Prey adopt similar adaptive strategies with different molecular response mechanisms: How do ciliates respond to different predation risk cues?

The adaptive strategies of prey against predation risk have been pivotal in non-consumptive effects. However, the adaptive strategies of prey and the response mechanisms to various predation risk cues remain unclear. We hypothesized that prey adopt a similar adaptive strategy to address different predation risks, but their response mechanisms depend on the attributes of predation risk cues. To test this hypothesis, we used Didinium-Paramecium as a predator-prey model to first evaluate the effects of various predation risk cues, including homogenates of predators (TD) and prey (TP) alone or in combination (TM), and the density of conspecific prey on the specific growth rate (SGR) to determine their adaptive strategies and their underlying response mechanisms based on transcriptomic analysis. Our results show that predation risk cues considerably enhance prey SGR, with effects in descending order of TM, TD, and TP. However, such effects gradually decrease as the density of conspecific prey increases until they disappear. The trend in the levels of differentially expressed genes (DEGs) in prey exposed to different predation risk cues is similar to that of SGRs. In particular, pathways based on DEG enrichment mediated by predation risk cues involve cellular processes, nutrient metabolism, and synthesis of biological macromolecules. However, they differ in type, number, and location within specific biological processes. In conclusion, Paramecium uses early reproduction strategies to address the risk of predation caused by different cues, and differences in its response mechanisms depend on the attributes of the cues of predation risk, resulting in differences in the SGR of the prey.