Molecular mechanisms of tolerance of Daphnia to microcystins

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Man-made eutrophication of lakes has caused increased frequencies of cyanobacterial blooms, which go along with a reduction in abundance of Daphnia, the major herbivore of the phytoplankton. This reduced abundance of Daphnia is a major cause for the fact that food web manipulations frequently cause only short term improvements of water quality. The most frequently reported toxins of cyanobacterial blooms are microcystins, which are hepatotoxic to mammals and increase mortality in Daphnia. Although it has frequently been shown that Daphnia coexisting with cyanobacteria show increased tolerance to toxic cyanobacteria, the genetic basis of this tolerance is entirely unknown. Here the genetic basis of tolerance to microcystins in Daphnia will be investigated. A microcystin-producing strain and a mutant of that strain, which is deficient in microcystins, will be fed to a microcystin-tolerant and a microcystin-sensitive clone of Daphnia magna. Transcriptome analysis based on RNASeq will be used to identify differentially expressed genes that either differ constitutively in expression among the two Daphnia clones or are differentially expressed upon ingestion of microcystins. Bioinformatic analysis of the data and metabolic mapping will be used to identify key enzymes and genes that mediate tolerance to microcystins. Expression of selected differentially expressed genes will be validated in an independent experiment using qPCR. Further evidences for a mediation of microcystin tolerance by selected differentially expressed genes will be gained by investigating how the concentration of dietary microcystins affect gene expression. This will be achieved by feeding liposomes that are loaded with microcystins to Daphnia. Understanding the molecular mechanisms is indispensable for the understanding of the evolution of microcystin tolerance in Daphnia and is of potentially pivotal importance for the management of lakes with toxic cyanobacterial blooms.