School of BioSciences - Theses
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ItemInterspecific hybridization as a tool for enhancing climate resilience of reef-building corals( 2018)The world’s coral reefs are facing unprecedented changes in temperature and carbonate chemistry caused by the increasing concentration of atmospheric CO2. Recent massive loss of corals across the world suggests that their rate of adaptation and/or acclimatization is unlikely fast enough to keep pace with climate change. This thesis examines interspecific hybridization as a conservation management tool to develop coral stock with enhanced climate resilience and adaptive potential. I start this thesis by discussing the potential benefits and risks of hybridization, and exploring the legal framework associated with hybrids and hybridization (chapter 1). Next, I present the results of interspecific fertilization trials, as well as stress experiments on coral larvae (chapter 2) and recruits (chapter 3) conducted to compare fitness of purebred and hybrid offspring. To understand mechanisms that may have contributed to the observed holobiont fitness differences, bacterial and algal endosymbiont communities associated with these corals were examined using 16S rRNA gene and ITS2 metabarcoding (chapter 4), and coral host gene expression patterns were assessed using RNA sequencing (chapter 5). The following findings and key conclusions have emerged from this thesis. Firstly, all four tested pairs of Acropora species from the Great Barrier Reef were cross-fertile, but the degree of prezygotic barriers varied (chapters 2, 3). In both years in which hybridization was attempted (2015, 2016), the majority of the target species pairs had no or limited temporal isolation (i.e., similar spawning dates and times). The only clear temporal isolation was between the ‘early spawner’ A. tenuis and the ‘late spawner’, A. loripes, although their gametes were still compatible. Gametic incompatibility varied between species pairs and the year of hybridization tests (which involved the same coral species collected from different locations). Levels of cross fertility ranged from no prezygotic barriers in both directions (chapter 3), to successful fertilization in one direction only, and in once case, unsuccessful fertilization in both directions (chapter 2). The observed variations in gametic incompatibility may be a consequence of differences in gamete-gamete recognition molecules. Secondly, hybrid corals were generally as fit as or more fit than parental purebred species (chapters 2, 3). At the embryonic stage, hybrid embryos developed normally and at similar rates as purebred embryos (chapter 3). At the larval stage, survival and settlement of hybrid larvae under 10 days exposure to ambient and elevated temperatures were mostly similar to that of purebreds, but higher than purebreds in a small number of cases (chapter 2). Hybrid recruits also had similar algal endosymbiont uptake rates and photochemical efficiency as that of purebred recruits (chapter 3). Under seven months exposure to ambient and elevated temperature and pCO2 conditions, however, some hybrids showed higher survival and grew larger than parental purebred species under both conditions (chapter 3). Overall, maternal effects were observed in hybrids of the A. tenuis x A. loripes cross (i.e., hybrids had similar fitness to the maternal parent species), and over-dominance in hybrids of the A. sarmentosa x A. florida cross (i.e., hybrids had higher fitness than both parental species), with some variations between traits and treatment conditions. While fitness of these hybrids in the field and their reproductive potential are yet to be investigated, these findings provide proof-of concept that interspecific hybridization may enhance coral resilience and this approach may therefore increase the success of coral reef restoration programs. Thirdly, the observed holobiont fitness differences between offspring groups were likely due to host-related factors (chapter 5), but not the microbial communities associated with these corals (chapter 4). No differences in the bacterial and microalgal endosymbiont community composition were found between hybrid and purebred corals (chapter 4). Microbial communities of these seven months old recruits were highly diverse and lacked host specificity. Winnowing of the communities occurred over time, resulting in less diverse microbial communities that differed between the two species pair crosses by two years of age. Transcriptome-wide gene expression analysis for the A. tenuis x A. loripes cross showed clear maternal patterns (chapter 5), consistent with the observed fitness results. Hybrids had similar gene expression patterns to their material parents, and only up to 10 differentially expressed genes were observed between them. In contrast, hundreds of genes were found differentially expressed between purebred A. tenuis and A. loripes, as well as between hybrids that had different maternal parents. Due to insufficient material available for the A. sarmentosa x A. florida cross at the end of the seven months aquarium experiment, transcriptome analysis was not conducted for this cross. Findings from this thesis support the notion that interspecific hybridization may improve coral resilience and facilitate adaptation to climate change. Further, as genetic diversity within species is predicted to decline as a consequence of high mortality disturbances such as mass bleaching events, interspecific hybridization can be used to restore losses in genetic diversity. If future studies can demonstrate high fitness of hybrid corals in the field and in advanced generations, hybrid corals may serve as a stock for reef managers for reseeding degraded reefs and/or enhancing resilience of healthy reefs.