Transcriptomic analysis dissects the mechanistic insight into the Daphnia clonal variation in tolerance to toxic Microcystis

Cyanobacteria have become more prevalent than other phytoplankton in freshwater assemblages during summer. In such conditions, cyanobacterial traits may reduce zooplankton fitness and the energy flow efficiency from primary producers to aquatic herbivores. Cladocerans, as the dominant zooplankton grazers in freshwater ecosystems, exhibit clonal variation in their tolerance to cyanobacteria with an increasing gradient in eutrophication history. Hitherto, research on the full modes of action (MoAs) of Daphnia clonal differences in tolerance to toxic Microcystis still remains in its infancy. We conducted fitness and transcriptome analyses on two Daphnia clones, clone TH09 and TH14. A significant decline in body growth rate was detected in the sensitive clone TH09 compared with the tolerant clone TH14 at the presence of toxic Microcystis. Furthermore, transcriptional analysis indicated that major MoAs such as glutathione metabolism, protein processing in endoplasmic reticulum, amino sugar/nucleotide sugar metabolism, and arachidonic acid metabolism were linked to the tolerance fitness in Daphnia similoides. These results provided mechanistic insights into the pathways of genetic and biological processes involved in cyanobacteria tolerance in the Daphnia clonal variation, and propose that the genetic architecture of this fitness-related trait would be helpful to clarify how zooplankton clones adapt to harmful algal blooms.