Molecular effects of the ubiquitous diatom Skeletonema marinoi on the copepods Calanus helgolandicus

Asai, Sneha P (2015). Molecular effects of the ubiquitous diatom Skeletonema marinoi on the copepods Calanus helgolandicus. PhD thesis The Open University.



Diatoms are the dominant photo synthetic organisms in the world’s oceans. Upon damage, several species produce secondary metabolites such as polyunsaturated aldehydes (PUAs) and other oxylipins deriving from the oxidative metabolism of fatty acids. These cytotoxic compounds are responsible for reproductive failure and teratogenesis in crustacean copepods, potentially sabotaging future generations of grazers by inducing poor recruitment during diatom blooms.

The aim of the present PhD thesis was to understand the molecular effects of the well-known oxylipin-producing diatom Skeletonema marinoi on the widespread copepod Calanus helgolandicus using transcriptome sequencing (RNA-Seq). Despite its ecological and physiological relevance, the molecular targets of diatoms and their metabolites in copepods are still poorly understood. A RNA extraction method for obtaining high quality and quantity of RNA from C. helgolandicus was successfully optimized. Using Illumina sequencing, a total of 606 million cleaned short reads were obtained, which were assembled into 30,339 high quality unigenes. Based on their sequence similarity to known proteins, about 13,000 genes were identified, providing a global view of C. helgolandicus transcriptome and enriching the genomic resources of this non-model organism, a dominant copepod species in European waters. Differential expression analysis revealed several metabolic, reproductive and developmental processes altered in S. marinoi-fed C. helgolandicus females. In particular, genes significantly affected played roles in carbohydrate and glycerolipid metabolism, stress response (protein and DNA damage repair and free radical detoxification), immune response, one carbon metabolism by folate and methionine metabolism, oogenesis and developmental signal transduction. Accuracy of Illumina sequencing data and expression profiles of several related genes of interest (GOIs), was confirmed by RT-qPCR. Additionally, time-dependent and dose-dependent changes in expression levels of GOIs, observed in S. marinoi feeding and PUAs incubation experiments, respectively, supported the hypothesis that the altered C. helgolandicus gene expression is related to diatom consumption and presence of oxylipins. Taken together, the results of the present RNA-Seq study revealed an unexpected complexity in the molecular response of the copepod to diatom oxylipins. These results provide insights into the mechanism of reproductive failure and teratogenesis induced by diatoms in copepods at the transcriptome level.

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