My doctoral thesis: Emdosymbiotic Interaction Between Hydra and Green Algae, and its Evolutionary Significance(細胞内共生関係におけるヒドラと緑藻の相互作用とその進化的重要性

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Title

Emdosymbiotic Interaction Between Hydra and Green Algae, and its Evolutionary Significance
(細胞内共生関係におけるヒドラと緑藻の相互作用とその進化的重要性)

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Abstract

Endosymbiosis is defined as a phenomenon in which one organism lives inside the cells of another organism. Clearly, endosymbiosis has played a monumental role in the evolution of organisms, such as the generation of mitochondria and chloroplast. Therefore, it is quite important to understand how endosymbiotic interactions are established. Hydra is a freshwater cnidarian animal that has been used as a model organism for 300 years. In genus Hydra, two species (H. vulgaris and H. viridissima) show endosymbiosis with green algae. In culture collection of Hydra at the National Institute of Genetics, all of the six H. viridissima strains show endosymbiosis, and the endosymbiosis has been considered as a key characteristic of this species. In H. vulgaris, on the other hand, only two of twenty-five strains show endosymbiosis. These two Hydra species with different endosymbiotic status could be useful to understand evolutionary process of endosymbiotic interaction. However, the evolution of the endosymbiosis is not fully understood. A previous study suggested that the endosymbiosis in H. viridissima occurred in the ancestor of H. viridissima strains, but it remains obscure about evolution of the endosymbiosis in H. vulgaris. It is possible that the twenty-three aposymbiotic H. vulgaris strains are also able to harbor the algae, therefore H. vulgaris may have acquired the potential for harboring algae before or during radiation of H. vulgaris strains. In order to understand evolution of endosymbiosis in H. vulgaris, I examined the endosymbiotic potential of the H. vulgaris strains by artificially introducing the algae. As a result, twelve of the twenty-three non-symbiotic H. vulgaris strains were also able to harbor the algae. Moreover, my phylogeneticanalysis by sequencing these mitochondrial genomes of the twenty-five H. vulgaris strains showed that the strains with endosymbiotic potential were grouped into one of the three clusters. These results suggest that the endosymbiotic potential obtained once during radiation of the H. vulgaris strains, but most of H. vulgaris strains remains non-symbiotic. This implies that the endosymbiosis in H. vulgaris is not stable compared with that in H. viridissima. Therefore, I next examined whether the endosymbiotic interaction with algae is different between the two Hydra species or not. As typical cases for investigating the endosymbiotic interaction, I compared survival rates between symbiotic polyps and aposymbiotic polyps in which algae were removed. The result showed that symbiotic H. viridissima was more tolerant to starvation than aposymbiotic polyp, whereas symbiotic H. vulgaris was less tolerant than aposymbiotic polyp, which is in contrast to H. viridissima. To understand the interaction at molecular level, I compared gene expression profiless between symbiotic and aposymbiotic polyps by using RNA-seq method. The analysis showed that the differential gene expression pattern in H. viridissima was extensively different from that in H. vulgaris, even though the differential gene expression pattern in H. viridissima is similar to that in other endosymbiotic organisms such as Paramecium bursaria and Ciona varians. Considering oxidative stress response, H. viridissima seems to have the mechanisms to manage the oxidative stress, such as down-regulation of the respiratory chain genes and up-regulation of calcium ion binding genes, whereas these mechanisms were unlikely to exist in H. vulgaris. These results suggest that H. viridissima has already established stable endosymbiotic relationships with green algae, whereas the mechanisms of stable endosymbiosis in H. vulgaris still seem to be immature. Through the present studies, I found that although both hydra species (H. vulgaris and H. viridissima) show endosymbiosis with algae, the interaction with algae was substantially different between the two Hydra species. H. viridissima showed a mutualistic relationship with the algae, whereas H. vulgaris did not show. These results suggest that the endosymbiosis in H. vulgaris is still in the course of an evolutionary process toward stable endosymbiosis compared with H. viridissima. The endosymbioses of hydras with different stages of evolution will therefore provide deeper insight into the evolutionary process of endosymbiosis, from non-symbiotic to stable endosymbiosis.

#evolution of symbiotic relationship