Evolution and plasticity of thermal performance: an analysis of variation in thermal tolerance and fitness in 22 Drosophila species

The thermal biology of ectotherms is often used to infer species' responses to changes in temperature. It is often proposed that temperate species are more cold-tolerant, less heat-tolerant, more plastic, have broader thermal performance curves (TPCs) and lower optimal temperatures when compared to tropical species. However, relatively little empirical work has provided support for this using large interspecific studies. In the present study, we measure thermal tolerance limits and thermal performance in 22 species of Drosophila that developed under common conditions. Specifically, we measure thermal tolerance (CT_min and CT_max) as well as the fitness components viability, developmental speed and fecundity at seven temperatures to construct TPCs for each of these species. For 10 of the species, we also measure thermal tolerance and thermal performance following developmental acclimation to three additional temperatures. Using these data, we test several fundamental hypotheses about the evolution and plasticity of heat and cold resistance and thermal performance. We find that cold tolerance (CT_min) varied between the species according to the environmental temperature in the habitat from which they originated. These data support the idea that the evolution of cold tolerance has allowed species to persist in colder environments. However, contrary to expectation, we find that optimal temperature ( T_opt) and the breadth of thermal performance ( T_breadth) are similar in temperate, widespread and tropical species and we also find that the plasticity of TPCs was constrained. We suggest that the temperature range for optimal thermal performance is either fixed or under selection by the more similar temperatures that prevail during growing seasons. As a consequence, we find that T_opt and T_breadth are of limited value for predicting past, present and future distributions of species. This article is part of the theme issue 'Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen'.

Cold acclimation modulates voltage gated Ca2+ channel currents and fiber excitability in skeletal muscles of Locusta migratoria

Cold exposure is known to induce stressful imbalances in chill susceptible insects, including loss of hemolymph water, hyperkalemia and cell depolarization. Cold induced depolarization induces uncontrolled Ca²⁺ influx and accumulation of injury through necrosis/apoptosis. Conversely cold induced Ca²⁺ influx has been shown to induce rapid cold hardening and therefore also play a role to reduce cold injury. Cold acclimation is known to reduce cold injury in insects and due to the involvement of depolarization and Ca²⁺ in the pathophysiology of hypothermia, we hypothesized that cold acclimation modulates voltage gated Ca²⁺ channels and fiber excitability. Using intracellular electrodes or force transducers, we measured the Ca²⁺ currents, fiber excitability and muscle contractility in warm (31 °C) and cold (11 °C) acclimated locusts. Experiments were performed under conditions ranging from mild conditions where the membrane potential is well regulated to stressful conditions, where the membrane potential is very depolarized and the tissue is at risk of accumulating injury. These experiments found that cold acclimation modulates Ca²⁺ currents and fiber excitability in a manner that depends on the cold exposure. Thus, under mild conditions, Ca²⁺ currents and fiber excitability was increased whilst muscle contractility was unaffected by cold acclimation. Conversely, fiber excitability and muscle contractility was decreased under stressful conditions. Further work is required to fully understand the adaptive effects of these modulations. However, we propose a model which reconciles the dualistic role of the Ca²⁺ ion in cold exposure and cold acclimation. Thus, increased Ca²⁺ currents at mild temperatures could help to enhance cold sensing capacity whereas reduced fiber excitability under stressful conditions could help to reduce catastrophic Ca²⁺ influx during periods of severe cold exposure.

The central nervous system and muscular system play different roles for chill coma onset and recovery in insects

When insects are cooled, they initially lose their ability to perform coordinated movements at their critical thermal minima (CT_min). At a slightly lower temperature, they enter a state of complete paralysis (chill coma onset temperature - CCO) and if they are returned to permissive temperatures they regain function after a recovery period which is termed chill coma recovery time (CCRT). These three phenotypes (CT_min, CCO, and CCRT) are all popular measures of insect cold tolerance and it is therefore important to characterize the physiological processes that are responsible for these phenotypes. In the present study we measured extracellular field potentials in the central nervous system (CNS) and muscle membrane potential (V_m) during cooling and recovery in three Drosophila species that have different cold tolerances. With these measurements we assess the role of the CNS and muscle V_m in setting the lower thermal limits (CT_min and CCO) and in delaying chill coma recovery (CCRT). The experiments suggest that entry into chill coma is primarily caused by the onset of a spreading depolarization in the CNS for all three species. In the two most cold-sensitive species we observed that the loss of CNS function was followed closely by a depolarization of muscle V_m which is known to compromise muscle function. When flies are returned to benign temperature after a cold exposure we observe a rapid recovery of CNS function, but functional recovery was delayed by a slower recovery of muscle polarization. Thus, we demonstrate the primacy of different physiological systems (CNS vs. muscle) as determinants of the most commonly used cold tolerance measures for insects (CT_min vs. CCRT).

