Transcriptome Analysis Reveals the Molecular Basis of Anti-Predation Defence in Daphnia Pulex Simultaneously Responding to Microcystis Aeruginosa

1. The influence of cyanobacteria on interspecies interactions has received growing attention as a predictor of aquatic ecosystem function. It is well-known that cyanobacteria greatly affect predator-induced phenotypic plasticity in planktonic animals. However, the underlying molecular mechanism remains unclear.2. Here, we used Daphnia pulex as a model organism for inducible defence, examining its morphological and life-history responses to fish kairomone while feeding on 100% Chlorella pyrenoides, or while feeding on 70% Chlorella and either microcystin-producing or -free Microcystis aeruginosa, separately. Transcriptome profiles of kairomone-exposed D. pulex fed different types of food were detected using RNA-seq. How the differentially expressed genes and pathways identified are responsible for the altered adaptive traits under Microcystis stress was discussed.3. Daphnia pulex fed 100% Chlorella matured at a smaller body size, elongated tail spine length, and produced smaller offspring as adaptive morphological and reproductive responses to fish kairomone. These anti-predation defences entailed a cost in decreased somatic growth rate. Microcystis stress caused defence trade-offs in both morphology and reproduction: reduction of body size at maturity was stronger whereas tail spine elongation was unchanged or inhibited. Given that reproductive investment was reduced overall in Daphnia exposed to fish kairomones, there was a trade-off for unchanged offspring size by reducing offspring number in Daphnia fed Microcystis. Defence-induced costs to growth were increased by Microcystis exposure.4. Transcriptome analysis revealed that neuronal signals including acetylcholine and glutamate signalling implicated in kairomone reception and/or transmission exhibited stronger responses under Microcystis stress. Cuticle development associated with the biosynthesis of wax esters and chitin, modification of lipo-chitin saccharides, and the stability of collagen triple helix apparently participated in the morphological changes. The arachidonic acid pathway and biosynthesis of cholesterol and steroid hormones mediated altered reproductive performance. In addition, metabolic processes such as detoxification, food assimilation, lysosome dysfunction and apoptosis activation were involved in the survival and growth of Daphnia, which may contribute to the increased defence-induced cost in growth when fed Microcystis.5. Taken together, these findings advance our understanding of the molecular mechanisms underlying anti-predation defensive responses by Daphnia to cyanobacterial stress. This work also provides a reference for further exploring the functional genes that mediate zooplankton-fish coevolution dynamics under eutrophic conditions.