Reef environments shape microbial partners in a highly connected coral population.

Evidence is mounting that composition of microorganisms within a host can play an essential role in total holobiont health. In corals, for instance, studies have identified algal and bacterial taxa that can significantly influence coral host function and these communities depend on environmental context. However, few studies have linked host genetics to algal and microbial partners across environments within a single coral population. Here, using 2b-RAD sequencing of corals and metabarcoding of their associated algal (ITS2) and bacterial (16S) communities, we show evidence that reef zones (locales that differ in proximity to shore and other environmental characteristics) structure algal and bacterial communities at different scales in a highly connected coral population (Acropora hyacinthus) in French Polynesia. Fore reef (FR) algal communities in Mo'orea were more diverse than back reef (BR) communities, suggesting that these BR conditions constrain diversity. Interestingly, in FR corals, host genetic diversity correlated with bacterial diversity, which could imply genotype by genotype interactions between these holobiont members. Our results illuminate that local reef conditions play an important role in shaping unique host-microbial partner combinations, which may have fitness consequences for dispersive coral populations arriving in novel environments.

Profiling gene expression responses of coral larvae (Acropora millepora) to elevated temperature and settlement inducers using a novel RNA-Seq procedure.

Elevated temperatures resulting from climate change pose a clear threat to reef-building corals; however, the traits that might influence corals' survival and dispersal during climate change remain poorly understood. Global gene expression profiling is a powerful hypothesis-forming tool that can help elucidate these traits. Here, we applied a novel RNA-Seq protocol to study molecular responses to heat and settlement inducers in aposymbiotic larvae of the reef-building coral Acropora millepora. This analysis of a single full-sibling family revealed contrasting responses between short- (4-h) and long-term (5-day) exposures to elevated temperatures. Heat shock proteins were up-regulated only in the short-term treatment, while the long-term treatment induced the down-regulation of ribosomal proteins and up-regulation of genes associated with ion transport and metabolism (Ca(2+) and CO(3)(2-)). We also profiled responses to settlement cues using a natural cue (crustose coralline algae, CCA) and a synthetic neuropeptide (GLW-amide). Both cues resulted in metamorphosis, accompanied by differential expression of genes with known developmental roles. Some genes were regulated only by the natural cue, which may correspond to the recruitment-associated behaviour and morphology changes that precede metamorphosis under CCA treatment, but are bypassed under GLW-amide treatment. Validation of these expression profiles using qPCR confirmed the quantitative accuracy of our RNA-Seq approach. Importantly, qPCR analysis of different larval families revealed extensive variation in these responses depending on genetic background, including qualitative differences (i.e. up-regulation in one family and down-regulation in another). Future studies of gene expression in corals will have to address this genetic variation, which could have important adaptive consequences for corals during global climate change.

Mutual orientation of tRNAs and interactions between the codon-anticodon duplexes within the ribosome: a stereochemical analysis.

Analysis of the available data demonstrated that a codon in the i(th) codon-anticodon duplex should interact with the wobble pair of the i - 1(th) duplex. This interduplex interaction should take place throughout the ribosomal elongation cycle in order to prevent unprogrammed frameshifting. An experimentally observed flexibility of tRNA allows to conserve the interduplex interaction at different mutual orientations of tRNAs, including conventional R and S. Moreover, the tRNA flexibility allows novel mutual orientations of tRNAs in which tRNA molecules, as in conventional R and S orientations, also form the codon-anticodon duplexes, and the CCA-ends are located adjacently. The R and S orientations do not offer any advantages over the novel orientations. Therefore, besides the conventional R and S orientations, the novel orientations should also be considered, i.e. the interpretations of the available experimental data on the mutual orientations of tRNAs should be reconsidered. All mutual orientations of tRNAs that are compatible with the available experimental data are given.