The problem
An onion’s genome contains roughly sixteen billion DNA letters in one chromosome set; much of its assembled sequence consists of repeats. More DNA does not simply mean more kinds of protein-making instructions. How do genomes gain so much material, and what determines whether it stays or is lost over evolution?[1] [3]
The inherited DNA of animals and plants can vary greatly in amount, including extensive repeated sequence and material outside protein-coding genes. What predicts their long-term expansion or contraction? Transposable elements are DNA sequences that can move or produce new copies; deletions remove material. Natural selection changes the success of variants with different effects, while genetic drift changes their frequencies through chance sampling. A 2025 analysis of 807 animal species tested population-size proxies against genome size and repeat content. Its broad negative result challenges a general drift-based prediction, rather than showing that drift never affects particular lineages.[1] [2]
Scope
Relative contributions of DNA gain, deletion, selection and genetic drift to eukaryotic genome-size evolution, especially noncoding and repetitive DNA.
What remains open
Repeated DNA explains much expansion. The forces deciding what is retained or lost remain incomplete.[1]
Why it matters
DNA amount is part of how a cell stores and copies its inherited material, not a ranking of how complex an organism is. Explaining its variation connects the history of repeats and gene organization to the evolutionary forces acting on them.[1]
DNA amount is associated with cell size and growth rates, linking genome evolution to the physical scale and pace of living cells. Those associations raise a consequential question: does accumulating DNA alter those traits enough to affect reproductive success, or do other changes explain the pattern? Separating these possibilities helps distinguish DNA that persists despite its effects from DNA whose effects favor retention or loss. This does not give every noncoding sequence the same function or cost.[1] [2]
Definitions
3 terms
transposable element
A DNA sequence able to move to a new genomic location or generate additional copies. Some types expand genomes by copying themselves.[1]
Sources
- Genome size evolution: towards new model systems for old questions
Notes on Genome size evolution: towards new model systems for old questions
Review distinguishes DNA gains and losses from the evolutionary forces affecting their retention. Repeats, gene counts and genome amount are different quantities; observed genome-size diversity cannot simply rank organism complexity.
- Effective population size does not explain long-term variation in genome size and transposable element content in animals
Notes on Effective population size does not explain long-term variation in genome size and transposable element content in animals
Version-of-record analysis combines an 807-species animal compilation with metric-specific subsets to test population-size proxies. Its general result challenges a long-term drift-based explanation without excluding drift in particular lineages. July 18, 2025 identifies this published version, following earlier reviewed preprints.
- Insights from the first genome assembly of Onion (Allium cepa)
Notes on Insights from the first genome assembly of Onion (Allium cepa)
Onion assembly provides a concrete example of a roughly 16-billion-letter genome and extensive repetitive assembled sequence. Estimated total genome size and assembled sequence length differ; this case does not equate DNA amount with protein-coding gene number.