Results
General metabolomic responses
We used mass spectrometry-based metabolomics to explore the metabolomes of D. magna . We obtained 2152 metabolites for the positive ion mode and 1519 metabolites for the negative ion mode. A PCA showed that fish kairomones induced a clear separation of the metabolomes along PC2 (Figure 1a, b). While there was also a separation along PC1, this was not linked to the treatments or clonal differences (see details in Supplementary Information). A two-way ANOVA on the PC2 scores showed significant effects of the fish kairomone treatment and the fish kairomone × subpopulation interaction (all P < 0.001, Table S1). These significant effects were confirmed by the ASCA model results (all P < 0.0001, see details in Supplementary Information). Because of the significant fish kairomone × subpopulation effect, we conducted separate PLS-DA for each subpopulation, and found the fish kairomone treatment significantly changed the metabolomes in each subpopulation (allP < 0.001, Figure 1c-h, Table S2).
Using MetaboAnalyst we found that four metabolic pathways for the positive ion mode and five pathways for the negative ion mode were affected by fish kairomones, these were mostly linked to amino acid and sugar metabolism (Figure 2).