Deciphering the genetic basis of microcystin tolerance.

Anke Schwarzenberger,Susanne Motameny,Kathryn Konrad,Thomas Sadler,Eric Von Elert,Kamel Ben-Khalifa,Peter Frommolt,Janine Altmüller

doi:10.1186/1471-2164-15-776

Abstract

BackgroundCyanobacteria constitute a serious threat to freshwater ecosystems by producing toxic secondary metabolites, e.g. microcystins. These microcystins have been shown to harm livestock, pets and humans and to affect ecosystem service and functioning. Cyanobacterial blooms are increasing worldwide in intensity and frequency due to eutrophication and global warming. However, Daphnia, the main grazer of planktonic algae and cyanobacteria, has been shown to be able to suppress bloom-forming cyanobacteria and to adapt to cyanobacteria that produce microcystins. Since Daphnia’s genome was published only recently, it is now possible to elucidate the underlying molecular mechanisms of microcystin tolerance of Daphnia.ResultsDaphnia magna was fed with either a cyanobacterial strain that produces microcystins or its genetically engineered microcystin knock-out mutant. Thus, it was possible to distinguish between effects due to the ingestion of cyanobacteria and effects caused specifically by microcystins. By using RNAseq the differentially expressed genes between the different treatments were analyzed and affected KOG-categories were calculated. Here we show that the expression of transporter genes in Daphnia was regulated as a specific response to microcystins. Subsequent qPCR and dietary supplementation with pure microcystin confirmed that the regulation of transporter gene expression was correlated with the tolerance of several Daphnia clones.ConclusionsHere, we were able to identify new candidate genes that specifically respond to microcystins by separating cyanobacterial effects from microcystin effects. The involvement of these candidate genes in tolerance to microcystins was validated by correlating the difference in transporter gene expression with clonal tolerance. Thus, the prevention of microcystin uptake most probably constitutes a key mechanism in the development of tolerance and adaptation of Daphnia. With the availability of clear candidate genes, future investigations examining the process of local adaptation of Daphnia populations to microcystins are now possible.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-776) contains supplementary material, which is available to authorized users.

Highlights

Cyanobacteria constitute a serious threat to freshwater ecosystems by producing toxic secondary metabolites, e.g. microcystins
For the transcriptome analyses we chose a clone of Daphnia magna that has been shown to be relatively tolerant to dietary microcystins and was assumed to have distinct mechanisms for coping with dietary microcystins
A significant reduction in somatic growth rate was observed in comparison to high quality food, while no differences in D. magna growth between microcystincontaining and microcystin-free cyanobacterial food was found (Tukey HSD after one-way ANOVA, p = 0.84). This indicates that D. magna was still able to cope with such a low concentration of microcystins without additional detectable costs besides costs associated with feeding on cyanobacteria in comparison to control food

Summary

Introduction

Cyanobacteria constitute a serious threat to freshwater ecosystems by producing toxic secondary metabolites, e.g. microcystins. The generality of this negative correlation between cyanobacterial and Daphnia biomass has recently been questioned in an experiment [19] and in several field studies [20,21], demonstrating that Daphnia have the potential to adapt to increasingly tolerate dietary cyanobacteria Cyanobacteria and their toxins are becoming more and more of an ecological threat due to global warming and eutrophication [22], and new solutions for the management of freshwater ecosystems are needed. Toxic cyanobacterial secondary metabolites that frequently occur in cyanobacterial blooms are the well-studied microcystins and serine protease inhibitors [23,24] Both toxin types have been shown to negatively affect Daphnia [13,14]

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