Investigating low frequency somatic mutations in Arabidopsis with Duplex Sequencing
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Mutations are the source of novel genetic diversity but can also lead to disease and maladaptation. The conventional view is that mutations occur randomly with respect to their environment-specific fitness consequences. However, intragenomic mutation rates can vary dramatically due to transcription coupled repair and based on local epigenomic modifications, which are non-uniformly distributed across genomes. One sequence feature associated with decreased mutation is higher expression level, which can vary depending on environmental cues. To understand whether the association between expression level and mutation rate creates a systematic relationship with environment-specific fitness effects, we perturbed expression through a heat treatment in Arabidopsis thaliana . We quantified gene expression to identify differentially expressed genes, which we then targeted for mutation detection using Duplex Sequencing. This approach provided a highly accurate measurement of the frequency of rare somatic mutations in vegetative plant tissues, which has been a recent source of uncertainty in plant mutation research. We included mutant lines lacking mismatch repair (MMR) and base excision repair (BER) capabilities to understand how repair mechanisms may drive biased mutation accumulation. We found wild type (WT) and BER mutant mutation frequencies to be very low (mean variant frequency 1.8×10 -8 and 2.6×10 -8 , respectively), while MMR mutant frequencies were significantly elevated (1.13×10 -6 ). These results show that somatic variant frequencies are extremely low in WT plants, indicating that larger datasets will be needed to address the fundamental evolutionary question as to whether environmental change leads to gene-specific changes in mutation rate.
SIGNIFICANCE
Accurately measuring mutations in plants grown under different environments is important for understanding the determinants of mutation rate variation across a genome. Given the low rate of de novo mutation in plant germlines, such measurements can take years to obtain, hindering tests of mutation accumulation under varying environmental conditions. We implemented highly accurate Duplex Sequencing to study somatic mutations in plants grown in two different temperatures. In contrast to plants with deficiencies in DNA mismatch repair machinery, we found extremely low mutation frequencies in wild type plants. These findings help resolve recent uncertainties about the somatic mutation rate in plant tissues and indicate that larger datasets will be necessary to understand the interaction between mutation and environment in plant genomes.
