Secondary endosymbioses and evolution of unicellular eukaryotes
Bibliographic record
Abstract
Many lines of evidence support the idea that the first chloroplast was the result of an endosymbiotic relationship between a cyanobacterium and a non‐photosynthetic eukaryote. Most of the cyanobacterial genes were lost, but a few remained in the chloroplast genome and as many as a thousand were transferred to the host nucleus. Genes encoding functions required by the chloroplast had to acquire presequences to target the products to the chloroplast. The situation gets more complicated when we consider the algae with chlorophyll c. They are the product of secondary endosymbiosis, where a putative red algal ancestor was engulfed by another non‐photosynthetic eukaryote, which retained the red algal chloroplast but eventually got rid of the rest of the cell. This left the chloroplast surrounded by two additional membranes: one derived from the red algal plasma membrane and the other from the host's phagocytic vacuole. In order for the endosymbiotic relationship to work, there must have been a substantial amount of gene transfer from the red algal nucleus to the host nucleus to support chloroplast functions. In the cryptophytes we even see an intermediate stage in this process, a relict nucleus (nucleomorph) in the periplastidal space between the outer two membranes and the original chloroplast envelope. Now that the draft genome sequence of the diatom Thalassiosira pseudonana (Diatom Genome Consortium) as well as genomes of rhodophyte Cyanidioschyzon merolae and green plants are available, it is possible to investigate the evolutionary history of plastid localized metabolic pathways. Phylogenetic analyses of nuclear‐encoded putatively plastid‐targeted enzymes showed that plastids obviously utilize enzymes not only of expected plastid (cyanobacterial) origin. Within the diatom, apicomplexan, plant and rhodophyte genomes, we have identified several enzymes that originate in α‐proteobacteria (mitochondria) or even in eukaryotic nucleus, but possess N‐terminal plastid‐targeting presequences. Although diatoms are, according to multiprotein phylogeny, related to Alveolates, some plastid‐related metabolic pathways show substantially different evolutionary pattern as well as, in silico, predicted localizations of involved enzymes.
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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 | 0.001 | 0.001 |
| 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 | 0.002 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.002 |
| Research integrity | 0.001 | 0.001 |
| 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".