Fitness surfaces and local thermal adaptation in Drosophila along a latitudinal gradient.

Local adaptation is commonly cited to explain species distribution, but how fitness varies along continuous geographical gradients is not well understood. Here, we combine thermal biology and life-history theory to demonstrate that Drosophila populations along a 2500 km latitudinal cline are adapted to local conditions. We measured how heat tolerance and viability rate across eight populations varied with temperature in the laboratory and then simulated their expected cumulative Darwinian fitness employing high-resolution temperature data from their eight collection sites. Simulations indicate a trade-off between annual survival and cumulative viability, as both mortality and the recruitment of new flies are predicted to increase in warmer regions. Importantly, populations are locally adapted and exhibit the optimal combination of both traits to maximize fitness where they live. In conclusion, our method is able to reconstruct fitness surfaces employing empirical life-history estimates and reconstructs peaks representing locally adapted populations, allowing us to study geographic adaptation in silico.

Application of the thermal death time model in predicting thermal damage accumulation in plants.

The thermal death time (TDT) model suggests that the duration for which an organism can tolerate thermal stress decreases exponentially as the intensity of the temperature becomes more extreme. This model has been used to predict damage accumulation in ectothermic animals and plants under fluctuating thermal conditions. However, the critical assumption of the TDT model, which is additive damage accumulation, remains unverified for plants. We assessed thermal damage in Thymus vulgaris under different heat and cold treatments, and used TDT models to predict time to thermal failure of PSII. Additionally, thermal tolerance estimates from previous studies were used to create TDT models to assess the applicability of this framework in plants. We show that thermal damage is additive between 44 °C and 47 °C and between -6.5 °C and -8 °C, and that the TDT model can predict damage accumulation at both temperature extremes. Data from previous studies indicate a broad applicability of this approach across plant species and traits. The TDT framework reveals a thermal tolerance landscape describing the relationship between exposure duration, stress intensity, and percentage damage accumulation. The extreme thermal sensitivity of plants emphasizes that even a 1 °C increase in future extreme temperatures could impact their mortality and distribution.

Beyond a single temperature threshold: Applying a cumulative thermal stress framework to plant heat tolerance.

Most plant thermal tolerance studies focus on single critical thresholds, which limit the capacity to generalise across studies and predict heat stress under natural conditions. In animals and microbes, thermal tolerance landscapes describe the more realistic, cumulative effects of temperature. We tested this in plants by measuring the decline in leaf photosynthetic efficiency (FV/FM) following a combination of temperatures and exposure times and then modelled these physiological indices alongside recorded environmental temperatures. We demonstrate that a general relationship between stressful temperatures and exposure durations can be effectively employed to quantify and compare heat tolerance within and across plant species and over time. Importantly, we show how FV/FM curves translate to plants under natural conditions, suggesting that environmental temperatures often impair photosynthetic function. Our findings provide more robust descriptors of heat tolerance in plants and suggest that heat tolerance in disparate groups of organisms can be studied with a single predictive framework.

Thermal limits of survival and reproduction depend on stress duration: A case study of Drosophila suzukii.

Studies of ectotherm responses to heat extremes often rely on assessing absolute critical limits for heat coma or death (CTmax), however, such single parameter metrics ignore the importance of stress exposure duration. Furthermore, population persistence may be affected at temperatures considerably below CTmax through decreased reproductive output. Here we investigate the relationship between tolerance duration and severity of heat stress across three ecologically relevant life-history traits (productivity, coma and mortality) using the global agricultural pest Drosophila suzukii. For the first time, we show that for sublethal reproductive traits, tolerance duration decreases exponentially with increasing temperature (R2 > 0.97), thereby extending the Thermal Death Time framework recently developed for mortality and coma. Using field micro-environmental temperatures, we show how thermal stress can lead to considerable reproductive loss at temperatures with limited heat mortality highlighting the importance of including limits to reproductive performance in ecological studies of heat stress vulnerability.

Interspecific differences in thermal tolerance landscape explain aphid community abundance under climate change.

