Journal of Experimental Biology

Powers, Matthew J.; Baty, James A.; Dinga, Alexis M.; Mao, James H.; Hill, Geoffrey E.

doi: 10.1242/jeb.244230pmid: 35695335

The shared-pathway hypothesis offers a cellular explanation for the connection between ketocarotenoid pigmentation and individual quality. Under this hypothesis, ketocarotenoid metabolism shares cellular pathways with mitochondrial oxidative phosphorylation such that red carotenoid-based coloration is inextricably linked mitochondrial function. To test this hypothesis, we exposed Tigriopus californicus copepods to a mitochondrially targeted protonophore, 2,4-dinitrophenol (DNP), to induce proton leak in the inner mitochondrial membranes. We then measured whole-animal metabolic rate and ketocarotenoid accumulation. As observed in prior studies of vertebrates, we observed that DNP treatment of copepods significantly increased respiration and that DNP-treated copepods accumulated more ketocarotenoid than control animals. Moreover, we observed a relationship between ketocarotenoid concentration and metabolic rate, and this association was strongest in DNP-treated copepods. These data support the hypothesis that ketocarotenoid and mitochondrial metabolism are biochemically intertwined. Moreover, these results corroborate observations in vertebrates, perhaps suggesting a fundamental connection between ketocarotenoid pigmentation and mitochondrial function that should be explored further.

Przybylska-Piech, Anna S.; Jefimow, Małgorzata

doi: 10.1242/jeb.244222pmid: 35615921

Nonresponding Siberian hamsters (Phodopus sungorus) do not develop the winter phenotype of white fur, low body mass (Mb) and spontaneous torpor in response to short photoperiod. However, their thermoregulatory response to fasting remains unknown. We measured body temperature and Mb of 12 nonresponders acclimated to short photoperiod and then to cold and fasted four times for 24 h. Four individuals used torpor, and in total, we recorded 19 torpor bouts, which were shallow, short and occurred at night. Moreover, fasting increased the heterothermy index in all hamsters. Low Mb was not a prerequisite for torpor use and Mb loss did not correlate with either heterothermy index or torpor use. This is the first evidence that individuals which do not develop the winter phenotype can use torpor or increase body temperature variability to face unpredictable, adverse environmental conditions. Despite the lack of seasonal changes, thermoregulatory adjustments may increase the probability of winter survival in nonresponders.

Ramos, Cara D.; Bohnert, K. Adam; Johnson, Alyssa E.

doi: 10.1242/jeb.244282pmid: 35620964

Sex-specific differences in animal behavior commonly reflect unique reproductive interests. In the nematode Caenorhabditis elegans, hermaphrodites can reproduce without a mate and thus prioritize feeding to satisfy the high energetic costs of reproduction. However, males, which must mate to reproduce, sacrifice feeding to prioritize mate-searching behavior. Here, we demonstrate that these behavioral differences influence sexual dimorphism at the organelle level; young males raised on a rich food source show constitutive induction of gut tubular lysosomes, a non-canonical lysosome morphology that forms in the gut of hermaphrodites when food is limited or as animals age. We found that constitutive induction of gut tubular lysosomes in males results from self-imposed dietary restriction through DAF-7/TGFβ, which promotes exploratory behavior. In contrast, age-dependent induction of gut tubular lysosomes in hermaphrodites is stimulated by self-fertilization activity. Thus, separate reproductive tradeoffs influence tubular lysosome induction in each sex, potentially supporting different requirements for reproductive success.

Lyons, Sulayman A.; McClelland, Grant B.

doi: 10.1242/jeb.244080pmid: 35552735

Highland native deer mice (Peromyscus maniculatus) have greater rates of lipid oxidation during maximal cold challenge in hypoxia (hypoxic cold-induced V̇O2,max) compared with their lowland conspecifics. Lipid oxidation is also increased in deer mice acclimated to simulated high altitude (cold hypoxia), regardless of altitude ancestry. The underlying lipid metabolic pathway traits responsible for sustaining maximal thermogenic demand in deer mice is currently unknown. The objective of this study was to characterize key steps in the lipid oxidation pathway in highland and lowland deer mice acclimated to control (23°C, 21 kPa O2) or cold hypoxic (5°C, 12 kPa O2) conditions. We hypothesized that capacities for lipid delivery and tissue uptake will be greater in highlanders and further increase with cold hypoxia acclimation. With the transition from rest to hypoxic cold-induced V̇O2,max, both highland and lowland deer mice showed increased plasma glycerol concentrations and fatty acid availability. Interestingly, acclimation to cold hypoxia led to increased plasma triglyceride concentrations at cold-induced V̇O2,max, but only in highlanders. Highlanders also had significantly greater delivery rates of circulatory free fatty acids and triglycerides due to higher plasma flow rates at cold-induced V̇O2,max. We found no population or acclimation differences in fatty acid translocase (FAT/CD36) abundance in the gastrocnemius or brown adipose tissue, suggesting that fatty acid uptake across membranes is not limiting during thermogenesis. Our data indicate that circulatory lipid delivery plays a major role in supporting the high thermogenic rates observed in highland versus lowland deer mice.

