimental work on live animals or measuring physiological responses on the students themselves. Since motivation is a critical variable for academic learning and achievement, then we must concern ourselves with questions that examine how students engage in laboratory work and persist at such activities. The purpose of the present study was to investigate how laboratory work influences student motivation in physiology. We administered the Lab Motivation Scale to assess our students' levels of interest, willingness to engage (effort), and confidence in understanding (self-efficacy). We also asked students about the role of laboratory work for their own learning and their experience in the physiology laboratory. Our results documented high levels of interest, effort, and self-efficacy among the students. Correlation analyses were performed on the three motivation scales and exam results, yet a significant correlation was only found between self-efficacy in laboratory work and academic performance at the final exam. However, almost all students reported that laboratory work was very important for learning difficult concepts and physiological processes (e.g., action potential), as the hands-on experiences gave a more concrete idea of the learning content and made the content easier to remember. These results have implications for classroom practice as biology students find laboratory exercises highly motivating, despite their different personal interests and subject preferences. This highlights the importance of not replacing laboratory work by other nonpractical approaches, for example, video demonstrations or computer simulations.

Concurrent effects of cold and hyperkalaemia cause insect chilling injury

Chilling injury and death are the ultimate consequence of low temperature exposure for chill susceptible insects, and low temperature tolerance is considered one of the most important factors determining insect distribution patterns. The physiological mechanisms that cause chilling injury are unknown, but chronic cold exposure that causes injury is consistently associated with elevated extracellular [K(+)], and cold tolerant insects possess a greater capacity to maintain ion balance at low temperatures. Here, we use the muscle tissue of the migratory locust (Locusta migratoria) to examine whether chill injury occurs during cold exposure or following return to benign temperature and we specifically examine if elevated extracellular [K(+)], low temperature, or a combination thereof causes cell death. We find that in vivo chill injury occurs during the cold exposure (when extracellular [K(+)] is high) and that there is limited capacity for repair immediately following the cold stress. Further, we demonstrate that that high extracellular [K(+)] causes cell death in situ, but only when experienced at low temperatures. These findings strongly suggest that that the ability to maintain ion (particularly K(+)) balance is critical to insect low temperature survival, and highlight novel routes of study in the mechanisms regulating cell death in insects in the cold.

Failure pattern and survival after breast conserving therapy. Long-term results of the Danish Breast Cancer Group (DBCG) 89 TM cohort

Based on the results from the DBCG 82 trial, breast conserving therapy (BCT) has been implemented as standard in Denmark since 1989, and today constitutes more than 70% of the primary treatment. Our aim was to evaluate the implementation of BCT as a routine procedure in patients treated according to the DBCG 89 program and compare recurrence pattern and survival both overall and when separated in age groups, with the results from the randomized DBCG 82 TM trial.

MATERIAL AND METHODS

A total of 1847 patients treated between 1989 and 1999 were included in a retrospective population-based cohort study. Data from the DBCG database were completed via search through the Danish Pathology Data Bank and medical records.

RESULTS

Median follow-up time was 17 years. At 20 years the cumulative incidences of local recurrence (LR) and disease-specific mortality (DSM) were 15.3% and 25.8%, respectively. Twenty-year overall survival (OS) and recurrence-free survival were 63.7% and 43.1%, respectively. Subdivided by age groups cumulative incidences at 20 years were LR: 18.9%, 10.5% and 12.4%, and DSM: 28.9%, 18.9% and 28.4% in young (≤45 years), middle-aged (46-55 years) and older (≥56 years) women, respectively. In an adjusted analysis age maintained a significant and independent effect on both LR and DSM.

CONCLUSION

The DBCG 82 TM program was successfully implemented. The women treated with BCT in the DBCG 89 program displayed equal failure pattern and improved survival in comparison with women from the DBCG 82 TM protocol. Occurrence of first failure and mortality varied with age; demonstrated by increased risk of LR, DM and DSM in the young patients and increased risk of DM and DSM in the older patients, compared to the middle-aged patients.