A single critical thermal limit is often used to explain and infer the impact of climate change on geographic range and population abundance. However, it has limited application in describing the temporal dynamic and cumulative impacts of extreme temperatures. Here, we used a thermal tolerance landscape approach to address the impacts of extreme thermal events on the survival of co-existing aphid species (Metopolophium dirhodum, Sitobion avenae and Rhopalosiphum padi). Specifically, we built the thermal death time (TDT) models based on detailed survival datasets of three aphid species with three ages across a broad range of stressful high (34-40 °C) and low (-3∼-11 °C) temperatures to compare the interspecific and developmental stage variations in thermal tolerance. Using these TDT parameters, we performed a thermal risk assessment by calculating the potential daily thermal injury accumulation associated with the regional temperature variations in three wheat-growing sites along a latitude gradient. Results showed that M. dirhodum was the most vulnerable to heat but more tolerant to low temperatures than R. padi and S. avenae. R. padi survived better at high temperatures than Sitobion avenae and M. dirhodum but was sensitive to cold. R. padi was estimated to accumulate higher cold injury than the other two species during winter, while M. dirhodum accrued more heat injury during summer. The warmer site had higher risks of heat injury and the cooler site had higher risks of cold injury along a latitude gradient. These results support recent field observations that the proportion of R. padi increases with the increased frequency of heat waves. We also found that young nymphs generally had a lower thermal tolerance than old nymphs or adults. Our results provide a useful dataset and method for modelling and predicting the consequence of climate change on the population dynamics and community structure of small insects.

Heat tolerance of marine ectotherms in a warming Antarctica.

Global warming is affecting the Antarctic continent in complex ways. Because Antarctic organisms are specialized to living in the cold, they are vulnerable to increasing temperatures, although quantitative analyses of this issue are currently lacking. Here we compiled a total of 184 estimates of heat tolerance belonging to 39 marine species and quantified how survival is affected concomitantly by the intensity and duration of thermal stress. Species exhibit thermal limits displaced toward colder temperatures, with contrasting strategies between arthropods and fish that exhibit low tolerance to acute heat challenges, and brachiopods, echinoderms, and molluscs that tend to be more sensitive to chronic exposure. These differences might be associated with mobility. A dynamic mortality model suggests that Antarctic organisms already encounter temperatures that might be physiologically stressful and indicate that these ecological communities are indeed vulnerable to ongoing rising temperatures.

El calentamiento global está afectando al continente antártico de formas complejas. Dado que los organismos antárticos están especializados a vivir en el frío, son vulnerables al aumento de las temperaturas, aunque en la actualidad hay carencia de análisis cuantitativos al respecto. Aquí recopilamos un total de 184 estimaciones de tolerancia al calor pertenecientes a 39 especies marinas, y cuantificamos cómo la supervivencia de estos organismos se ve afectada concomitantemente por la intensidad y la duración de un estrés térmico. Efectivamente las especies antárticas muestran límites térmicos desplazados hacia temperaturas más frías, con estrategias contrastadas entre los artrópodos y los peces que muestran una baja tolerancia a los desafíos térmicos agudos, y los braquiópodos, equinodermos y moluscos que tienden a ser más sensibles a la exposición crónica. Estas diferencias podrían estar asociadas con la movilidad. Un modelo dinámico de mortalidad sugiere que los organismos antárticos ya se enfrentan a temperaturas que podrían ser fisiológicamente estresantes e indican que estas comunidades ecológicas son realmente vulnerables al aumento continuo de las temperaturas.

Finding the right thermal limit: a framework to reconcile ecological, physiological and methodological aspects of CTmax in ectotherms.

Upper thermal limits (CTmax) are frequently used to parameterize the fundamental niche of ectothermic animals and to infer biogeographical distribution limits under current and future climate scenarios. However, there is considerable debate associated with the methodological, ecological and physiological definitions of CTmax. The recent (re)introduction of the thermal death time (TDT) model has reconciled some of these issues and now offers a solid mathematical foundation to model CTmax by considering both intensity and duration of thermal stress. Nevertheless, the physiological origin and boundaries of this temperature-duration model remain unexplored. Supported by empirical data, we here outline a reconciling framework that integrates the TDT model, which operates at stressful temperatures, with the classic thermal performance curve (TPC) that typically describes biological functions at permissive temperatures. Further, we discuss how the TDT model is founded on a balance between disruptive and regenerative biological processes that ultimately defines a critical boundary temperature (Tc) separating the TDT and TPC models. Collectively, this framework allows inclusion of both repair and accumulation of heat stress, and therefore also offers a consistent conceptual approach to understand the impact of high temperature under fluctuating thermal conditions. Further, this reconciling framework allows improved experimental designs to understand the physiological underpinnings and ecological consequences of ectotherm heat tolerance.

