Schwarzenberger Anke, Sadler Thomas, Motameny Susanne, Ben-Khalifa Kamel, Frommolt Peter, Altmüller Janine, Konrad Kathryn, von Elert Eric
University of Cologne, Cologne Biocenter, Aquatic Chemical Ecology, Zuelpicher Str, 47b, 50674 Cologne, Germany.
BMC Genomics. 2014 Sep 9;15(1):776. doi: 10.1186/1471-2164-15-776.
Cyanobacteria 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.
Daphnia 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.
Here, 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.
蓝藻通过产生有毒的次生代谢产物（如微囊藻毒素）对淡水生态系统构成严重威胁。这些微囊藻毒素已被证明会危害家畜、宠物和人类，并影响生态系统服务和功能。由于富营养化和全球变暖，蓝藻水华在全球范围内的强度和频率都在增加。然而，水蚤作为浮游藻类和蓝藻的主要捕食者，已被证明能够抑制形成水华的蓝藻，并适应产生微囊藻毒素的蓝藻。由于水蚤的基因组直到最近才公布，现在有可能阐明水蚤对微囊藻毒素耐受性的潜在分子机制。
用产生微囊藻毒素的蓝藻菌株或其基因工程微囊藻毒素敲除突变体喂养大型溞。因此，可以区分由于摄入蓝藻引起的影响和由微囊藻毒素特异性引起的影响。通过使用RNA测序分析不同处理之间差异表达的基因，并计算受影响的KOG类别。我们在此表明，水蚤中转运蛋白基因的表达作为对微囊藻毒素的特异性反应受到调节。随后的qPCR和用纯微囊藻毒素进行饮食补充证实，转运蛋白基因表达的调节与几个水蚤克隆的耐受性相关。
在这里，我们通过将蓝藻影响与微囊藻毒素影响分开，能够鉴定出对微囊藻毒素有特异性反应的新候选基因。通过将转运蛋白基因表达的差异与克隆耐受性相关联，验证了这些候选基因在对微囊藻毒素耐受性中的作用。因此，防止微囊藻毒素的摄取很可能是水蚤耐受性和适应性发展的关键机制。有了明确的候选基因，现在就有可能对水蚤种群对微囊藻毒素的局部适应过程进行未来的研究。
