Time and time again: unisexual salamanders (genus Ambystoma) are the oldest unisexual vertebrates
Bibliographic record
Abstract
BACKGROUND: The age of unisexual salamanders of the genus Ambystoma is contentious. Recent and ancient evolutionary histories of unisexual Ambystoma were proposed by a few separate studies that constructed phylogenies using mitochondrial DNA markers (cytochrome b gene vs. non-coding region). In contrast to other studies showing that unisexual Ambystoma represent the most ancient unisexual vertebrates, a recent study by Robertson et al. suggests that this lineage has a very recent origin of less than 25,000 years ago. RESULTS: We re-examined the phylogenetic relationship of the unisexuals to A. barbouri from various populations using both mitochondrial markers as well as the complete mitochondrial genomes of A. barbouri and a unisexual individual from Kentucky. Lineage dating was conducted using BEAST and MultiDivTime on a complete mitochondrial genome phylogeny. Our results support a monophyletic lineage for unisexual Ambystoma that shares its most recent common ancestor with an A. barbouri lineage from western Kentucky. In contrast to the Robertson et al.'s study, no A. barbouri individual shared an identical or almost identical cytochrome b haplotype with any unisexual. Molecular dating supports an early Pliocene origin for the unisexual linage (approximately 5 million years ago). We propose that a unisexual-like cytochrome b numt (or pseudogene) exists in the controversial A. barbouri individuals from Kentucky, which was likely the cause of an erroneous phylogeny and time estimate in Robertson et al.'s study. CONCLUSION: We reject a recent origin of unisexual Ambystoma and provide strong evidence that unisexual Ambystoma are the most ancient unisexual vertebrates known to exist. The likely presence of an ancient cytochrome b numt in some Kentucky A. barbouri represents a molecular "fossil" reinforcing the hypothesis that these individuals are some of the closest extant relatives to unisexual Ambystoma.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| 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 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
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 teacher head, 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".