Thorstensen, Matt J.; Turko, Andy J.; Heath, Daniel D.; Jeffries, Ken M.; Pitcher, Trevor E.

doi: 10.1242/jeb.244162pmid: 35673877

Transcriptomic research provides a mechanistic understanding of an organism's response to environmental challenges such as increasing temperatures, which can provide key insights into the threats posed by thermal challenges associated with urbanization and climate change. Differential gene expression and alternative splicing are two elements of the transcriptomic stress response that may work in tandem, but relatively few studies have investigated these interactions in fishes of conservation concern. We studied the imperilled redside dace (Clinostomus elongatus) as thermal stress is hypothesized to be an important cause of population declines. We tested the hypothesis that gene expression–splicing interactions contribute to the thermal stress response. Wild fish exposed to acute thermal stress were compared with both handling controls and fish sampled directly from a river. Liver tissue was sampled to study the transcriptomic stress response. With a gene set enrichment analysis, we found that thermally stressed fish showed a transcriptional response related to transcription regulation and responses to unfolded proteins, and alternatively spliced genes related to gene expression regulation and metabolism. One splicing factor, prpf38b, was upregulated in the thermally stressed group compared with the other treatments. This splicing factor may have a role in the Jun/AP-1 cellular stress response, a pathway with wide-ranging and context-dependent effects. Given large gene interaction networks and the context-dependent nature of transcriptional responses, our results highlight the importance of understanding interactions between gene expression and splicing for understanding transcriptomic responses to thermal stress. Our results also reveal transcriptional pathways that can inform conservation breeding, translocation and reintroduction programs for redside dace and other imperilled species by identifying appropriate source populations.

Knight, Kathryn

doi: 10.1242/jeb.244545pmid: N/A

View largeDownload slide A deer mouse (Peromyscus maniculatus) in a respirometry chamber. Photo credit: Sulayman Lyons. View largeDownload slide A deer mouse (Peromyscus maniculatus) in a respirometry chamber. Photo credit: Sulayman Lyons. They may not look like it, but high-altitude deer mice are metaphorically on fire. With an astronomically fast metabolism evolved to combat the twin challenges of living at high altitude –maintaining their warm body temperature while living in chilly thin air – the tiny mammals are well prepared for their mountain existence. To stoke their heating, the deer mice fuel their internal inferno with fat: ‘Highland deer mice burn fats faster for maximum heat production than when they are exercising’, says Sulayman Lyons from McMaster University, Canada. But it wasn't clear exactly how they managed to mobilise enough fatty fuel to meet their extraordinary heat production demands. So, Lyons and Grant McClelland (McMaster University) collected deer mice from the summit of Mount Evans, CO, USA (4350 m) and low altitude Nine-mile Prairie, NE, USA (320 m), to find out how the animals manage their fuel supplies in warm low altitude conditions and in a chilly mountain top simulation. ‘We simulate high altitude conditions by keeping mice in special chambers, which lower the air pressure, in a cold room’, says Lyons, who kept deer mice originating from both the mountain and prairie at 5°C in the air pressure experienced at ∼4300 m for 6 weeks, while other deer mice basked in warm (∼23°C) lab air. He then measured the metabolic rates of some of the animals at the temperature and air pressure at which they had been living in the lab, while also measuring the metabolic rates of the animals as he chilled them to extreme mountain temperatures (–10°C) in thin air. Then he collected blood samples, fat and muscle from all of the deer mice, to find out how they managed and mobilised their fat reserves. Comparing the deer mice that originated from the lowland prairies with the highland deer mice as they adjusted to life on the simulated mountain top, both groups of animals increased the amount of fat they released from their white fat stores into their blood to fuel their higher metabolisms in the thin cold air. In addition, the deer mice that had originally lived at high altitude in Colorado carried more triglyceride fats in their blood, which could be used as fuel by their muscles and brown fat to produce warmth. Lyons also measured the quantities of two different types of fat (non-esterified fatty acids and triglycerides) in the blood of resting deer mice and when the animals had to work harder because they were breathing high-altitude thin cold air. The deer mice that had been born at high altitude were able to boost the amount of fat in their blood enormously, increasing the non-esterified fatty acids 3.1-fold and triglycerides by a massive 7.1-fold, when their bodies needed the fuel to keep warm in the chilly mountain conditions. However, when he checked for evidence of the proteins that are responsible for transporting fuel fats into the muscle cells and brown fat that generate heat, the low- and high-altitude deer mice had essentially the same quantities of fat-transporting proteins within the tissues. The differences in the animals’ metabolisms were entirely down to their abilities to transport fats in their blood, with deer mice that make their homes amongst the mountain peaks supplying more fuel through the blood to their muscle and brown fat furnaces. Lyons also suspects that the mini mountaineers devote more muscle to shivering than they use when moving, which could account for the deer mouse's extraordinarily high rates of fuel consumption when keeping warm at altitude. Lyons , S. A. and McClelland , G. B. ( 2022 ). Thermogenesis is supported by high rates of circulatory fatty acid and triglyceride delivery in highland deer mice . J. Exp. Biol. 225 , jeb244080 . https://doi.org/10.1242/jeb.244080 Google Scholar Crossref Search ADS © 2022. Published by The Company of Biologists Ltd 2022

