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.