Abstract S2-01: Importance of margin width and re-excision in breast conserving treatment of early breast cancer; a Danish breast cancer cooperative group study of 11,900 women

Background: The majority of women with invasive breast cancer are treated surgically by breast conserving surgery (BCS). A significant proportion subsequently undergo re-excision to obtain clear margins. However what constitutes a sufficient negative margin continues to be subject of controversy. The purpose of the study was to investigate the association between margin width and ipsilateral breast tumour recurrence (IBTR) as well as identifying factors associated with residual disease after repeat surgery, and to determine the effect of re-excision on IBTR in a population-based nationwide cohort.

Method: 11,900 patients treated with breast conserving therapy for unilateral invasive cancer in Denmark between 2000 and 2009 were included. All patients received whole breast irradiation and were offered systemic adjuvant treatment according to the guidelines of the Danish Breast Cancer Cooperative Group.

Results: The median follow-up was 4.9 years. The cumulative incidence of IBTR at 5 and 9 year was 2.4% and 5.9%, respectively. No decrease in IBTR with a wider negative margin compared to a narrow but negative margin was seen in adjusted analysis (>0-1 mm vs. 2-4 mm vs. ≥5 mm (reference): HR 1.54 (CI 95% 0.81-2.93) vs 0.95 (CI 95% 0.56-1.62) vs. 1). A final positive margin did however increase the risk of IBTR (HR 2.51; 95% CI 1.02-6.23). Other factors associated with increased IBTR were young age (HR 3.10; 95% CI 1.89-5.10), more than 4 positive lymph nodes (HR 1.80; 95% CI 1.24-2.62), and re-excision (HR 1.53; 95% CI 1.16-2.02). Receiving chemotherapy (HR 0.45; 95% CI 0.33-0.61) or boost (HR 0.43; 95% CI 0.31-0.60) reduced risk of IBTR as did being oestrogen receptor positive treated with (HR 0.35; 95% CI 0.25-0.49) or without (HR 0.43; 95% CI 0.31-0.60) adjuvant endocrine therapy.

Within two months of initial BCS 1342 women (11%) had a re-excision. Residual disease was found in 20% of re-excisions. In adjusted analysis DCIS outside the invasive tumour (OR 2.69; 95% CI 1.99-3.63), positive initial margin (OR 2.26, 95% CI 1.70-2.99, p<0.001), and age <50 years (OR 1.53; 95% CI 1.00-2.31) was associated with increased risk of residual disease. Patients with residual disease after re-excision had in the adjusted analysis an increased risk of ipsilateral breast tumour recurrence (IBTR), regardless of whether residual findings were invasive carcinoma (HR 2.97, CI 95% 1.57-5.62) or DCIS (HR 2.58, CI 95% 1.50-4.45). However no difference was seen for overall survival comparing one procedure with repeat surgery with or without residual disease (p=0.96).

Conclusion: An overall low rate of IBTR was seen. While a final positive margin was associated with a more than two-fold risk of IBTR, no evidence of improved local control was found with wider negative margins compared to narrow. However the finding of residual disease at re-excision was associated with an increased risk of IBTR.

Citation Format: Bodilsen A, Bjerre K, Offersen BV, Vahl P, Mele M, Dixon MJ, Ejlertsen B, Overgaard J, Christiansen P. Importance of margin width and re-excision in breast conserving treatment of early breast cancer; a Danish breast cancer cooperative group study of 11,900 women. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr S2-01.

Benefits of Group Living Include Increased Feeding Efficiency and Lower Mass Loss during Desiccation in the Social and Inbreeding Spider Stegodyphus dumicola