Effects of anoxia on ATP, water, ion and pH balance in an insect (Locusta migratoria)

When exposed to anoxia, insects rapidly go into a hypometabolic coma from which they can recover when exposed to normoxia again. However, prolonged anoxic bouts eventually lead to death in most insects, although some species are surprisingly tolerant. Anoxia challenges ATP, ion, pH and water homeostasis, but it is not clear how fast and to what degree each of these parameters is disrupted during anoxia, nor how quickly they recover. Further, it has not been investigated which disruptions are the primary source of the tissue damage that ultimately causes death. Here, we show, in the migratory locust (Locusta migratoria), that prolonged anoxic exposures are associated with increased recovery time, decreased survival, rapidly disrupted ATP and pH homeostasis and a slower disruption of ion ([K⁺] and [Na⁺]) and water balance. Locusts could not fully recover after 4 h of anoxia at 30°C, and at this point hemolymph [K⁺] was elevated 5-fold and [Na⁺] was decreased 2-fold, muscle [ATP] was decreased to ≤3% of normoxic values, hemolymph pH had dropped 0.8 units from 7.3 to 6.5, and hemolymph water content was halved. These physiological changes are associated with marked tissue damage in vivo and we show that the isolated and combined effects of hyperkalemia, acidosis and anoxia can all cause muscle tissue damage in vitro to equally large degrees. When locusts were returned to normoxia after a moderate (2 h) exposure of anoxia, ATP recovered rapidly (15 min) and this was quickly followed by recovery of ion balance (30 min), while pH recovery took 2-24 h. Recovery of [K⁺] and [Na⁺] coincided with the animals exiting the comatose state, but recovery to an upright position took ∼90 min and was not related to any of the physiological parameters examined.

Fluctuating thermal regime preserves physiological homeostasis and reproductive capacity in Drosophila suzukii

Drosophila suzukii, an invasive species recently introduced in Europe, lays eggs in thin-skinned fruits and causes huge financial losses to fruit growers. One potential way to control this pest is the sterile insect technique (SIT) which demands a large stock of reproductive females to produce millions of sterile males to be released on demand. Unfortunately, Drosophila stocks age quickly, show declining fecundity when maintained at warm temperatures and conversely, they die from chill injury if they are maintained at constant low temperature. Here we investigate the potential of fluctuating thermal regime (FTR) as a storage method that harness the benefits of both warm and cold storage. Using a FTR with a daily warm period (1 h 20 at 25 °C) and cold period (20 h at 3 °C), interspaced by gradual heating and cooling, we compared longevity, fecundity and physiological condition between FTR females and females exposed to constant 25 °C and 3 °C. As hypothesised, FTR flies experienced much slower senescence (>3-fold increase in lifespan) and they preserved fecundity to a much higher age than flies from constant 25 °C. Flies maintained at constant 3 °C quickly died from chill injuries caused by a gradual loss of ion and water balance. In contrast, FTR flies were able to maintain ion and water balance (similar to 25 °C flies) as they were allowed to recover homeostasis during the short warm periods. Together these results demonstrate that FTR represents a useful protocol for storage of Drosophila stocks, and more broadly, this shows that the benefits of FTR are tightly linked with the insect ability to recover physiological homeostasis during the short warm periods.

Plasticity for desiccation tolerance across Drosophila species is affected by phylogeny and climate in complex ways

Comparative analyses of ectotherm susceptibility to climate change often focus on thermal extremes, yet responses to aridity may be equally important. Here we focus on plasticity in desiccation resistance, a key trait shaping distributions of Drosophila species and other small ectotherms. We examined the extent to which 32 Drosophila species, varying in their distribution, could increase their desiccation resistance via phenotypic plasticity involving hardening, linking these responses to environment, phylogeny and basal resistance. We found no evidence to support the seasonality hypothesis; species with higher hardening plasticity did not occupy environments with higher and more seasonal precipitation. As basal resistance increased, the capacity of species to respond via phenotypic plasticity decreased, suggesting plastic responses involving hardening may be constrained by basal resistance. Trade-offs between basal desiccation resistance and plasticity were not universal across the phylogeny and tended to occur within specific clades. Phylogeny, environment and trade-offs all helped to explain variation in plasticity for desiccation resistance but in complex ways. These findings suggest some species have the ability to counter dry periods through plastic responses, whereas others do not; and this ability will depend to some extent on a species' placement within a phylogeny, along with its basal level of resistance.

