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Skeletal muscle performance is attributed to work producing crossbridge interactions between the contractile proteins, actin and myosin. Myosin kinetics are through to underpin the isotonic force velocity relationship observed in skeletal muscle. Variation in maximum shortening velocity (Vmax ) is attributed to variation in myosin isoform and is thought to dictate organismal performance. However, Vmax  varies in ways that cannot be explained by actin-myosin interactions, decreasing with decreasing muscle recruitment irrespective of the myosin isoform recruited. This effect of recruitment on Vmax  has been suggested to be due to the physical properties of the muscle, such as mass, viscosity and elasticity, limiting its capacity to accelerate and so reach a myosin-defined Vmax . Here we determined the effect of recruitment on Vmax  in 2 anuran plantaris muscles (cane toad and tree frog), with an ~40-fold difference in muscle mass, in vitro. If mass is an important determinant of Vmax , then we would expect a greater effect of recruitment in the larger muscle.  There was a significant effect of recruitment on Vmax  in both muscles (p&lt; .005), and this effect was significantly greater in the larger cane toad muscle (p&lt; .005). This has implications for the scaling of locomotor performance as the ratio between the force available for acceleration and mass to be accelerated systematically declines with increasing body size. Hence, larger species may be limited in their locomotor performance irrespective of the myosin isoform expressed, potentially explaining effects such as the scaling of maximum running speed.

SEB Conference 2025

The effects of muscle mass on maximum shortening velocity and the implications for scaling of locomotor performance

scaling

velocity

muscle

Skeletal muscle performance is attributed to work producing crossbridge interactions between the contractile proteins, actin and myosin. Myosin kinetics are through to underpin the isotonic force velocity relationship observed in skeletal muscle. Variation in maximum shortening velocity (Vmax ) is attributed to variation in myosin isoform and is thought to dictate organismal performance. However, Vmax  varies in ways that cannot be explained by actin-myosin interactions, decreasing with decreasing muscle recruitment irrespective of the myosin isoform recruited. This effect of recruitment on Vmax  has been suggested to be due to the physical properties of the muscle, such as mass, viscosity and elasticity, limiting its capacity to accelerate and so reach a myosin-defined Vmax . Here we determined the effect of recruitment on Vmax  in 2 anuran plantaris muscles (cane toad and tree frog), with an ~40-fold difference in muscle mass, in vitro. If mass is an important determinant of Vmax , then we would expect a greater effect of recruitment in the larger muscle.  There was a significant effect of recruitment on Vmax  in both muscles (p< .005), and this effect was significantly greater in the larger cane toad muscle (p< .005). This has implications for the scaling of locomotor performance as the ratio between the force available for acceleration and mass to be accelerated systematically declines with increasing body size. Hence, larger species may be limited in their locomotor performance irrespective of the myosin isoform expressed, potentially explaining effects such as the scaling of maximum running speed.

technical paper

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The radial lattice spacing (LS) of actin and myosin filaments is a fundamental determinant of muscle force generation, yet its regulation remains an open question. While the assumption of volume constancy provides a phenomenological plausibility check, it lacks a mechanistic basis for LS changes during fiber contraction. We introduce a novel model demonstrating that electrostatic repulsion between charged filaments, governed by Debye-Hückel theory, dictates LS adaptations. Under the simple assumption of energy conservation, the model predicts LS variations across sarcomere lengths with remarkable accuracy, aligning closely with experimental data and outperforming traditional volume-based models.By explaining macroscopic muscle behavior with molecular interactions, the model offers deeper insight into the physics of muscle function. The refined understanding of LS dynamics enhances predictive capabilities in musculoskeletal modeling, with implications for physiology, biomechanics, and robotics. Adaptability to different muscle types further broadens its relevance, offering potential applications in understanding pathological conditions affecting sarcomere structure or the development of targeted drugs.Providing a first-principles explanation for LS regulation, this work challenges conventional assumptions and establishes a new foundation for multi-scale muscle research, from molecular mechanisms to whole-body movement.