Group living carries a price: it inherently entails increased competition for resources and reproduction, and may also be associated with mating among relatives, which carries costs of inbreeding. Nonetheless, group living and sociality is found in many animals, and understanding the direct and indirect benefits of cooperation that override the inherent costs remains a challenge in evolutionary ecology. Individuals in groups may benefit from more efficient management of energy or water reserves, for example in the form of reduced water or heat loss from groups of animals huddling, or through reduced energy demands afforded by shared participation in tasks. We investigated the putative benefits of group living in the permanently social spider Stegodyphus dumicola by comparing the effect of group size on standard metabolic rate, lipid/protein content as a body condition measure, feeding efficiency, per capita web investment, and weight/water loss and survival during desiccation. Because energetic expenditure is temperature sensitive, some assays were performed under varying temperature conditions. We found that feeding efficiency increased with group size, and the rate of weight loss was higher in solitary individuals than in animals in groups of various sizes during desiccation. Interestingly, this was not translated into differences in survival or in standard metabolic rate. We did not detect any group size effects for other parameters, and group size effects did not co-vary with experimental temperature in a predictive manner. Both feeding efficiency and mass loss during desiccation are relevant ecological factors as the former results in lowered predator exposure time, and the latter benefits social spiders which occupy arid, hot environments.

Reduced L-type Ca2+ current and compromised excitability induce loss of skeletal muscle function during acute cooling in locust

Low temperature causes most insects to enter a state of neuromuscular paralysis, termed chill coma. Susceptibility of insect species to enter chill coma is tightly correlated to the species distribution limits and for this reason it is important to understand the cellular processes that underlie chill coma. It is known that muscle function is markedly depressed at low temperature and this suggests that chill coma is partly caused by impairment in the muscle per se. To find the cellular mechanism(s) underlying muscle dysfunction at low temperature, we examined the effect of low temperature (5°C) on several events in the excitation-contraction-coupling in the migratory locust (Locusta migratoria). Intracellular membrane potential recordings during single nerve stimulations showed that 70% of fibers at 20°C produced an action potential (AP), while only 55% of the fibers were able to fire AP at 5°C. Reduced excitability at low temperature was caused by ∼80% drop in L-type Ca2+ current and a depolarizing shift in its activation of around 20 mV, which means that a larger endplate potential would be needed to activate the muscle AP at low temperature. In accordance we showed that intracellular Ca2+ transients were largely absent at low temperature following nerve stimulation. In contrast, maximum contractile force was unaffected by low temperature in chemically skinned muscle bundles which demonstrates that the function of the contractile filaments are preserved at low temperature. These findings demonstrate that reduced L-type Ca2+ current is likely the most important factor contributing to loss of muscle function at low temperature in locust.

Cold-acclimation improves chill tolerance in the migratory locust through preservation of ion balance and membrane potential

Most insects have the ability to alter their cold tolerance in response to temporal temperature fluctuations, and recent studies have shown that insect cold tolerance is closely tied to the ability to maintain transmembrane ion-gradients that are important for the maintenance of cell membrane potential (Vm). Accordingly, several studies have suggested a link between preservation of Vm and cellular survival after cold stress, but none have measured Vm in this context. We tested this hypothesis by acclimating locusts (Locusta migratoria) to high (31°C) and low temperature (11°C) for four days before exposing them to cold stress (0°C) for up to 48 hours and subsequently measuring ion balance, cell survival, muscle Vm, and whole animal performance. Cold stress caused gradual muscle cell death which coincided with a loss of ion balance and depolarisation of muscle Vm. The loss of ion-balance and cell polarisation were, however, dampened markedly in cold-acclimated locusts such that the development of chill injury was reduced. To further examine the association between cellular injury and Vm we exposed in vitro muscle preparations to cold buffers with low, intermediate, or high [K+]. These experiments revealed that cellular injury during cold exposure occurs when Vm becomes severely depolarised. Interestingly we found that cellular sensitivity to hypothermic hyperkalaemia was lower in cold-acclimated locusts that were better able to defend Vm whilst exposed to high extracellular [K+]. Together these results demonstrate a mechanism of cold-acclimation in locusts that improves survival after cold stress: Increased cold tolerance is accomplished by preservation of Vm through maintenance of ion homeostasis and decreased K+-sensitivity.