Thermal tolerance in Drosophila: Repercussions for distribution, community coexistence and responses to climate change.

Here we combined controlled experiments and field surveys to determine if estimates of heat tolerance predict distributional ranges and phenology of different Drosophila species in southern South America. We contrasted thermal death time curves, which consider both magnitude and duration of the challenge to estimate heat tolerance, against the thermal range where populations are viable based on field surveys in an 8-year longitudinal study. We observed a strong correspondence of the physiological limits, the thermal niche for population growth, and the geographic ranges across studied species, which suggests that the thermal biology of different species provides a common currency to understand how species will respond to warming temperatures both at a local level and throughout their distribution range. Our approach represents a novel analytical toolbox to anticipate how natural communities of ectothermic organisms will respond to global warming.

Acclimation, duration and intensity of cold exposure determine the rate of cold stress accumulation and mortality in Drosophila suzukii.

The spotted wing drosophila (SWD), Drosophila suzukii, is a major invasive fruit pest. There is strong consensus that low temperature is among the main drivers of SWD population distribution, and the invasion success of SWD is also linked to its thermal plasticity. Most studies on ectotherm cold tolerance focus on exposure to a single stressful temperature but here we investigated how cold stress intensity affected survival duration across a broad range of low temperatures (-7 to +3 °C). The analysis of Lt50 at different stressful temperatures (Thermal Death Time curve - TDT) is based on the suggestion that cold injury accumulation rate increases exponentially with the intensity of thermal stress. In accordance with the hypothesis, Lt50 of SWD decreased exponentially with temperature. Further, comparison of TDT curves from flies acclimated to 15, 19 and 23 °C, respectively, showed an almost full compensation with acclimation such that the temperature required to induce mortality over a fixed time decreased almost 1 °C per °C lowering of acclimation temperature. Importantly, this change in cold tolerance with acclimation was uniform across the range of moderate to intense cold stress exposures examined. To understand if cold stress at moderate and intense exposures affects the same physiological systems we examined how physiological markers/symptoms of chill injury developed at different intensities of the cold stress. Specifically, hsp23 expression and extracellular [K+] were measured in flies exposed to different intensities of cold stress (-6, -2 and +2 °C) and at various time points corresponding to the same progression of injury (equivalent to 1/3, 2/3 or 3/3 of Lt50). The different cold stress intensities all triggered hsp23 expression following 2 h of recovery, but patterns of expression differed. At the most intense cold stress (-6 and -2 °C) a gradual increase with time was found. In contrast, at +2 °C an initial increase was followed by a dissipating expression. A gradual perturbation of ion balance (hyperkalemia) was also found at all three cold stress intensities examined, with only slight dissimilarities between treatment temperatures. Despite some differences between the three cold intensities examined, the results generally support the hypothesis that intense and moderate cold stress induces the same physiological perturbation. This suggests that cold stress experienced during natural fluctuating conditions is additive and the results also illustrate that the rate of injury accumulation increases dramatically (exponentially) with decreasing temperature (increasing stress).

Divergence in Thermal Physiology Could Contribute to Vertical Segregation in Intertidal Ecotypes of Littorina saxatilis.

AbstractThermal stress is a potentially important selective agent in intertidal marine habitats, but the role that thermal tolerance might play in local adaptation across shore height has been underexplored. Northwest Spain is home to two morphologically distinct ecotypes of the periwinkle Littorina saxatilis, separated by shore height and subject to substantial differences in thermal stress exposure. However, despite other biotic and abiotic drivers of ecotype segregation being well studied, their thermal tolerance has not been previously characterized. We investigated thermal tolerance across multiple life history stages by employing the thermal death time (TDT) approach to determine (i) whether the two ecotypes differ in thermal tolerance and (ii) how any differences vary with life history stage. Adults of the two ecotypes differed in their thermal tolerance in line with their shore position: the upper-shore ecotype, which experiences more extreme temperatures, exhibited greater endurance of thermal stress compared with the lower-shore ecotype. This difference was most pronounced at the highest temperatures tested. The proximate physiological basis for these differences is unknown but likely due to a multifarious interaction of traits affecting different parts of the TDT curve. Differences in tolerance between ecotypes were less pronounced in early life history stages but increased with ontogeny, suggesting partial divergence of this trait during development. Thermal tolerance could potentially play an important role in maintaining population divergence and genetic segregation between the two ecotypes, since the increased thermal sensitivity of the lower-shore ecotype may limit its dispersal onto the upper shore and so restrict gene flow.

