Clonal thermal preferences affect the strength of the temperature-size rule

Temperature-driven trade-off between mitochondrial activity and efficiency in live rotifers representing different thermal histories

Mitochondria generate up to 90% of cellular ATP, making it critical to understand how abiotic factors affect mitochondrial function under varying conditions. Using clones of the rotifer Lecane inermis with known thermal preferences, we investigated mitochondrial bioenergetic responses to four thermal regimes: standard temperature, optimal temperature, low suboptimal temperature and high suboptimal temperature. The study aimed to determine how mitochondrial parameters in intact organisms vary with temperature shifts and whether these responses differ across experimental populations. We assessed key bioenergetic parameters: routine respiration (representing overall metabolic rate), electron transport system capacity (indicative of oxidative phosphorylation potential) and proton leak rates (reflecting the energetic costs of maintaining mitochondrial membrane potential). Our results showed that populations with different thermal preferences displayed distinct mitochondrial responses to temperature changes, particularly at suboptimal temperatures. In contrast, responses were more uniform under standard and optimal conditions. Our findings demonstrated that metabolic plasticity in changing environments often involves trade-offs between mitochondrial efficiency and maintenance. By studying mitochondrial respiration at the whole-organism level, we revealed the complex temperature dependence of bioenergetic traits, providing insights beyond isolated mitochondria studies. This research highlights how a cascade of plastic responses spanning from mitochondrial responses to overall growth patterns is triggered by temperature changes, offering a valuable perspective in the context of global warming and organismal adaptation.

Rapid evolution of unimodal but not of linear thermal performance curves in Daphnia magna

Species may cope with warming through both rapid evolutionary and plastic responses. While thermal performance curves (TPCs), reflecting thermal plasticity, are considered powerful tools to understand the impact of warming on ectotherms, their rapid evolution has been rarely studied for multiple traits. We capitalized on a 2-year experimental evolution trial in outdoor mesocosms that were kept at ambient temperatures or heated 4°C above ambient, by testing in a follow-up common-garden experiment, for rapid evolution of the TPCs for multiple key traits of the water flea Daphnia magna. The heat-selected Daphnia showed evolutionary shifts of the unimodal TPCs for survival, fecundity at first clutch and intrinsic population growth rate toward higher optimum temperatures, and a less pronounced downward curvature indicating a better ability to keep fitness high across a range of high temperatures. We detected no evolution of the linear TPCs for somatic growth, mass and development rate, and for the traits related to energy gain (ingestion rate) and costs (metabolic rate). As a result, also the relative thermal slope of energy gain versus energy costs did not vary. These results suggest the overall (rather than per capita) top-down impact of D. magna may increase under rapid thermal evolution.

Experimental Evolution Shows Body Size Decrease in Response to Hypoxia, with a Complex Effect on Plastic Size Response to Temperature

AbstractThere is a scientific debate whether oxygen concentration may be a factor driving the pattern of size decrease at higher temperature. Central to this debate is the fact that oxygen availability relative to demand for living organisms decreases with increasing temperature. We examined whether rotifers Lecane inermis exposed to hypoxic conditions would evolve smaller sizes than rotifers exposed to normoxic conditions, using experimental evolution with the same fluctuating temperature but differentiated by three regimes of oxygen availability: normoxia, hypoxia throughout the whole thermal range, and hypoxia only at the highest temperature. Immediately after the six-month experiment (more than 90 generations), we tested the plasticity of size responses to temperature in three post-evolution groups, and we related these responses to fitness. The results show that normoxic rotifers had evolved significantly larger sizes than two hypoxic rotifer groups, which were similar in size. All three groups displayed similar plastic body size reductions in response to warming over the range of temperatures they were exposed to during the period of experimental evolution, but they showed different and complex responses at two temperatures below this range. Any type of plastic response to different temperatures resulted in a similar fitness pattern across post-evolution groups. We conclude that (i) these rotifers showed a genetic basis for the pattern of size decrease following evolution under both temperature-dependent and temperature-independent hypoxia; and (ii) plastic body size responds consistently to temperatures that are within the thermal range that the rotifers experienced during their evolutionary history, but responses become more noisy at novel temperatures, suggesting the importance of evolutionary responses to reliable environmental cues.