RESUMEN
Background: Pocillopora acuta is a hermatypic coral with strong ecological importance. Anthropogenic disturbances and global warming are major threats that can induce coral bleaching, the disruption of the mutualistic symbiosis between the coral host and its endosymbiotic algae. Previous works have shown that somaclonal colonies display different levels of survival depending on the environmental conditions they previously faced. Epigenetic mechanisms are good candidates to explain this phenomenon. However, almost no work had been published on the P. acuta epigenome, especially on histone modifications. In this study, we aim at providing the first insight into chromatin structure of this species. Methods: We aligned the amino acid sequence of P. acuta core histones with histone sequences from various phyla. We developed a centri-filtration on sucrose gradient to separate chromatin from the host and the symbiont. The presence of histone H3 protein and specific histone modifications were then detected by western blot performed on histone extraction done from bleached and healthy corals. Finally, micrococcal nuclease (MNase) digestions were undertaken to study nucleosomal organization. Results: The centri-filtration enabled coral chromatin isolation with less than 2% of contamination by endosymbiont material. Histone sequences alignments with other species show that P. acuta displays on average ~90% of sequence similarities with mice and ~96% with other corals. H3 detection by western blot showed that H3 is clipped in healthy corals while it appeared to be intact in bleached corals. MNase treatment failed to provide the usual mononucleosomal digestion, a feature shared with some cnidarian, but not all; suggesting an unusual chromatin structure. Conclusions: These results provide a first insight into the chromatin, nucleosome and histone structure of P. acuta. The unusual patterns highlighted in this study and partly shared with other cnidarian will need to be further studied to better understand its role in corals.
RESUMEN
BACKGROUND: Although the term holobiont has been popularized in corals with the advent of the hologenome theory of evolution, the underlying concepts are still a matter of debate. Indeed, the relative contribution of host and environment and especially thermal regime in shaping the microbial communities should be examined carefully to evaluate the potential role of symbionts for holobiont adaptation in the context of global changes. We used the sessile, long-lived, symbiotic and environmentally sensitive reef-building coral Pocillopora damicornis to address these issues. RESULTS: We sampled Pocillopora damicornis colonies corresponding to two different mitochondrial lineages in different geographic areas displaying different thermal regimes: Djibouti, French Polynesia, New Caledonia, and Taiwan. The community composition of bacteria and the algal endosymbiont Symbiodinium were characterized using high-throughput sequencing of 16S rRNA gene and internal transcribed spacer, ITS2, respectively. Bacterial microbiota was very diverse with high prevalence of Endozoicomonas, Arcobacter, and Acinetobacter in all samples. While Symbiodinium sub-clade C1 was dominant in Taiwan and New Caledonia, D1 was dominant in Djibouti and French Polynesia. Moreover, we also identified a high background diversity (i.e., with proportions < 1%) of A1, C3, C15, and G Symbiodinum sub-clades. Using redundancy analyses, we found that the effect of geography was very low for both communities and that host genotypes and temperatures differently influenced Symbiodinium and bacterial microbiota. Indeed, while the constraint of host haplotype was higher than temperatures on bacterial composition, we showed for the first time a strong relationship between the composition of Symbiodinium communities and minimal sea surface temperatures. CONCLUSION: Because Symbiodinium assemblages are more constrained by the thermal regime than bacterial communities, we propose that their contribution to adaptive capacities of the holobiont to temperature changes might be higher than the influence of bacterial microbiota. Moreover, the link between Symbiodinium community composition and minimal temperatures suggests low relative fitness of clade D at lower temperatures. This observation is particularly relevant in the context of climate change, since corals will face increasing temperatures as well as much frequent abnormal cold episodes in some areas of the world.
