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The ability of corals to form novel partnerships with symbionts that may be better suited to new environmental conditions is an important factor when assessing the ability for corals to adapt to climate change. However, relatively little attention has been given to the effects of hosting different symbiont types on holobiont physiology, competitive interactions between these symbionts, or the capacity of the host to regulate populations of different symbionts. Such factors likely play an important role in patterns of host-symbiont specificity and flexibility, and hence the potential for corals to respond to climate change. The aim of this research was to characterise the cellular and physiological events associated with hosting different symbiont species in Exaiptasia pallida (commonly referred to as ‘Aiptasia’), a model cnidarian-dinoflagellate symbiosis, and how these events might contribute to host-symbiont specificity. The specific objectives were to: (1) determine the effect of symbiont species on the population dynamics of host colonisation and holobiont physiology; (2) measure the competitiveness of the homologous symbiont versus heterologous symbionts, under both control and elevated temperatures; and quantify the ability of the host to regulate its symbiont population in response to homologous versus heterologous symbiont taxa (3) via host-cell apoptosis and (4) via symbiont cell cycle regulation.  To explore this, aposymbiotic (i.e. symbiont-free) individuals of Aiptasia were first inoculated with one of five Symbiodinium taxa (the homologous S. minutum or heterologous S. microadriaticum, phylotype C3, S. trenchii or S. voratum), and the rates and patterns of colonisation assessed. Proliferation success inside the anemone was different between symbionts, with the homologous S. minutum being the most successful species, while Symbiodinium C3 and S. voratum struggled or failed to form a long-lasting symbiosis. The spatial pattern of symbiont colonisation was identical for all the successful Symbiodinium taxa, however the timing differed between these different symbionts. Symbiont identity also had an effect on holobiont fitness, as S. microadriaticum and S. trenchii were less beneficial to the host compared to S. minutum, as indicated by lower rates of photosynthesis, anemone growth and pedal laceration (i.e. asexual reproduction). The taxon-specific differences demonstrated here provided a basis for the subsequent thesis chapters, leading to questions about how the different symbionts might compete with one another and be regulated by the host.  The competitiveness of the homologous symbiont relative to heterologous ones, and hence the ability of the host to ‘switch’ and ‘shuffle’ its symbiont population, was tested by inoculating aposymbiotic sea anemones either with simultaneous or sequential mixtures of thermally tolerant and sensitive Symbiodinium and exposing them to control versus elevated temperatures. The homologous species was dominant regardless of temperature, outcompeting the heterologous, thermally tolerant S. microadriaticum and S. trenchii. This result indicates that the high level of specificity seen between Aiptasia and S. minutum in the Pacific Ocean may result, in part, from a reluctance to form new symbioses, even if such associations have the potential to confer a degree of thermal tolerance that may be beneficial under future climate change.  The differential success of the different symbionts was also reflected in the host’s apoptotic response to their presence in its tissues, as measured via caspase-3 activity. In particular, anemones hosting S. minutum and S. microadriaticum exhibited lower levels of caspase activity than those hosting S. trenchii and S. voratum throughout symbiosis establishment, consistent with symbiont proliferation success. The general pattern of caspase activity during the 28-days colonisation period was similar, however, with induction of caspase-3 activity upon inoculation, followed by a marked decline in activity over the subsequent week, and then an increase (either moderate or marked depending upon symbiont identity) across the remainder of the colonisation period measured. Host cell apoptosis therefore likely plays an important role in determining the compatibility and fate of different Symbiodinium taxa in a host, and the potential for establishing novel symbioses.  In contrast to the apparent importance of host apoptosis, symbiont cell cycle control did not seem to play an important role in determining the different rates of symbiont colonisation observed. Flow cytometry was used to determine the relative proportion of cells in the different phases of the cell cycle (i.e. G1, G2, S, M), with all symbiont taxa exhibiting the same pattern of cell cycle progression. In particular, more cells were in the S and G2/M phases combined than in G1 during the first two weeks of colonisation, but this changed as colonisation progressed, when a greater proportion cells were in the G1 phase. This indicates that symbiont cell division becomes limited in the later stages of colonisation as symbiont density increases, consistent with increasing resource limitation.  This thesis provides valuable insights into the regulation of the cnidarian-dinoflagellate symbiosis, and the events that contribute to host-symbiont specificity. In particular, it suggests that through cellular control and physiological impacts on the host (and hence the overall symbiosis), there is likely limited potential to establish new host-symbiont partnerships that allow for adaptation to our warming climate. The next step is now to further elucidate the relative importance of post-phagocytosis control mechanisms, and to test the generality of my findings by extending them from the model Aiptasia system to reef corals.

The Cellular and Physiological Basis of Host-Symbiont Specificity in a Model Cnidarian-Dinoflagellate Symbiosis