Food quality shapes gradual phenotypic plasticity in ectotherms facing temperature variability

Organisms exhibit reversible physiological adjustments as a response to rapidly changing environments. Yet, such plasticity of the phenotype is gradual and may lag behind the environmental fluctuations, thereby affecting the long-term average performance of the organisms. By supplying energy and essential compounds for optimal tissue building, food determines the range of possible phenotypic changes and potentially the rate at which they occur. Here, we assess how differences in the dietary supply of essential lipids modulate the phenotypic plasticity of an ectotherm facing thermal fluctuations. We use three phytoplankton strains to create a gradient of polyunsaturated fatty acids and sterols supply for Daphnia magna under constant and fluctuating temperatures. We used three different fluctuation periodicities to unravel the temporal dynamics of gradual plasticity and its long-term consequences on D. magna performance measured as juvenile somatic growth rate. In agreement with gradual plasticity theory, we show that in D. magna, fluctuation periodicity determines the differential between observed growth rates and those expected from constant conditions. Most importantly, we show that diet modulates both the size and the direction of the growth rate differential. Overall, we demonstrate that the nutritional context is essential for predicting ectotherm consumers’ performance in fluctuating thermal environments.

Methods
To obtain the different data, we ran two experiments:

First, Daphnia magna individuals were fed on three different diets and incubated in climate chambers (POL-EKO ST3/3C SMART) with temperature alternating between 20°C and 28°C with 3 different periodicities (every 12 hrs, 24hrs or 48hrs) while trying to keep the same mean temperature and variance across treatments.

Then, in order to have a reference for performance without temperature fluctuations and to model the Thermal Performance Curve (TPC), we additionally grew Daphnia at constant temperatures of 18°C, 20°C, 24°C, 26°C, 28°C and 30°C, fed on the three diets.

We measured for each experiment and condition, the growth rate at day 6 of individuals,

The data were collected in an Excell file and analysed with an R script.