Normal operating range (NOR) in Enchytraeus albidus - Transcriptional responses to control conditions

Abstract

Gene expression studies provided a major step forward in ecotoxicology and ecological functional genomics, among other areas. How to classify changes in gene expression to be significant compared to its normal operating range (NOR) requires a number of biological and statistical criteria. Doubts have been casted about when a change in gene expression has ecologically relevant meaning and what is the normal biological variation or NOR. Because genes are highly sensitive and self-regulating pieces, how can we design our experiments to control normal variation? The main aim of the present paper is to discuss such questions, while providing suggestions for improvement and interpretation. We have analysed several gene control data sets of Enchytraeus albidus comparing standard control test conditions, i.e. (1) from experiments performed at different occasions in time, (2) to culture media, (3) to acetone control, (4) from exposure time series (2, 4 and 8 days) and (5) between replicates. We discriminated between some normal variation sources. For instance, the transference of organisms from cultures to standard testing condition suggests a lowered general metabolism. Chemical carriers like acetone seem to cause changes related with soil structure and hypoxia conditions. Organisms have a “time-like” transcription signature when sampled along a time series. The coefficient of variance (CV) between biological replicates was 0.1 < CV < 1.3 for 17% of the genes. Variability will partly be inherent to the species, but most important is to understand the ground for interpretations as discussed. Here, the first attempt for the establishment of a NOR for E. albidus gene expression was made which represents a naturally varying ecological window – this contributes for the discrimination between stress responses and natural ecological variations in soil studies.