Prey are under selection to minimize predation losses. In aquatic environments, many prey use chemical cues released by predators, which initiate predator avoidance. A prominent example of behavioral predator-avoidance constitutes diel vertical migration (DVM) in the freshwater microcrustacean Daphnia spp., which is induced by chemical cues (kairomones) released by planktivorous fish. In a bioassay-guided approach using liquid chromatography and mass spectrometry, we identified the kairomone from fish incubation water as 5α-cyprinol sulfate inducing DVM in Daphnia at picomolar concentrations. The role of 5α-cyprinol sulfate in lipid digestion in fish explains why from an evolutionary perspective fish has not stopped releasing 5α-cyprinol sulfate despite the disadvantages for the releaser. The identification of the DVM-inducing kairomone enables investigating its spatial and temporal distribution and the underlying molecular mechanism of its perception. Furthermore, it allows to test if fish-mediated inducible defenses in other aquatic invertebrates are triggered by the same compound.

A huge variety of organisms respond to the presence of predators with inducible defences, each of which is associated with costs. Many genotypes have the potential to respond with more than one defence, and it has been argued that it would be maladaptive to exhibit all possible responses at the same time. Here, we test how a well-known anti-fish defence in Daphnia, life-history changes (LHC), is controlled by light. We show that the kairomone-mediated reduction in size at first reproduction is inversely coupled to the light intensity. A similar effect was found for the kairomonemediated expression of candidate genes in Daphnia. We argue that the light intensity an individual is exposed to determines the degree of LHC, which allows for plastic adjustment to fluctuating environments and simultaneously minimizes the associated costs of multiple alternately deployable defences. It is hypothesized that this allows for a coupling of multiple defences, i.e. LHC and diel vertical migration.

Daphnia, an important model organism for studies on ecology and evolution, has become a textbook example for inducible defenses against predators. Inducible defenses are widespread in nature, and the underlying molecular mechanisms for this plasticity in general and in particular in Daphnia are not fully understood. Here, we provide for the first time a combination of established life-history changes (LHC), which are induced by chemical cues of a predator (fish kairomones), in Daphnia with differential peptide labeling (iTRAQ) in LC-MS/MS based proteomics. The aim of the present study is the elucidation of proteins involved in specific antipredator responses in a predator-prey system of ecological relevance by high-throughput proteomics. To obtain a highly specific antifish response of Daphnia, highly purified fish kairomones were applied in the presence or absence of light. We were able to identify a set of functional proteins, which are likely to explain the kairomone-mediated and light-dependent LHC in Daphnia.

No abstract

No abstract