Tryptophan hydroxylase (TRH) loss of function mutations in Daphnia deregulated growth, energetic, serotoninergic and arachidonic acid metabolic signalling pathways.

Serotonin has a pivotal function regulating development, growth, reproduction and behavior in animals. In this paper, we studied the deregulatory effects of the deprivation of serotonin in Daphnia magna TRH CRISPR-Cas9 mutants. Bi-allelic in-del THR mutants and, to a lesser extent, mono-allelic ones grew less, reproduced later, and produced smaller clutches than wild type clones. Transcriptomic and functional gene analyses showed a down-regulation of growth/molting and energy metabolism signaling pathways in TRH mutants, while revealing marked differences between mono- and bi-allelic clones. Bi-allelic mutants, lacking serotonin, presented the serotonergic synapse and arachidonic acid metabolic pathways down-regulated while the tryptophan to kynurenine was upregulated, thus indicating a cross-talk between the serotonergic and arachidonic acid metabolic pathways. Finally, the effects on the insulin growth factor-mediated signaling pathway were marginal. These changes in functional and metabolic pathways are consistent with previously reported effects in D. magna exposed to pharmaceuticals that inhibited arachidonic metabolism or enhanced the levels of serotonin.