The Complete Mitochondrial DNA Sequence of Mesostigma viride Identifies This Green Alga as the Earliest Green Plant Divergence and Predicts a Highly Compact Mitochondrial Genome in the Ancestor of All Green Plants
Bibliographic record
Abstract
To gain insights into the nature of the mitochondrial genome in the common ancestor of all green plants, we have completely sequenced the mitochondrial DNA (mtDNA) of Mesostigma viride. This green alga belongs to a morphologically heterogeneous class (Prasinophyceae) that includes descendants of the earliest diverging green plants. Recent phylogenetic analyses of ribosomal RNAs (rRNAs) and concatenated proteins encoded by the chloroplast genome identified Mesostigma as a basal branch relative to the Streptophyta and the Chlorophyta, the two phyla that were previously thought to contain all extant green plants. The circular mitochondrial genome of Mesostigma resembles the mtDNAs of green algae occupying a basal position within the Chlorophyta in displaying a small size (42,424 bp) and a high gene density (86.6% coding sequences). It contains 65 genes that are conserved in other mtDNAs. Although none of these genes represents a novel coding sequence among green plant mtDNAs, four of them (rps1, sdh3, sdh4, and trnL[caa]) have not been reported previously in chlorophyte mtDNAs, and two others (rpl14 and trnI[gau]) have not been identified in the streptophyte mtDNAs examined so far (land-plant mtDNAs). Phylogenetic analyses of 19 concatenated mtDNA-encoded proteins favor the hypothesis that Mesostigma represents the earliest branch of green plant evolution. Four group I introns (two in rnl and two in cox1) and three group II introns (two in nad3 and one in cox2), two of which are trans-spliced at the RNA level, reside in Mesostigma mtDNA. The insertion sites of the three group II introns are unique to this mtDNA, suggesting that trans-splicing arose independently in the Mesostigma lineage and in the Streptophyta. The few structural features that can be regarded as ancestral in Mesostigma mtDNA predict that the common ancestor of all green plants had a compact mtDNA containing a minimum of 75 genes and perhaps two group I introns. Considering that the mitochondrial genome is much larger in size in land plants than in Mesostigma, we infer that mtDNA size began to increase dramatically in the Streptophyta either during the evolution of charophyte green algae or during the transition from charophytes to land plants.
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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 | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 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".