Delayed transcriptional response of Daphnia pulex to thermal stress.

Temperature fluctuations impose significant physiological challenges on aquatic invertebrates, with far-reaching consequences that span from cellular to ecosystem levels. Even low to moderate heat stress can activate molecular responses that reshape development, metabolism, and reproduction. In this study, we investigated the transcriptional response of Daphnia pulex, a common grazer in lentic freshwater systems, to sublethal temperature stress (a temperature below the acute lethal limit, allowing for survival during chronic exposure). D. pulex were exposed to control (20 °C) and elevated sublethal (25 °C) temperatures to simulate an increased water temperature from a mild heat wave for 168 h. Our findings indicate a dynamic transcriptional response to elevated temperatures. Notably, differential gene expression between the control and temperature-elevated treatment increased throughout the experiment with a 3-fold increase in counts of differentially expressed genes (DEGs) from 247 at 96 h to 743 at 168 h. Changes in gene expression were related to development, specifically reproduction, at 96 h, and a shift toward metabolic processes occurred at 168 h. D. pulex within the experimental treatment generally had higher mean cumulative offspring produced compared to the control treatment. Given D. pulex's role as a foundational species in aquatic food webs, the observed transcriptional response provides insight into the potential for both plastic and adaptive responses in the face of environmental change.