A widespread coral-infecting apicomplexan contains a plastid encoding chlorophyll biosynthesis
Bibliographic record
Abstract
The Apicomplexa are an important group of obligate intracellular parasites that include the causative agents of human diseases like malaria and toxoplasmosis. They evolved from free-living, phototrophic ancestors, and how this transition to parasitism occurred remains an outstanding question. One potential clue lies in coral reefs, where environmental DNA surveys have uncovered several lineages of uncharacterized, basally-branching apicomplexans. Reef-building corals form a well-studied symbiotic relationship with the photosynthetic dinoflagellate Symbiodinium , but identification of other key microbial symbionts of corals has proven elusive. Here, we used community surveys, genomics, and microscopy to identify an apicomplexan lineage, which we name ‘corallicola’, that was found in high prevalence (>80%) across all major groups of corals. In-situ fluorescence and electron microscopy confirmed that corallicola lives intracellularly within the tissues of the coral gastric cavity, and possesses clear apicomplexan ultrastructural features. We sequenced the plastid genome, which lacked all genes for photosystem proteins, indicating that corallicola harbours a non-photosynthetic plastid (an apicoplast). However, the corallicola plastid differed from all other known apicoplasts because it retains all four genes involved in chlorophyll biosynthesis. Hence, corallicola shares characteristics with both its parasitic and free-living relatives, implicating it as an evolutionary intermediate, and suggesting that a unique ancestral biochemistry likely operated during the transition from phototrophy to parasitism.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".