Knight, Kathryn

doi: 10.1242/jeb.244588pmid: N/A

View largeDownload slide A fluorescing desert grassland scorpion (Paruroctonus utahensis) with a crystal on its back. Photo credit: Mariëlle Hoefnagels. View largeDownload slide A fluorescing desert grassland scorpion (Paruroctonus utahensis) with a crystal on its back. Photo credit: Mariëlle Hoefnagels. Since the advent of satellites and Google Maps, most of us just plug in to find our way home. But not the intrepid birds, bees and ants that negotiate the planet. They depend on more traditional means of navigation, orienting by the stars, sun and local views. But Douglas Gaffin from the University of Oklahoma, USA, suspects that scorpions may use another strategy. The arachnids are equipped with a pair of touch- and taste-sensitive brushes, known as pectines, located beneath their abdomens, which can rub along the ground as the animals tour around. Could the fearsome creatures be tasting the ground and reading the surface by touch with their pectines to navigate home? To find out, Gaffin with colleagues Maria Muñoz and Mariëlle Hoefnagels (both from the University of Oklahoma) visited the University of New Mexico Sevilleta Field Station to gather desert grassland scorpions (Paruroctonus utahensis) to bring back to the lab, to find out how the animals navigate when they have to depend on their pectines alone. ‘We tested various combinations of sand, soil and arena configurations before we settled on our final design of water heater drainage pans surrounded with blackout curtains to prevent the scorpions from seeing landmarks for visual navigation’, says Gaffin, who also provided the scorpions with a mound of sand to burrow in. Then, the team glued a small crystal to the back of each animal that would reflect light during the day and infrared light at night, to allow the team to film and keep track of each individual as it ambled around an enclosure after dark. If the animals were able to navigate using their pectines alone for guidance, Gaffin reasoned that the scorpions would circulate in ever expanding circles around their new homes to define the lie of the land to build a map of their surroundings for later use. Collecting 1500 h of movies as 23 scorpions constructed burrows in the mound of sand and in a featureless flat arena, Gaffin, Muñoz and Hoefnagels saw 18 of the arachnids emerge from their new burrows and begin exploring the region around their home, looping and completing up to 10 exploratory circuits. The scorpions appeared to be performing learning walks, just like navigating ants that depend on local knowledge, learned when they first emerge from their home and circulate around their burrow. The scorpions were probably gleaning information about their surroundings from their touch- and taste-sensitive pectines alone, as they were unable to see any landmarks. And, when the trio cunningly rotated the scorpions’ arenas – first by 90 deg, and then a day later by a further 90 deg – the pioneering arachnids successfully found their repositioned burrows, instead of sticking stubbornly to exploring the burrows' original location. They were tasting and reading the feel of the sand with their pectines to return continually to their burrow homes, even though the homes had moved by ∼15 cm when the scientists rotated the desert arenas. So, the desert scorpions’ pectines appear to provide the animals with their sense of place, but that does not rule out the possibility that the arachnids also depend on vision for navigation when provided with a more informative view. And Gaffin suspects that scorpions also use their own internally calibrated odometer to keep track of their location relative to home when they venture out, once they have a sense of the lie of the land. Gaffin , D. D., Muñoz , M. G. and Hoefnagels , M. H. ( 2022 ). Evidence of learning walks related to scorpion home burrow navigation . J. Exp. Biol. 225 , jeb243947 . https://doi.org/10.1242/jeb.243947 Google Scholar Crossref Search ADS © 2022. Published by The Company of Biologists Ltd 2022