Filaments repel while muscles propel: A bio-physical explanation of lattice spacing

As global warming progresses, organismal pace-of-life (POL) traits evolve in response to shifting climates. A new hypothesis states that, apart from the food digestive metabolism, the mitochondrial phenotype (efficiency and density) contributes to POL-variations across thermal regimes. We investigated the thermal evolution and plasticity of the POL and its underlying mechanisms by making use of the southward range expansion of the damselfly Ischnura elegans, from France into warmer regions of Spain. Alongside the ancestral French region, we studied two Spanish regions: a ‘new edge’ region founded 10 years ago (Salamanca) and an ‘old edge’ region founded 70 years ago (Murcia-Alicante). In a common garden experiment, larvae and adults from all regions were exposed to French and Spanish summer temperatures. Our results confirmed that the old edge region evolved a faster POL than the ancestral region when exposed to Spanish temperatures. As hypothesized, the old edge individuals had higher mitochondrial and digestive efficiencies supporting their evolved fast POL. Additionally, the larvae and adults showed higher mitochondrial densities, further sustaining this pace. In contrast, the new edge region showed unexpectedly maladaptive traits, exhibiting a slower POL than the ancestral region when exposed to Spanish temperatures. These individuals increased their food intake, likely prioritizing energy allocation toward repair mechanisms to cope with thermal stress. Our findings suggest that the traditional POL framework, which links food intake to fast POL traits, may require refinement as we show for the first time that the mitochondrial phenotype is a driver of the POL during thermal evolution.

Energized and efficient: digestive efficiency and mitochondrial phenotype underlie the rapid evolution of a faster pace-of-life in a range-expanding species

Environmental temperature affects organisms’ physiology, behaviour and reproduction with costs for fitness and survival. Under climate change, extreme weather events are predicted to increase in frequency and intensity, yet it is not well known how organisms adjust to these rapid variations. Short-term fluctuating temperatures can alter an endotherms’ thermoregulatory capacity, which can be influenced by energy metabolism. On the organismal level, metabolic rate can vary with rapid temperature variations, especially outside the thermoneutral zone, and at the cellular level, mitochondrial function adjusts to meet energy demands. We investigated if repeated exposure to short-term thermal fluctuations can hinder or alleviate metabolic costs of responding to cold temperatures. To achieve this, wild great tits (Parus major) were caught in late autumn and separated into three groups: control, heatwave, and cold spell. First, all birds were kept in a controlled ambient temperature (16°C). Birds in the heatwave and cold spell groups were then subjected to three short-term temperature fluctuations (for 48 hours, 7 days apart), with temperature increasing or decreasing by 10°C from the ambient temperature. Overnight whole-body metabolic rate (basal metabolic rate and metabolic rate below thermoneutrality) and mitochondrial function were assessed at the beginning and end of the experiment. Additionally, after the first wave, daytime metabolic rate was measured below thermoneutrality to study immediate changes. The results of this study could provide insights into how animals can cope with increased climate instability, by investigating the responses to temperature fluctuations and capacity for metabolic plasticity, at both the whole-body and cellular level.

AThe impact of short-term temperature fluctuations on whole-body metabolism and mitochondrial function

Across invertebrates, the offspring of older parents exhibit a tendency for reduced longevity compared to those of younger parents (parental effect senescence). Such a reduction in total lifespan may not impact fitness if shorter-lived individuals can compensate by accelerating development and maximising early-life fecundity. However, whether the offspring can compensate for reduced longevity depends on how parental age affects offspring mortality. If parental effect senescence manifests through increased baseline mortality, then offspring fitness will be constrained from birth. Meanwhile, if parental effect senescence only accelerates the offspring’s age-specific mortality in late life, this could then facilitate the switch to a fast pace-of-life. Here, using individually-housed two-spotted crickets (Gryllus bimaculatus) in a within-individual longitudinal study, we observed that as parental age at reproduction increased, offspring post-natal developmental rates and adult body mass decreased, while adult lifespan increased. Surprisingly, we observed no effect of parental age on offspring total lifespan, with the baseline and age-specific mortality rates being comparable among offspring from young-and old-aged parents. The benign laboratory conditions in which our study population was maintained could have masked any negative effects of parental age on offspring longevity, which may only manifest under certain environments (e.g. with increased competition, pathogens, predation). However, our initial results tentatively suggest that the offspring from old-aged parents’ are shifting towards a fast pace-of-life strategy, as evidenced by their accelerated development and reduced body size. Future work will focus on establishing if any parental age effects persist trans-generationally to similarly influence the grand offspring’s life-histories.

