Control of toxigenic dinoflagellates through parasitism : Implications for host-parasite coevolution

Abstract

Dinoflagellates are among the most important primary producers in the ocean and represent highly diverse life forms. This group practices a wide variety of alternative nutritional modes; there are phototrophic and heterotrophic, as well as mixotrophic, free-living forms, and the obligated symbiotic and parasitic members. Two major lineages of dinoflagellates have been defined - the core and basal groups, the latter including the obligated parasitic syndinean dinoflagellates. Several dinoflagellates belonging to the core dinoflagellates produce dangerous toxins. Among these, Alexandrium fundyense is one of the most prominent harmful algal bloom-forming genera with its negative impact on the ecosystem, aquaculture seafood and causing serious hazard to human health. Increasing evidence shows that Alexandrium populations can be affected by parasitic attack by the basal syndinean dinoflagellate Amoebophrya. However, regulatory mechanisms of the infection of core dinoflagellates by their parasites are largely unknown. The aim of the thesis was to provide insights into the infection dynamics among Alexandrium and its parasite Amoebophrya, and to better understand the infection processes with implications for host-parasite coevolution. To investigate the susceptibility of the dinoflagellate to infection by the parasite on an intra-specific level, different populations of the host Alexandrium from very distant geographical origins (Alaska, the Gulf of Maine and the North Sea) were provided to the parasite Amoebophrya. There was a strong negative effect of parasitism on the development of host populations, but no apparent adaptation of the host Alexandrium was observed. Cellular toxin contents were examined, showing that neither toxin concentration nor composition changed within each geographical population. Therefore, the results indicated that the host Alexandrium likely does not use toxins as a potential defense strategy against the parasite. In this thesis, a whole genome sequencing of the parasite Amoebophrya was performed for the first time and a transcriptomic dataset from the infection cycle of this parasite-host system was generated. The basal dinoflagellate Amoebophrya has a relatively small genome in size around 90 Mbp. Besides the reduction of genome size, several parasitic features were observed in the genome including loss of duplicated genes and function loss (e.g. inability to generate certain amino acids) that indicates the parasite dependent on the host. Notably, the genome also exhibits novel features. The shikimate and tryptophan synthesis pathways are physically linked that may constitute an unknown mechanism of pathway regulation. Mitochondria are observed, but the mitochondrial genome is completely lost in Amoebophrya. The established cDNA library (>900,000 reads/313 Mbp) consists of 14,455 ESTs. Differentially expressed genes point to general mechanisms in host-parasite recognition and infection. Particular surface lectins are expressed in the parasite Amoebophrya at early infection processes, and these lectins likely mediate the attachment to the host cell, followed by processes involved in host recognition, adhesion, and invasion. During maturation, cell division and proliferation related genes reflect fast cell growth of the parasite. These findings indicate the presence of fundamental processes that have remained stable throughout evolution. By contrast, the host Alexandrium reacts differently towards parasite infection and respective parasitic waterborne cues, but both treatments exhibited significant changes in gene expression associated with specific metabolic pathways. A total of 14,882 Alexandrium genes were differentially expressed over the whole-parasite infection cycle at three different time points (0, 6 and 96 h). The results from RNA sequencing analyses indicate that parasite infection increases the energy demand of the host, as a large amount of genes involved in photosynthesis, ATP synthesis through glycolysis and fatty acid production were upregulated. The stimulation of signal transduction chains by waterborne cues from the parasite alone could prime the host s defense or induce host s adaptive responses to the parasite activity.