Browsing by Author "Ye, Siao"
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Item Stress tolerance of the endosymbiotic cnidarian Hydra viridissima: Contributions of the host and the symbiont(2020-05-07) Ye, Siao; Siemann, EvanEndosymbiosis is a universal relationship in nature, in which the symbiont is tightly integrated into the host and alters the host’s ecological niche and evolutionary trajectory. However, endosymbiosis can be easily disrupted by stress, which raises concerns about holobiont (symbiont plus host) persistence with global climate change, because massive endosymbiosis breakdown could have severe ecological consequences. The hologenome theory proposes that the symbiont and the host jointly determine the holobiont phenotype, and so rapid holobiont phenotype alteration can be achieved through changes in symbiont composition via strain acquisition, adaptation or acclimation. Previous studies have confirmed that switching symbionts can change holobionts’ stress tolerance, yet the relative roles of the symbiont and the host in controlling the holobiont phenotype are unclear as is whether symbiont adaptation or acclimation alone could allow holobionts to persist in stressful environments. My thesis addressed factors affecting stress tolerance of the holobiont green hydra (Hydra viridissima) from both the symbiont (green algae) and the host (hydra) perspectives, and experimentally explored how manipulations of the symbiont or the host alone could alter holobiont stress tolerance. Furthermore, I used agent-based modeling to simulate the holobiont adaptation via symbiont adaptation. My experimental results showed unequal contributions of the host and the symbiont to holobiont stress tolerance, with the host playing a dominant role. Furthermore, I detected positive transgenerational effects in the host. Although host traits are usually overlooked in holobiont studies, my studies suggest they are equally or even more important in determining holobiont stress tolerance. On the other hand, symbiont adaptation or acclimation provides the holobiont an extra route to acquire stress tolerance if the host fails to acquire it directly, and such acquired stress tolerance can be inherited. In addition, my experiments proved the feasibility of altering holobiont phenotypes by symbiont manipulations in vivo. Simulations further demonstrated that holobionts with either vertical or horizontal symbiont transmission are able to evolve solely through adaption of the symbiont. These findings support the hologenome theory and have implications for forestry, agriculture and conservation by providing insights into the determination and management of holobiont stress tolerance.Item The Roles of Endosymbionts and Hosts in Adaptive Response to Stress(Rice University, 2017) Ye, SiaoEndosymbiosis is such an intimate interaction that impacts on either partner may disrupt the other, which makes endosymbiotic species quite sensitive to environmental changes, such as elevated temperature, decreasing pH, etc. In some circumstances, the changes are so rapid that the whole ecosystem is under threaten as the endosymbiosis is disrupted, for example, the coral reefs experience bleaching when the sea temperature exceeds certain threshold. The hologenome theory proposed by E. Rosenberg and I. Zilber Rosernberg suggests that endosymbionts can adapt to such changes and confer their resistance to hosts, as they usually have shorter generation time and larger population size compared with hosts, and thus the holobiont, which is the host plus the endosymbiont, is the unit under selection. My dissertation research uses green hydra Hydra viridissima as the model system, in which green algae provide carbonate to hydra while the hydra provide CO2 and proteins in returns. The hydra can be bleached and survive without algae, and can re-associate with algae later, allowing manipulations on both the host and the endosymbiont. To date I have compared survival rate of symbiotic hydra and aposymbiotic hydra (i.e bleached hydra that algae have been deprived) under heat stress, and tried to select for UV-B tolerant algae. The result shows high variation exist across both symbiotic and aposymbiotic hydra populations, and there is correlation between the thermal tolerance of these two groups. In addition, the aposymbiotic hydra is more tolerant than their symbiotic ancestral strains. This provides a direct evidence that holobiont stress tolerance is correlated to host stress tolerance, which is usually overlooked in the past. For the artificial selection experiment, the selected algae didn’t show improved tolerance under UV-B. However, different mutated algal strains exhibit various UV-B tolerance, indicating there is possibility that a stress tolerant mutation can be induced. The next step would be testing UV-B resistance of hydra receiving these selected algae to see if hologbiont stress tolerant is related to algal stress tolerance. I will also compare acclimation/deacclimation rate of symbiotic/aposymbiotic hydra, as well as adaptation rate of algae within/without host in the future.