A Comparison of Three Methods for Determining Thermal Inactivation Kinetics: A Case Study on Salmonella enterica in Whole Milk Powder.

ABSTRACT: Different methods for determining the thermal inactivation kinetics of microorganisms can result in discrepancies in thermal resistance values. In this study, thermal resistance of Salmonella in whole milk powder was determined with three methods: thermal death time (TDT) disk in water bath, pouches in water bath, and the TDT Sandwich system. Samples from three production lots of whole milk powder were inoculated with a five-strain Salmonella cocktail and equilibrated to a water activity of 0.20. The samples were then subjected to three isothermal treatments at 75, 80, or 85°C. Samples were removed at six time points and cultures were enumerated for survivors. The inactivation data were fitted to two consolidated models: two primary models (log linear and Weibull) and one secondary model (Bigelow). Normality testing indicated that all the model parameters were normally distributed. None of the model parameters for both consolidated models were significantly different (α = 0.05). The amount of inactivation during the come-up time was also not significantly different among the methods (α = 0.05). However, the TDT Sandwich resulted in less inactivation during the come-up time and overall less variation in model parameters. The survivor data from all three methods were combined and fitted to both consolidated models. The Weibull had a lower root mean square error and a better fit, according to the corrected Akaike's information criterion. The three thermal treatment methods produced results that were not significantly different; thus, the methods are interchangeable, at least for Salmonella in whole milk powder. Comparisons with more methods, other microorganisms, and larger varieties of food products using the same framework presented in this study could provide guidance for standardizing thermal inactivation kinetics studies for microorganisms in foods.

TDT Sandwich: An open source dry heat system for characterizing the thermal resistance of microorganisms.

The determination of the thermal death kinetics of microorganisms has traditionally been performed with liquid baths which have some disadvantages such as liquid spillage and liquid infiltration into samples. The TDT Sandwich was developed as a free, open source alternative that utilizes dry heat. The system is capable of heating samples up to 140 °C and maintaining it within 0.2 °C of the target temperature. Other features of the TDT Sandwich include adjustable heating rates up to approximately 100 °C/min, real-time display and recording of temperature readings at a nominal rate of 5 Hz, an optional thermocouple for acquiring temperature of samples, built-in heating timer, and customizable operating parameters. The modular nature of the TDT Sandwich allows multiple units to be connected to a computer or laptop. Operation of the TDT Sandwich is done through a computer program which, along with the build instructions and microcontroller program, are open source and are available for free to the public at https://doi.org/10.17605/OSF.IO/5Q3Y7.

Effects of developmental plasticity on heat tolerance may be mediated by changes in cell size in Drosophila melanogaster.

There is a growing interest in the physiology underpinning heat tolerance of ectotherms and their responses to the ongoing rise in temperature. However, there is no consensus about the underlying physiological mechanisms. According to "the maintain aerobic scope and regulate oxygen supply" hypothesis, responses to warming at different organizational levels contribute to the ability to safeguard energy metabolism via aerobic pathways. At the cellular level, a decrease in cell size increases the capacity for the uptake of resources (e.g., food and oxygen), but the maintenance of electrochemical gradients across cellular membranes implies greater energetic costs in small cells. In this study, we investigated how different rearing temperatures affected cell size and heat tolerance in the fruit fly Drosophila melanogaster. We tested the hypothesis that smaller-celled flies are more tolerant to acute, intense heat stress whereas larger-celled flies are more tolerant to chronic, mild heat stress. We used the thermal tolerance landscape framework, which incorporates the intensity and duration of thermal challenge. Rearing temperatures strongly affected both cell size and survival times. We found different effects of developmental plasticity on tolerance to either chronic or acute heat stress. Warm-reared flies had both smaller cells and exhibited higher survival times under acute, intense heat stress when compared to cold-reared flies. However, under chronic, mild heat stress, the situation was reversed and cold-reared flies, consisting of larger cells, showed better survival. These differences in heat tolerance could have resulted from direct effects of rearing temperature or they may be mediated by the correlated changes in cell size. Notably, our results are consistent with the idea that a smaller cell size may confer tolerance to acute temperatures via enhanced oxygen supply, while a larger cell may confer greater tolerance to chronic and less intense heat stress via more efficient use of resources.

