Theory 021/Life & Evolution /Genomics
Genome Size Is a Deletion Balance
Genome size is explained by cell size and by junk. The mechanism may be a tug of war over deletions.
Genome size is set by the balance between deletional bias and the cost of DNA replication rather than by cell size, so species with the same cell size can differ in genome size by an order of magnitude if their deletion machinery differs.
01The evidence
Genome sizes vary enormously among organisms with similar cell sizes and similar lifestyles, and the variation correlates with the efficiency of deletional repair.
02The tension
The cell-size explanation works within groups and fails across them, and the junk-DNA explanation predicts no relationship at all. Neither accounts for the correlation with repair efficiency.
03The mechanism
Most mutations that change genome size are deletions, and their fixation depends on population size and on the cost of replicating extra DNA. A lineage with efficient deletion and small populations drifts toward compact genomes; one with weak deletion drifts toward expansion.
That makes genome size a balance point rather than an adaptation, and predicts that genome size should track deletion efficiency more closely than cell size.
04What this is not
Those accounts tie size to function or to absence of function. This ties it to a mutation bias, which predicts differences within functional classes.
05Predictions
- P1Genome size should correlate with deletion bias across lineages.
- P2Species matched on cell size but differing in repair should differ in genome size.
- P3Population size should modulate the balance.
06Refutation
Genome size tracking cell size at controlled deletion bias would restore the functional account.
07What it changes
It would make genome size a readout of mutation bias.