Hemolymph metabolites and osmolality are tightly linked to cold tolerance of Drosophila species: a comparative study

Drosophila, like most insects, are susceptible to low temperatures, and will succumb to temperatures above the freezing point of their hemolymph. For these insects, cold exposure causes a loss of extracellular ion and water homeostasis, leading to chill injury and eventually death. Chill tolerant species are characterized by lower hemolymph [Na+] than chill susceptible species and this lowered hemolymph [Na+] is suggested to improve ion and water homeostasis during cold exposure. It has therefore also been hypothesized that hemolymph Na+ is replaced by other “cryoprotective” osmolytes in cold tolerant species. Here, we compare the hemolymph metabolite profiles of five drosophilid species with marked difference in chill tolerance. All species were examined under “normal” thermal conditions (i.e. 20°C) and following cold exposure (4 hours at 0°C). Under benign conditions total hemolymph osmolality was similar among all species despite chill tolerant species having lower hemolymph [Na+]. Using NMR spectroscopy we found that chill tolerant species instead have higher levels of sugars and free amino acids in their hemolymph, including classical “cryoprotectants” such as trehalose and proline. In addition, we found that chill tolerant species maintain a relatively stable hemolymph osmolality and metabolite profile when exposed to cold stress while sensitive species suffer from large increases in osmolality and massive changes in their metabolic profiles during a cold stress. We suggest that the larger contribution of classical “cryoprotectants” in chill tolerant Drosophila play a non-colligative role for cold tolerance that contributes to osmotic and ion homeostasis during cold exposures and in addition we discuss how these comparative differences may represent an evolutionary pathway toward more extreme cold tolerance of insects.

Does oxygen limit thermal tolerance in arthropods? A critical review of current evidence

Over the last decade, numerous studies have investigated the role of oxygen in setting thermal tolerance in aquatic animals, and there has been particular focus on arthropods. Arthropods comprise one of the most species-rich taxonomic groups on Earth, and display great diversity in the modes of ventilation, circulation, blood oxygen transport, with representatives living both in water (mainly crustaceans) and on land (mainly insects). The oxygen and capacity limitation of thermal tolerance (OCLTT) hypothesis proposes that the temperature dependent performance curve of animals is shaped by the capacity for oxygen delivery in relation to oxygen demand. If correct, oxygen limitation could provide a mechanistic framework to understand and predict both current and future impacts of rapidly changing climate.

In arthropods, most studies testing the OCLTT hypothesis have considered tolerance to thermal extremes. These studies likely operate from the philosophical viewpoint that if the model can predict these critical thermal limits, then it is more likely to also explain loss of performance at less extreme, non-lethal temperatures, for which much less data is available. Nevertheless, the extent to which lethal temperatures are influenced by limitations in oxygen supply remains unresolved. Here we critically evaluate the support and universal applicability for oxygen limitation being involved in lethal temperatures in crustaceans and insects.

The relatively few studies investigating the OCLTT hypothesis at low temperature do not support a universal role for oxygen in setting the lower thermal limits in arthropods. With respect to upper thermal limits, the evidence supporting OCLTT is stronger for species relying on underwater gas exchange, while the support for OCLTT in air-breathers is weak. Overall, strongest support was found for increased anaerobic metabolism close to thermal maxima. In contrast, there was only mixed support for the prediction that aerobic scope decreases near critical temperatures, a key feature of the OCLTT hypothesis. In air-breathers, only severe hypoxia (< 2 kPa) affected heat tolerance. The discrepancies for heat tolerance between aquatic and terrestrial organisms can to some extent be reconciled by differences in the capacity to increase oxygen transport. As air-breathing arthropods are unlikely to become oxygen limited under normoxia (especially at rest), the oxygen limitation component in OCLTT does not seem to provide sufficient information to explain lethal temperatures. Nevertheless, many animals may simultaneously face hypoxia and thermal extremes and the combination of these potential stressors is particularly relevant for aquatic organisms where hypoxia (and hyperoxia) is more prevalent. In conclusion, whether taxa show oxygen limitation at thermal extremes may be contingent on their capacity to regulate oxygen uptake, which in turn is linked to their respiratory medium (air vs. water).

Fruitful directions for future research include testing multiple predictions of OCLTT in the same species. Additionally, we call for greater research efforts towards studying the role of oxygen in thermal limitation of animal performance at less extreme, sub-lethal temperatures, necessitating studies over longer timescales and evaluating whether oxygen becomes limiting for animals to meet energetic demands associated with feeding, digestion and locomotion.