Paralytic hypo-energetic state facilitates anoxia tolerance despite ionic imbalance in adult Drosophila melanogaster

Oxygen limitation plays a key role in many pathologies; yet, we still lack a fundamental understanding of the mechanisms responsible for variation in anoxia tolerance. Most vertebrate studies suggest that anoxia tolerance involves the ability to maintain cellular ATP despite the loss of aerobic metabolism. However, insects such as adult Drosophila melanogaster are able to survive long periods of anoxia (LT₅₀: ∼8 h) in a hypo-energetic state characterized by low [ATP]. In this study, we tested for possible mechanisms that allow D. melanogaster adults to survive long periods of anoxia. Adults are paralyzed within 30 s, and after 2 h of anoxia, ATP was 3% of normal, extracellular potassium concentration ([K⁺]_o) increased threefold, pH dropped 1 unit, yet survival was 100%. With 0.5-6 h of anoxia, adults maintained low but constant ATP levels while [K⁺]_o and pH_o continued to change. When returned to normoxia, adults restored [K⁺]_o and activity. With longer durations of anoxia, ATP levels decreased and [K⁺]_o rose further, and both correlated tightly with decreased survival. This response contrasts with the anoxia-sensitive larval stage (LT₅₀: ∼1 h). During anoxia, larvae attempted escape for up to 30 min and after 2 h of anoxia, ATP was <1% of resting, [K⁺]_o increased by 50%, hemolymph pH fell by 1 unit, and survival was zero. The superior anoxia tolerance of adult D. melanogaster appears to be due to the capacity to maintain a paralytic hypometabolic state with low but non-zero ATP levels, and to be able to tolerate extreme extracellular ionic variability.

Role of temperature on growth and metabolic rate in the tenebrionid beetles Alphitobius diaperinus and Tenebrio molitor

Insects are increasingly used as a dietary source for food and feed and it is therefore important to understand how rearing conditions affect growth and development of these agricultural animals. Temperature is arguably the most important factor affecting metabolism and growth rate in insects. Here, we investigated how rearing temperature affected growth rate, growth efficiency and macronutrient composition in two species of edible beetle larvae: Alphitobius diaperinus and Tenebrio molitor. Growth rates of both species were quantified at temperatures ranging from 15.2 to 38.0 °C after which we measured protein and lipid content of the different treatment groups. Metabolic rate was measured in a similar temperature range by measuring the rate of O₂ consumption (V·O₂) and CO₂ production (V·CO₂) using repeated measures closed respirometry. Using these measurements, we calculated the growth efficiency of mealworms by relating the energy assimilation rate to the metabolic rate. Maximum daily growth rates were 18.3% and 16.6% at 31 °C, for A. diaperinus and T. molitor respectively, and we found that A. diaperinus was better at maintaining growth at high temperatures while T. molitor had superior growth at lower temperatures. Both species had highest efficiencies of energy assimilation in the temperature range of 23.3-31.0 °C, with values close to 2 J assimilated/J metabolised in A. diaperinus and around 4 J assimilated/J metabolised in T. molitor. Compared to "conventional" terrestrial livestock, both species of insects were characterised by high growth rates and very high energy conversion efficiency at most experimental temperatures. For A. diaperinus, lipid content was approximately 30% of dry mass and protein content approximately 50% of dry mass across most temperatures. Temperature had a greater influence on the body composition of T. molitor. At 31.0 °C the lipid and protein content was measured to 47.4% and 37.9%, respectively but lipid contents decreased, and protein contents increased when temperatures were higher or lower than 31.0 °C. In summary, rearing temperature had large and independent effects on growth rate, energy assimilation efficiency and protein/lipid content. Accordingly, temperature is a critical parameter to control in commercial insect rearing regardless if the producer wants to optimise production speed, production efficiency or product quality.