Gene expression emerges as a genome-wide driver of mutation rate in flies and worms
A new preprint argues that gene expression is not just a downstream readout of genome regulation but a major determinant of where mutations accumulate across animal genomes. By reanalyzing published mutation-accumulation datasets from Drosophila melanogaster and Caenorhabditis elegans, the investigators found that loci under stronger purifying selection accumulated fewer de novo single nucleotide variants, even in noncoding and apparently neutral sequence classes such as introns and synonymous sites. The pattern points to a genome that is not mutationally uniform, but organized by transcriptional activity and DNA repair dynamics.
The result cuts against the simplest expectation that mutation rate is mostly a passive byproduct of local sequence chemistry. GC content, chromatin state, and other regulatory features did matter, but they did not fully explain the depletion of mutations in highly conserved genes. Gene expression provided the strongest account of that depletion, suggesting that the more a locus is transcribed, the more likely it is to benefit from repair processes that lower its local mutational burden. In practical terms for evolutionary genetics, the genome appears to protect some genes more than others, and transcription is a central part of that protection.
Transcription-coupled repair provides the most plausible mechanism
The strongest mechanistic clue came from excision repair sequencing data. Regions with higher levels of transcription-coupled repair were associated with lower mutation rates, a relationship that fits neatly with the idea that active transcription recruits repair machinery to the transcribed strand. In highly conserved genes, repeated transcription may therefore function as a kind of surveillance system, limiting the persistence of DNA lesions before they become fixed as mutations. The biology is straightforward: when RNA polymerase encounters damage, repair pathways are more likely to be summoned, and that can reduce the local rate at which errors survive replication.
This matters because the association was not confined to a single organism or a single annotation class. The pattern held across animal model systems with distinct life histories and genome architectures, which makes a purely idiosyncratic explanation less convincing. It also extends the earlier plant literature that had raised the possibility that important genes experience lower mutation rates than expected. Here, the same broad idea is carried into animals, where transcriptional activity seems to shape the mutational fate of specific loci under minimal selection pressure.
Mutation rate, in this framework, is partly written by expression.
Conserved genes accumulated fewer mutations under minimal selection
The preprint repurposed mutation accumulation experiments, a design that is particularly useful for separating spontaneous mutation from selection. Under those conditions, loci under stronger purifying selection still accumulated fewer mutations, including variants in introns and synonymous positions. Because those classes are usually treated as less constrained than protein-altering changes, their depletion at conserved loci is hard to dismiss as a simple artifact of coding importance. The signal suggests that genomic context, rather than only protein function, influences the baseline rate at which mutations arise and persist.
Sequence-specific factors remained relevant. GC content and chromatin modifications correlated with mutation rate, reinforcing the idea that the local DNA environment shapes vulnerability to damage and repair. Yet the authors report that gene expression best explained the specific depletion seen at strongly conserved genes, even after accounting for correlations among variables. In other words, expression did not merely travel with the other genomic features; it retained explanatory power when those features were placed side by side.
Evolutionary history appears to track changes in expression and mutation rate
One of the more interesting comparisons linked present-day expression to evolutionary change. By examining orthologs between C. elegans and its close relative Pristionchus pacificus, the investigators found that changes in gene expression over evolutionary time correlated with changes in mutation rate. This offers a coherent evolutionary narrative: if a gene becomes more highly expressed in one lineage, it may also become better protected by transcription-coupled repair, and its mutational input may shift accordingly.
That possibility creates a bridge between gene regulation and mutation bias. Rather than requiring direct selection on mutation rate itself, the mechanism allows mutation patterns to emerge indirectly from changes in transcriptional programs. For evolutionary theory, the implication is substantial. Nonrandom mutation rates need not contradict natural selection; they can arise as a byproduct of regulatory evolution, with transcription acting as the intermediary between gene function and DNA maintenance.
Why this model changes how mutation bias is interpreted
The preprint proposes a model in which developmental transcription modulates local mutation rates through repair-dependent pathways. That idea is attractive because it explains several otherwise separate observations with one mechanism: conserved genes mutate less, expressed genes are better protected, and repair activity maps onto mutation depletion. It also helps resolve a long-standing tension in evolutionary biology. If mutation rates were entirely random with respect to function, one would not expect such a consistent relationship between transcription and the genomic distribution of de novo variants.
There is a simpler bedside-style lesson for genetics, even though the work is not clinical in the medical sense. Genome maintenance is not evenly distributed, and transcriptional state appears to tilt the odds in favor of some genes over others. The genome is not only read; it is actively defended in a gene-specific way.
As with any preprint, these findings will need independent confirmation and deeper mechanistic testing in additional taxa and experimental settings. The current analysis is strongest where it connects mutation accumulation, repair profiling, and cross-species comparison, but it still leaves open how universal the expression-mutation relationship is across developmental stages, cell types, and environmental conditions. The most important remaining question is not whether transcription matters, but how far that influence extends across animal genomes and which repair pathways carry it forward.