Multiple Origins of Sex Chromosomes in <i>Nothobranchius</i> Killifishes
Bibliographic record
Abstract
ABSTRACT Sex chromosomes have evolved repeatedly across eukaryotes. The emergence of a sex‐determining (SD) locus is expected to progressively restrict recombination, driving convergent molecular differentiation. However, evidence from taxa like teleost fishes, representing over half of vertebrate species with unmatched diversity in SD systems, challenges this model. Teleost sex chromosomes are often difficult to detect as they experience frequent turnovers, resetting the differentiation process. Nothobranchius killifishes, which include the XY system shared by N. furzeri and N. kadleci and X 1 X 2 Y systems in six other species, offer a valuable model to study sex chromosome turnovers. We characterised X 1 X 2 Y systems in five killifish species and found that sex chromosomes evolved at least four times independently. Sex‐determining regions resided near centromeres or predicted chromosome rearrangement breakpoints in N. brieni and N. guentheri , suggesting recombination cold spots may facilitate sex chromosome evolution. Chromosomes representing the XY system in N. furzeri / N. kadleci were sex‐linked also in the outgroup Fundulosoma thierryi , with several genes, including gdf6 , residing in the region of differentiation. Although the X 1 X 2 Y systems of N. guentheri , N. lourensi (both Coastal clade), and N. brieni (Kalahari clade) involved different chromosomes, they shared a potential SD region. We uncovered two sex‐linked evolutionary strata of distinct age in N. guentheri . However, its potential SD gene amhr2 was located in the younger stratum and is hence unlikely to be the ancestral SD gene in this lineage. Our findings suggest recombination landscapes shape sex chromosome turnover and that certain synteny blocks are repeatedly co‐opted as sex chromosomes in killifishes.
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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 | 0.000 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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 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".