Thermal Death Kinetics of Cryptolestes pusillus (Schonherr), Rhyzopertha dominica (Fabricius), and Tribolium confusum (Jacquelin du Val) Using a Heating Block System.

Thermal treatment has been extensively used to control pests in stored grains for a long time. The objective of this study was to analyze thermal death kinetics of adult flat grain beetle, Cryptolestes pusillus (Schonherr), lesser grain borer, Rhyzopertha dominica (Fabricius), and confused flour beetle, Tribolium confusum (Jacquelin du Val), using a heating block system (HBS), at temperatures of 46, 48, 50, and 52 °C for C. pusillus and T. confusum, and 48, 50, 52, and 54 °C for R. dominica with a heating rate of 5 °C/min. Thermal death curves of those three insects followed a 0th-order reaction model. Complete mortality of C. pusillus, R. dominica, and T. confusum were observed after exposure to 1.4, 5.0, and 0.9 min at 52, 54 and 52 °C, respectively. The thermal death activation energy for controlling C. pusillus, R. dominica, and T. confusum was 689.91, 380.88, and 617.08 kJ/mol with z values of 2.88, 5.18, and 3.22 °C, respectively. The cumulative lethal time model can also be used to predict mortality of these three insects during a practical heating process. The information provided by this study on storage pests may be useful for developing effective thermal treatment protocols.

Heat tolerance in Drosophila subobscura along a latitudinal gradient: Contrasting patterns between plastic and genetic responses.

Susceptibility to global warming relies on how thermal tolerances respond to increasing temperatures through plasticity or evolution. Climatic adaptation can be assessed by examining the geographic variation in thermal-related traits. We studied latitudinal patterns in heat tolerance in Drosophila subobscura reared at two temperatures. We used four static stressful temperatures to estimate the thermal death time (TDT) curves, and two ramping assays with fast and slow heating rates. Thermal death time curves allow estimation of the critical thermal maximum (CT(max)), by extrapolating to the temperature that would knock down the flies almost "instantaneously," and the thermal sensitivity to increasing stressful temperatures. We found a positive latitudinal cline for CT(max), but no clinal pattern for knockdown temperatures estimated from the ramping assays. Although high-latitude populations were more tolerant to an acute heat stress, they were also more sensitive to prolonged exposure to less stressful temperatures, supporting a trade-off between acute and chronic heat tolerances. Conversely, developmental plasticity did not affect CT(max) but increased the tolerance to chronic heat exposition. The patterns observed from the TDT curves help to understand why the relationship between heat tolerance and latitude depends on the methodology used and, therefore, these curves provide a more complete and reliable measurement of heat tolerance.

Thermal death kinetics of fifth-instar Corcyras cephalonica (Lepidoptera: Galleriidae).

The infestation of rice moth, Corcyras cephalonica (Lepidoptera: Galleriidae), causes severe losses in postharvest walnuts. Heat has been studied as a phytosanitary treatment to replace chemical fumigation for controlling this pest. Information on kinetics for thermal mortality of C. cephalonica is needed for developing effective postharvest phytosanitary thermal treatments of walnuts. Thermal death kinetics of fifth-instar C. cephalonica were investigated at temperatures between 44°C and 50°C at a heating rate of 5°C min(-1) using a heating block system. The results showed that thermal-death curves for C. cephalonica larvae followed a 0 order of kinetic reaction. The time to reach 100% mortality decreased with increasing temperature from 150 min at 44°C to 2.5 min at 50°C. The activation energy for controlling C. cephalonica was 466-592 kJ/mol, and the z value obtained from the thermal death time curve was 3.3°C. This kinetic model prediction could be useful in designing the thermal treatment protocol for controlling C. cephalonica in walnuts.