The consequences of late-life reproduction: offspring from old-aged parents may shift to a fast pace-of-life

Climate change-induced heat waves significantly threaten biodiversity by disrupting critical behaviours, such as parental care, essential for offspring development and survival. While parental care can buffer offspring against environmental stressors, its components may respond differently to heat stress. In this study, we investigated how elevated temperatures affect parental care and its buffering role in reproductive performance in the burying beetle Nicrophorus nepalensis across critical life stages. Using experimental warming, we tested the effects of mild (20°C) and severe (22°C) heat stress, compared to ambient temperatures (18°C), on pre-hatching care (carcass preparation) and post-hatching care (offspring provisioning). Severe heat stress completely inhibited egg hatching, whereas mild heat stress reduced brood size and offspring weight. Reciprocal experiments, where carcasses prepared under ambient and heat-stressed conditions were exchanged between breeding females, revealed that heatwaves affecting only one care period did not significantly alter reproduction. However, when both pre- and post-hatching periods were exposed to heat stress, reproductive success declined sharply. Females breeding under heat stress exhibited higher energy expenditure, shown by greater body mass loss during provisioning. Notably, heat stress had long-lasting effects on offspring through carcass preparation, resulting in smaller adult sizes and higher mortality rates. These findings demonstrate that different parental care components respond variably to heat stress, with cascading effects on offspring development. This study highlights the importance of assessing parental care behaviours and their interactions across life stages to better understand species' vulnerability to rising temperatures.

Heat stress shapes parental care and reproductive success in burying beetles across critical life stages

The characteristic buzz of bees, produced by their indirect (thoracic) flight muscles, has been extensively studied in the context of flight. In particular, many studies have investigated how the wingbeat frequency is influenced by abiotic factors such as air density, temperature, and humidity but is primarily constrained by biomechanical factors such as resonance of the thorax and the need to generate lift. However, these same muscles also produce vibrations in non-flight contexts, including communication, defence, and buzz-pollination. During these behaviours, the wings are decoupled from the system, providing a more direct route to assessing the determinants of muscle contraction frequency. Interestingly, mass-dependence is absent in non-flight buzzing, suggesting that other factors dominate when the wings are removed from the system. Here, we summarise multiple experiments on wild and lab-reared bumblebees to explore these factors. In particular, we demonstrate the role of thorax temperature in determining muscle biomechanics, and show that exposure to heavy metals, which interfere with calcium signalling, can reduce contraction frequency. We highlight that studying non-flight vibrations in bees provides valuable insights into asynchronous muscle function, offering a rich avenue for future research.

What determines the pitch of a bee's buzz?

Freshwater ecosystems face growing threats from chemical pollution and climate warming. However, their combined effects on freshwater invertebrates remain poorly understood, particularly the mechanisms linking (sub)individual responses to broader ecological impacts.
To fill this gap, we assessed the effects of environmentally realistic concentrations of pharmaceutical contaminants (PhACs) and warming on freshwater invertebrates in outdoor mesocosms. We explored the physiological responses of Asellus aquaticus (Isopoda) and Cloeon dipterum (Ephemeroptera) in winter and summer, revealing seasonal differences in sensitivity: in winter, PhACs reduced energy availability and increased oxidative damage, while in summer, they boosted energy levels but reduced the population size. These physiological and population-level changes translated into ecosystem-level effects, including reduced leaf litter decomposition and altered insect emergence.
Building on these findings, we investigated the effects of a PhAC-and-pesticide mixture and warming to examine how sublethal effects of environmentally realistic pollutant mixtures cascade to population and ecosystem levels. Preliminary insights suggest that interactions between chemical stressors and warming drive unexpected shifts in zooplankton population size and structure, reinforcing the need to move beyond mortality-based assessments toward more ecologically relevant endpoints.
Our results highlight the vulnerability of aquatic invertebrates to warming in combination with low levels of pollution, demonstrating cascading impacts from individuals to ecosystems. These findings emphasize the need for integrative approaches to assess and mitigate the ecological consequences of emerging contaminants in a changing climate.

