Transcriptional response to kairomone exposure in Daphnia magna: A candidate gene approach

Abstract

Background: Daphnia (Brachiopoda, Cladocera) is a well-studied model organism providing unparalleled opportunity to test epigenetic regulation of predator avoidance mechanisms in aquatic ecosystems. The discovery of regulatory functions for microRNA molecules and recently described miRNA profiles of Daphnia make it an ideal system to probe for posttranslational regulatory mechanisms mediated by kairomone released by predatory fish. However, despite a number of studies that focused on mRNA transcript level differences, no miRNA studies associated with kairomone exposure have been reported. Results: Exposing D. magna to fish kairomone from birth to the first reproduction was found to result in the differential expression of the four miRNAs tested: miR-7, miR-34, miR-317, and miR-375. Normalized transcript levels for each miRNA were found to vary across the exposure period with no clear conserved pattern of expression despite functional target analyses by GO, COG and KEGG indicating that predicted miRNA target genes are likely involved in related biological activities. Analysis of six mRNA transcripts (Hsp70, Hsp90, actin, AKT, GYS and IGFR), identified in previous studies as kairomone-mediated genes in Daphnia magna, were also carried out. Similar to that obtained for miRNAs, the mRNA transcript levels showed varying degrees of temporal regulation across the exposure time course with the two heat shock transcripts exhibiting elevated levels at early and late time points of kairomone exposure while the AKT, GYS, and IGFR transcripts had an general decrease in expression during the first 96 hours. Conclusions: Differential mRNA expression data supports the premise of an ecological trade-off between the cost of general biological processes and that of survival under long-term kairomone stress. Transcript levels for the four miRNAs tested were found to vary across developmental time with kairomone exposure which suggests that they may have a role in regulating morphological, behavioral or physiological responses by altering target gene expression. These studies lay the foundation for future work aimed at linking miRNAs and their target transcripts to changes in the signaling events that govern Daphnia response to kairomone specific stress.