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Deciphering the nature of the coral-Chromera association

ABSTRACT: Since the discovery of Chromera velia as a novel coral-associated microalga, this organism has attracted interest because of its unique evolutionary position between the photosynthetic dinoflagellates and the parasitic apicomplexans. The nature of the relationship between Chromera and its coral host is controversial. Is it a mutualism, from which both participants benefit, or is Chromera a parasite, harming its host? To better understand the interaction, larvae of the common Indo-Pacific reef-building coral Acropora digitifera were experimentally infected with Chromera and the impact on the host transcriptome assessed at 4, 12, and 48 h post-infection using Illumina RNA-Seq technology. The transcriptomic response of the coral to Chromera was complex and implies that host immunity is strongly suppressed, and both phagosome maturation and the apoptotic machinery modified. These responses differ markedly from those described for infection with a competent strain of the coral symbiont Symbiodinium, instead resembling those of vertebrate hosts to parasites and/or pathogens such as Mycobacterium tuberculosis. Consistent with ecological studies suggesting that the association may be accidental, the transcriptional response of A. digitifera larvae leads us to conclude that Chromera is more likely to be a coral parasite, commensal, or accidental bystander, but certainly not a beneficial mutualist

ORGANISM(S): Acropora digitifera

PROVIDER: GSE102664 | GEO | 2017/08/16

SECONDARY ACCESSION(S): PRJNA398338

REPOSITORIES: GEO

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Project description:A mutualistic relationship between reef-building corals and endosymbiotic algae (Symbiodinium spp.) forms the basis for the existence of coral reefs. Genotyping tools for Symbiodinium spp. have added a new level of complexity to studies concerning cnidarian growth, nutrient acquisition, and stress. For example, the response of the coral holobiont to thermal stress is connected to the host-Symbiodinium genotypic combination, as different partnerships can have different bleaching susceptibilities. If, and to what extent, differences in algal symbiont clade contents can exert effects on the coral host transcriptome is currently unknown. In this study, we monitored algal physiological parameters and profiled the coral host transcriptional responses in acclimated, thermally stressed, and recovered coral fragments using a custom cDNA gene expression microarray. Combining these analyses with results from algal and host genotyping revealed a striking symbiont effect on both the acclimated coral host transcriptome and the magnitude of the thermal stress response. This is the first study that links coral host transcriptomic patterns to the clade content of their algal symbiont community. Our data provide a critical step to elucidating the molecular basis of the apparent variability seen among different coral-algal partnerships. Sample Collection and Tank Experiment. On July 31 2007, six replicate fragments (9.5 Â± 3.5cm2) were collected using hammer and chisel from the upper sun-exposed surface of five different, healthy-looking Montastraea faveolata colonies near Puerto Morelos, Quintana Roo, MÃ©xico (20o52â28.77âN and 86o51â04.53âW). The fragments were transported to the institute within 1 h and divided evenly between two 50 L aquaria (i.e., 3 fragments from each colony were placed into each tank) that received a constant flow of seawater (0.6 L/min). Each aquarium was fit with a water pump connected to a spray-bar to provide constant water movement and aeration. Both aquaria were placed in a common pond with flowing water to buffer diurnal temperature fluctuations, and both aquaria were acclimated to the same shaded ambient light condition. All coral fragments were mounted on plasticene and kept at a depth of 7 cm. From 10 to 19 August 2007, both aquaria received an average water temperature of 27.9 Â± 0.6oC (as recorded by HOBO Light/Temperature Data Loggers, Onset Corp.). Beginning on 11 August, dark-adapted maximum quantum yields for charge separation (Fv/Fm) were measured at dusk for all coral fragments using a DIVING-Pulse Amplitude Modulated (PAM) fluorometer (Walz, Germany). Photosynthetically active radiation (PAR) was measured at noon and averaged 318 Â± 129 Âµmol m-2 s-1. From 20 to 21 August 2007, all coral fragments were brought inside during the passage of Hurricane Dean. On 22 August, the experiment was reconstituted, and acclimation continued on 23 August and lasted until 1 September. During this time, both aquaria received a mean water temperature of 28.5 Â± 0.8oC, and mean PAR of 371 Â± 169 Âµmol m-2 s-1. On the night of 1 September, control time point samples from five different colonies were collected from each tank (Figure 1), before one 200-Watt aquarium heater was turned on in the treatment aquarium. A second heater was turned on 3 days later. During the thermal stress experiment, the control aquarium received mean water temperature of 28.8 Â± 1.2oC; the heated aquarium, 31.5 Â± 1.1oC. During the thermal stress experiment, tanks received mean PAR of 420 Â± 152 Âµmol m-2 s-1. On 11 September, and 19 October, one sample each from five different colonies was collected from each tank for the bleaching and recovery time point, respectively. Microarray Gene Expression Analysis of Coral Host Gene Expression. For the biologically replicated time series experiment, a pool of RNA from all control tank fragments was used as a reference RNA sample, against which each sample from the treatment tank was competitively hybridized. All control and heat-stressed samples were competitively hybridized against the reference RNA. Dye swaps were not performed, as any dye bias present is equal in all comparisons to the reference. Note: Samples VALUEs used for statistical analysis of differential expression. GSE15253_shini_cbr_matrix2_norm.txt used for hierarchical clustering.

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Deciphering the nature of the coral-Chromera association

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