Quality and shelf life of buffalo meat blocks processed in retort pouches.

The shelf life of buffalo meat blocks processed in 3-ply retort pouches at Fo = 12.13 in a stock sterilizer were evaluated at 15 days interval for physico-chemical, microbiological and sensory attributes for a period of 3 months. The pH of the product was 6.28 at 0 day and a gradual decline was noticed during storage. Texture of the product as indicated by shear force values had decreased slowly. The residual nitrite content had significantly declined from 82.67 ppm at 0 day to 45.00 ppm on 90th day of storage. The TBARS values were 0.24 and 0.67 mg malonaldehyde/kg, respectively at 0 day and 90 days of storage. Tyrosine value had significantly increased from 0.37 mg/100 g at 0 day to 0.58 mg/100 g during storage. Free aminoacid content increased gradually from an initial level of 124.32 to 217.51 at 90(th) day of storage. The SDS-PAGE hydrolysis pattern showed barely discernible 205 KDa protein and presence of subfragments in the molecular range of 63 KDa to 29 KDa protein. The sensory studies indicated that the products were well acceptable up to a period of 90 days. As the storage period increased pH, reidual nitrite, sensory attributes declined significantly and TBARS value, tyrosine value and free aminoacid content significantly increased. Mesophillic aerobes and anerobes were found to be absent. The shelf life study indicated that the products were well acceptable up to a period of 90 days based on the assessment of physico-chemical, microbiological and sensory attributes.

Determination of thermal process schedule for emulsion type buffalo meat block in retort pouch.

The process temperature for buffalo met blocks processed in retort pouches calculated based on the heat resistance of Clostridium sporogenes PA 3679 in Phosphate buffer saline (PBS- Ph 7.0) as reference medium and in buffalo meat block (pH 6.28) was in the range of 110-121°C. The D values and Z values calculated for C.sporogenes PA 3679 confirmed that the suspension was best suited for conducting thermal resistance studies. The experiment for indirect confirmation of microbial safety of the products involving inoculating the buffalo meat emulsion filled in pouches with C.sporogenes PA 3679 and processed at Fo 12.13 min showed no growth of microorganisms.

Thermal Inactivation of Escherichia coli O157: H7, Salmonella senftenberg , and Enzymes with Potential as Time-Temperature Indicators in Ground Beef.

The USDA has established processing schedules for beef products based on the destruction of pathogens. Several enzymes have been suggested as potential indicators of heat processing. However, no relationship between the inactivation rates of these enzymes and those of pathogenic microorganisms has been determined. Our objective was to compare the thermal inactivation of Escherichia coli O157:H7 and Salmonella senftenberg to those of endogenous muscle proteins. Inoculated and noninoculated ground beef samples were heated at four temperatures for predetermined intervals of time in thermal-death-time studies. Bacterial counts were determined and enzymes were assayed for residual activity. The D values for E. coli O157:H7 were 46.10, 6.44, 0.43, and 0.12 min at 53, 58, 63, and 68°C, respectively, with a z value of 5.60°C. The D values for S. senftenberg were 53.00, 15.17, 2.08, and 0.22 min at 53, 58, 63, and 68°C, respectively, with a z value of 6.24°C. Apparent D values at 53, 58, 63, and 68°C were 352.93, 26.31, 5.56, and 3.33 min for acid phosphatase; 6968.64, 543.48, 19.61, and 1.40 min for lactate dehydrogenase; and 3870.97, 2678.59, 769.23, and 42.92 min for peroxidase; with z values of 7.41,3.99, and 7.80°C, respectively. Apparent D values at 53, 58, 63, and 66°C were 325.03, 60.07, 3.07, and 1.34 min for phosphoglycerate mutase; 606.72, 89.86, 4.40, and 1.28 min for glyceraldehyde-3-phosphate dehydrogenase; and 153.06, 20.13, 2.25, and 0.74 min for triose phosphate isomerase; with z values of 5.18, 4.71, and 5.56°C, respectively. The temperature dependence of triose phosphate isomerase was similar to those of both E. coli O157 :H7 and S. senftenberg , suggesting that this enzyme could be used as an endogenous time-temperature indicator in beef products.