Rees, Bernard B.; Reemeyer, Jessica E.; Irving, Brian A.

doi: 10.1242/jeb.243680pmid: 35673886

This study asked whether interindividual variation in maximum and standard aerobic metabolic rates of the Gulf killifish, Fundulus grandis, correlates with gill morphology and cardiac mitochondrial bioenergetics, traits reflecting critical steps in the O2 transport cascade from the environment to the tissues. Maximum metabolic rate (MMR) was positively related to body mass, total gill filament length and myocardial oxygen consumption during maximum oxidative phosphorylation (multiple R2=0.836). Standard metabolic rate (SMR) was positively related to body mass, total gill filament length and myocardial oxygen consumption during maximum electron transport system activity (multiple R2=0.717). After controlling for body mass, individuals with longer gill filaments, summed over all gill arches, or greater cardiac respiratory capacity had higher whole-animal metabolic rates. The overall model fit and the explanatory power of individual predictor variables were better for MMR than for SMR, suggesting that gill morphology and myocardial bioenergetics are more important in determining active rather than resting metabolism. After accounting for body mass, heart ventricle mass was not related to variation in MMR or SMR, indicating that the quality of the heart (i.e. the capacity for mitochondrial metabolism) was more influential than heart size. Finally, the myocardial oxygen consumption required to offset the dissipation of the transmembrane proton gradient in the absence of ATP synthesis was not correlated with either MMR or SMR. The results support the idea that interindividual variation in aerobic metabolism, particularly MMR, is associated with variation in specific steps in the O2 transport cascade.

Capshaw, Grace; Christensen-Dalsgaard, Jakob; Carr, Catherine E.

doi: 10.1242/jeb.244130pmid: 35724322

The ability to sense and localize sound is so advantageous for survival that it is difficult to understand the almost 100 million year gap separating the appearance of early tetrapods and the emergence of an impedance-matching tympanic middle ear – which we normally regard as a prerequisite for sensitive hearing on land – in their descendants. Recent studies of hearing in extant atympanate vertebrates have provided significant insights into the ancestral state(s) and the early evolution of the terrestrial tetrapod auditory system. These reveal a mechanism for sound pressure detection and directional hearing in ‘earless’ atympanate vertebrates that may be generalizable to all tetrapods, including the earliest terrestrial species. Here, we review the structure and function of vertebrate tympanic middle ears and highlight the multiple acquisition and loss events that characterize the complex evolutionary history of this important sensory structure. We describe extratympanic pathways for sound transmission to the inner ear and synthesize findings from recent studies to propose a general mechanism for hearing in ‘earless’ atympanate vertebrates. Finally, we integrate these studies with research on tympanate species that may also rely on extratympanic mechanisms for acoustic reception of infrasound (<20 Hz) and with studies on human bone conduction mechanisms of hearing.

Ste-Marie, Eric; Grémillet, David; Fort, Jérôme; Patterson, Allison; Brisson-Curadeau, Émile; Clairbaux, Manon; Perret, Samuel; Speakman, John R.; Elliott, Kyle H.

doi: 10.1242/jeb.243252pmid: 35593255

Accelerometry has been widely used to estimate energy expenditure in a broad array of terrestrial and aquatic species. However, a recent reappraisal of the method showed that relationships between dynamic body acceleration (DBA) and energy expenditure weaken as the proportion of non-mechanical costs increases. Aquatic air breathing species often exemplify this pattern, as buoyancy, thermoregulation and other physiological mechanisms disproportionately affect oxygen consumption during dives. Combining biologging with the doubly labelled water method, we simultaneously recorded daily energy expenditure (DEE) and triaxial acceleration in one of the world's smallest wing-propelled breath-hold divers, the dovekie (Alle alle). These data were used to estimate the activity-specific costs of flying and diving and to test whether overall dynamic body acceleration (ODBA) is a reliable predictor of DEE in this abundant seabird. Average DEE for chick-rearing dovekies was 604±119 kJ day−1 across both sampling years. Despite recording lower stroke frequencies for diving than for flying (in line with allometric predictions for auks), dive costs were estimated to surpass flight costs in our sample of birds (flying: 7.24× basal metabolic rate, BMR; diving: 9.37× BMR). As expected, ODBA was not an effective predictor of DEE in this species. However, accelerometer-derived time budgets did accurately estimate DEE in dovekies. This work represents an empirical example of how the apparent energetic costs of buoyancy and thermoregulation limit the effectiveness of ODBA as the sole predictor of overall energy expenditure in small shallow-diving endotherms.