Intra-genomic variation in symbiotic dinoflagellates: Recent divergence or natural hybridization?

The perpetuity of coral reefs will ultimately depend on the ability of corals to adapt to changing conditions. Inter-specific hybridization can provide the raw genetic material necessary for adaptation, and stimulate macro-evolutionary leaps during periods of environmental upheaval. Though well-documented in corals, hybridization has yet to be identified in their dinoflagellate symbionts (genus Symbiodinium), despite growing evidence of sexual reproduction in this genus. The integral roles that these symbiotic algae play in coral productivity, reef accretion and ‘coral bleaching’ emphasize the need to better understand their short-term evolutionary potential. In this thesis, I develop new molecular and statistical methodology, and combine lab- and field-based analysis to explore the potential for hybridization between divergent Symbiodinium taxa. To screen for putative Symbiodinium hybrids, intra-genomic variation was examined within individual symbionts isolated from the reef-building coral Pocillopora damicornis at Lord Howe Island (Australia). A nested quantitative PCR (qPCR) assay was developed to quantify polymorphic internal transcribed spacer 2 (ITS2) sequences within the genome of each symbiont cell. Three genetically distinct Symbiodinium populations were detected co-existing within the symbiont consortium of P. damicornis. Mixed populations of ‘pure’ Symbiodinium types C100 and C109 coexisted with a population of cells hosting co-dominant C100 and C109 ITS2 repeats. Genetically heterogeneous Symbiodinium cells were more common than homogeneous symbionts in four of the six colonies analysed, with a maximum proportional abundance of 89%. Morphological, functional and ecological attributes of heterogeneous Symbiodinium cells were characterized to assess their candidacy as putative hybrids. The proportional abundance of genetically heterogeneous symbionts was spatially and temporally conserved within colonies, indicating a lack of competition between Symbiodinium populations. However, this abundance ratio varied considerably between colonies separated by metres to tens of metres, and to a greater extent between sites isolated by hundreds to thousands of metres. The local thermal maximum emerged as a significant predictor of the proportional abundance of genetically heterogeneous Symbiodinium cells, suggesting that the distribution of these ‘putative hybrids’ is influenced by a reduced affinity for thermal stress. Genetically heterogeneous Symbiodinium cells were around 50% larger (by volume) than homogeneous cells, occupied tissue of the coral host at reduced densities, and showed relatively poor light-harvesting efficiency. Colonies hosting a higher proportion of these symbionts suffered a reduction in overall photosynthetic performance (maximum gross photosynthesis normalised to respiration; P:R) at the ambient temperature of 25 °C. This disparity was maintained when the temperature was elevated to simulate the maximum experienced within the LHI lagoon (29 °C). Under these stressful conditions, colonies dominated by putative Symbiodinium hybrids were only marginally capable of net oxygen production. The influence of putative Symbiodinium hybrids on the growth and survival of P. damicornis was tested by reciprocally transplanting coral colonies between reef sites featuring distinct temperature regimes. Neither calcification nor mortality was influenced by the proportional abundance of genetically heterogeneous cells in the symbiont consortium. This uncoupling of symbiont performance and host fitness may be explained by stochastic events such as predation and disease, which substantially increase variation in growth and mortality in field experiments. Alternatively, it may represent some unknown benefit associated with hosting hybrid symbionts, belying their relatively poor photosynthetic performance, and explaining the widespread abundance of these heterogeneous Symbiodinium cells on the Lord Howe Island reef. Our inability to maintain many clade C Symbiodinium types in culture prevents direct observations of hybridization between C100 and C109. Unequivocal evidence of this phenomenon will therefore likely remain elusive until high-resolution, single-copy nuclear markers can be developed, since the incomplete displacement of ancestral polymorphisms can leave a similar genomic signature to that of hybridization. However, this study serves to provide an initial proof-of-principle for hybridization between divergent Symbiodinium taxa. In doing so, it highlights the need to better understand the evolutionary processes underpinning coral- and symbiont-adaptation in a changing climate.