Abstract

BackgroundRegulatory adjustments to acute and chronic temperature changes are highly important for aquatic ectotherms because temperature affects their metabolic rate as well as the already low oxygen concentration in water, which can upset their energy balance. This also applies to severe changes in food supply. Thus, we studied on a molecular level (transcriptomics and/or proteomics) the immediate responses to heat stress and starvation and the acclimation to different temperatures in two clonal isolates of the model microcrustacean Daphnia pulex from more or less stressful environments, which showed a higher (clone M) or lower (clone G) tolerance to heat and starvation.ResultsThe transcriptomic responses of clone G to acute heat stress (from 20 °C to 30 °C) and temperature acclimation (10 °C, 20 °C, and 24 °C) and the proteomic responses of both clones to acute heat, starvation, and heat-and-starvation stress comprised environment-specific and clone-specific elements. Acute stress (in particular heat stress) led to an early upregulation of stress genes and proteins (e.g., molecular chaperones) and a downregulation of metabolic genes and proteins (e.g., hydrolases). The transcriptomic responses to temperature acclimation differed clearly. They also varied depending on the temperature level. Acclimation to higher temperatures comprised an upregulation of metabolic genes and, in case of 24 °C acclimation, a downregulation of genes for translational processes and collagens. The proteomic responses of the clones M and G differed at any type of stress. Clone M showed markedly stronger and less stress-specific proteomic responses than clone G, which included the consistent expression of a specific heat shock protein (HSP60) and vitellogenin (VTG-SOD).ConclusionsThe expression changes under acute stress can be interpreted as a switch from standard products of gene expression to stress-specific products. The expression changes under temperature acclimation probably served for an increase in energy intake (via digestion) and, if necessary, a decrease in energy expenditures (e.g, for translational processes). The stronger and less stress-specific proteomic responses of clone M indicate a lower degree of cell damage and an active preservation of the energy balance, which allowed adequate proteomic responses under stress, including the initiation of resting egg production (VTG-SOD expression) as an emergency reaction.

Highlights

  • Regulatory adjustments to acute and chronic temperature changes are highly important for aquatic ectotherms because temperature affects their metabolic rate as well as the already low oxygen concentration in water, which can upset their energy balance

  • From the 24,679 genes represented on the microarray, between 85 and 543 differentially expressed genes (DEGs) (48-384 Eukaryotic orthologous groups (KOG)-identified DEGs, KOGs) were upregulated, and 13-1038 DEGs (7645 KOGs) were downregulated upon any type of temperature change (Fig. 1)

  • Log2-fold changes greater than 2 or lesser than − 2 were found for 4-58 DEGs (233 KOGs) or 5-37 DEGs (4-27 KOGs)

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Summary

Introduction

Regulatory adjustments to acute and chronic temperature changes are highly important for aquatic ectotherms because temperature affects their metabolic rate as well as the already low oxygen concentration in water, which can upset their energy balance. This applies to severe changes in food supply. The microcrustacean Daphnia pulex, which inhabits thermally volatile ponds or small lakes, experiences severe acute and chronic temperature changes during diurnal vertical migration (minimal 10 °C) [1] and during the seasons (up to 20 °C) [2] They face large fluctuations in nutrient (phytoplankton) availability [3]. One of its key functions is the provision of enough time for the later stressor-specific or even organism-specific CHR, which serves to recover cellular homeostasis

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