Apicomplexa is a group of obligate intracellular parasites that includes the causative agents of human diseases such as malaria and toxoplasmosis. Apicomplexans evolved from free-living phototrophic ancestors, but how this transition to parasitism occurred remains unknown. One potential clue lies in coral reefs, of which environmental DNA surveys have uncovered several lineages of uncharacterized basally branching apicomplexans1,2. Reef-building corals have a well-studied symbiotic relationship with photosynthetic Symbiodiniaceae dinoflagellates (for example, Symbiodinium3), but the identification of other key microbial symbionts of corals has proven to be challenging4,5. Here we use community surveys, genomics and microscopy analyses to identify an apicomplexan lineage—which we informally name ‘corallicolids’—that was found at a high prevalence (over 80% of samples, 70% of genera) across all major groups of corals. Corallicolids were the second most abundant coral-associated microeukaryotes after the Symbiodiniaceae, and are therefore core members of the coral microbiome. In situ fluorescence and electron microscopy confirmed that corallicolids live intracellularly within the tissues of the coral gastric cavity, and that they possess apicomplexan ultrastructural features. We sequenced the genome of the corallicolid plastid, which lacked all genes for photosystem proteins; this indicates that corallicolids probably contain a non-photosynthetic plastid (an apicoplast6). However, the corallicolid plastid differs from all other known apicoplasts because it retains the four ancestral genes that are involved in chlorophyll biosynthesis. Corallicolids thus share characteristics with both their parasitic and their free-living relatives, which suggests that they are evolutionary intermediates and implies the existence of a unique biochemistry during the transition from phototrophy to parasitism.
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The following are deposited in GenBank: the Rhodactis sp. wkC1 mitochondrial genome (accession number MH320096); corallicolid 18S, 5.8S and 28S rRNA genes from Rhodactis sp. wkC1 (MH304760, MH304761), Orbicella sp. TRC (MH304758) and Cyphastrea sp. 2 (MH304759); corallicolid mitochondrial genomes from Rhodactis sp. wkC1 (MH320093), Orbicella sp. 8CC (MH320094) and Cyphastrea sp. 2 (MH320095); and the corallicolid plastid genome from Rhodactis sp. wkC1 (MH304845).
The 18S rRNA and 16S rRNA gene amplicon reads are deposited in the NCBI Sequence Read Archive (PRJNA482746).
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We thank C. Zwimpfer and B. Ross for assistance with sample processing and electron microscopy. This work was funded by the Canadian Institutes for Health Research grant MOP-42517 (to P.J.K.), the Natural Sciences and Engineering Research Council of Canada Fellowship PDF-502457-2017 and a Killam Postdoctoral Research Fellowship (to W.K.K.) and the Marie Curie International Outgoing Fellowship FP7-PEOPLE-2012-IOF - 331450 CAARL and a Tula Foundation grant to the Centre for Microbial Biodiversity and Evolution (to J.d.C.).
Nature thanks Christopher Howe, Patrick Wincker and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Names denote the host coral from which the genomes were retrieved. The three mitochondria-encoded genes are shown in blue. Tick marks (moving clockwise) denote 1,000 bp. It is unclear whether the genomes are circular (as depicted), or tandem linear.
Extended Data Fig. 2 Distribution and diversity of corallicolid type-N from eukaryotic microbiome surveys.
a, Phylogenetic placement of short-amplicon OTUs (red) and near-full-length sequences (black), which show the diversity of the type-N clade. Coral host species are indicated. Values at nodes denote maximum likelihood bootstrap support (n = 1,000). Relationships between type-N lineages were generally poorly resolved. b, Presence of type-N reads in 18S rRNA gene surveys from environmental and host-associated samples, which shows that type-N is largely restricted to corals. Surveys included in this analysis are listed in Supplementary Table 5.
a, Geographical location. b, Water depth. Correlation calculated using ANOSIM with 999 permutations. N/A, not available.
Extended Data Fig. 4 Transmission electron micrographs of darkly stained organelles in corallicolid cells, showing distinctive internal structures.
Structure and orientations (sagittal and transverse sections illustrated at the top) were inferred from viewing multiple organelles from several cells. Imaging was conducted in triplicate; representative results are shown.
Extended Data Fig. 5 Tetrapyrrole and chlorophyll biosynthesis pathways, showing putative function of genes.
Genes retained in the corallicolid plastid genome (acsF, chlL, chlN and chlB) are highlighted. All enzymatic steps depicted here are inferred to occur within the apicomplexan plastid58.
a, Phylogenetic placement of corallicolids, based on mitochondria-encoded proteins. b, Phylogenetic placement of corallicolids, based on plastid-encoded proteins. c, Phylogenetic placement of corallicolid chlorophyll biosynthesis proteins (concatenation of proteins ChlL, ChlN and ChlB on the left; AcsF on the right). All phylogenetic trees shown were produced with the maximum likelihood algorithm; values at nodes denote maximum likelihood bootstrap support percentages (n = 1,000 replicates) and Bayesian posterior probabilities (see Methods).
Extended Data Fig. 7 Conservation of key amino acid residues implies conservation of protein function in chlorophyll biosynthesis genes.
a, b, Sequence alignment of AcsF (a) and ChlN (b) proteins. For ChlN alignment, a comparison to NifD is shown. ‘Corallicolid meta’ sequence is derived from metagenomics and metatranscriptomics assembly; ‘Corallicolid wkC1’ is from the complete plastid sequence.
Extended Data Fig. 8 Phylogenetic placement of corallicolids, based on putative nucleus-encoded proteins.
Based on concatenation of HSP90, RPL3, RPL27A, RPS8, RPS19, RPS21 and RPS27 proteins (Supplementary Table 6). The tree shown was produced with the maximum likelihood algorithm; values at nodes denote maximum likelihood bootstrap support percentages (n = 1,000 replicates) and Bayesian posterior probabilities (see Methods).
a, Pairwise amino acid identities and dN values support a close relationship between corallicolids and the Coccidia. b, Phylogenetic analysis of single plastid genes and proteins to test alternative topologies of corallicolid placement. Results vary by gene and methodology: although most plastid genes show a basal placement for corallicolids, a few support the grouping of corallicolids within the Apicomplexa. Tree construction methods are indicated at top, with the model of evolution in parentheses. NJ, neighbour joining; MP, maximum parsimony; ML, maximum likelihood. A dash indicates a lack of support for either topology. N/A, not applicable.
Supplementary Table 1: Coral samples used in this study. Includes data on wild coral samples and aquarium coral samples.
Supplementary Table 2: Eukaryote and prokaryote OTU tables and representative sequences, generated from this study. Includes data on 18S rRNA gene read abundances, 16S rRNA gene read abundances, assigned taxonomic identity of OTUs and representative sequences.
Supplementary Table 3: Summary of metagenomic and metatranscriptomic dataset survey results, with list of contigs corresponding to the corallicolid plastid genome.
Supplementary Table 4: Corallicolid prevalence from previous studies. Includes list of host coral species, dates and locations.
Supplementary Table 5: Sources and statistics of 18S rRNA gene amplicon datasets used for analysis of type-N distribution. Includes OTU tables and representative sequences from these data.
Supplementary Table 6: List of hits for potential nuclear-encoded corallicolid genes, and photosystem genes.
Supplementary Table 7: Nucleotide sequences used in this study (accession numbers).
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Kwong, W.K., del Campo, J., Mathur, V. et al. A widespread coral-infecting apicomplexan with chlorophyll biosynthesis genes. Nature 568, 103–107 (2019). https://doi.org/10.1038/s41586-019-1072-z
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