Sublethal Yet Significant: How Pollutants and Warming Impact Freshwater Invertebrates and Ecosystem Functioning

Foundation models have revolutionized artificial intelligence by providing versatile, pre-trained architectures that can be adapted to various downstream tasks. In the realm of plant biodiversity, Pl@ntNeth as emerged as a large-scale, AI-driven plant identification system. In this presentation, we investigate the potential of using Pl@ntnet as base for various computer vision tasks beyond species identification, combining it with generalist foundation models positionned upstream or downstream in the pipeline to enable zero-shot possiblity. We test our method on publicly available datasets and which allow comparison with existing supervised and other zero-shot methods.

Pl@ntNet as a foundation model

Photosynthesis is the workhorse for growth of all crops and is one of the key targets for improving agricultural productivity. Its efficiency differs both between and within species, with responses intensively measured under varying CO2  concentrations and light intensities (represented by A-Ci  and A-Q curves). While many published kinetic models of photosynthesis were used to simulate photosynthetic response and to propose different hypotheses, their kinetic parameters were collected from literature or estimated in different species. Yet, the differences in kinetic parameters of enzymes involved in photosynthesis between different genotypes are not fully explored. Moreover, photosynthetic traits are the integrated outcome of genetic factors, environmental conditions, and their complex interactions that renders their accurate prediction from genetic markers challenging. To address these problems, we parameterized a kinetic model for C4  photosynthesis allowing accurate simulation of A-Ci  or A-Q curves measured for 68 inbred lines from a maize population grown in two seasons. The genotype-specific kinetic parameters were then used to train a machine learning model capable of predicting parameter values for genotypes with only genomic data available. The predicted parameters were used to simulate photosynthesis for unseen genotypes and seasons by integrating environmental conditions. The photosynthetic rates simulated at saturating light using our approach, KineticGP, outperformed the predictions of baseline genomic models for unseen genotypes across seasons.

KineticGP, a computational framework for genomic prediction of leaf photosynthesis traits

Anthropogenic CO2  emissions are leading to rising global temperatures and increasingly extreme weather events. High vapour pressure deficit (VPD), which often occurs during heatwaves, reduces stomatal opening, which could negate crop photosynthetic gains associated with higher atmospheric CO2  concentrations. Studying the interactive effects of increased CO2  and VPD using free-air concentration enrichment (FACE) under field conditions is challenging due to technical difficulties associated with regulating temperature and moisture content. To bypass this limitation, we conducted high-VPD (3.14 kPa) heatwave experiments on spring wheat (Triticum Aestivum) at high temperatures (37 °C) and low relative air humidity (50%) in climate-controlled growth chambers set to ambient (450 ppm) or elevated (720 ppm) CO2  concentration.
We find that elevated CO2  concentration combined with high-VPD treatment leads to higher saturating light photosynthesis and maximum rate of rubisco carboxylation (Vcmax ), but this is also associated with much greater evaporative demand than might be expected. Transpiration and thermal profiling within chambers revealed high CO2  grown plants often had higher levels of water-flux and cooler leaves than ambient CO2  equivalent plants, and overall this resulted in higher plant water usage. Light-shift assays showed that photosynthetic performance was tightly linked with stomatal performance during heatwaves at either CO2  concentration, with stomatal conductance only reducing by 37-38% during complete darkness for both CO2  treatments, which could have considerable implications for future water-use efficiency. Together, our results highlight that high CO2  and rising VPD together could aid photosynthetic performance, but this will be at the expense of higher water usage.

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Filaments repel while muscles propel: A bio-physical explanation of lattice spacing

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