Subpopulation-specific metabolomes under predation risk
The metabolomic responses to fish kairomones differed strongly among subpopulations. Only a small part of the responsive peaks was shared by all three subpopulations: 15.9 % (176 of 1105, Figure 3a) for the positive ion mode, and 23.5 % (136 of 580, Figure 3b) for the negative ion mode. The total number of differentially regulated peaks to fish kairomones differed among the subpopulations (positive ion mode:\(\ x_{2}^{2}\) = 278.39, P < 0.001; negative ion mode: \(x_{2}^{2}\) = 79.40, P < 0.001, Table S3). The high-fish subpopulation had the highest number of responsive peaks (Figure 3, Table S3), indicating that the metabolome of the high-fish subpopulation changed most strongly in response to fish kairomones.
Projecting the phenotypic trajectories onto the metabolomic PCA landscape showed that for both ion modes the magnitude of the multivariate metabolomic reaction norm was greater for the high-fish subpopulation than the pre-fish (P < 0.001 for both ion modes) and the reduced-fish (P = 0.026 for the positive, P< 0.001 for the negative ion mode) subpopulations, while the latter two did not differ in magnitude (Figure 4a-d, Table S4). The direction of the multivariate plasticity differed considerably between the pre-fish subpopulation and the two other subpopulations (bothP < 0.001). In contrast, the high-fish and reduced-fish subpopulations did not differ in the direction of multivariate plasticity for the positive ion mode (P = 0.086), and only differed slightly for the negative ion mode (P = 0.047).