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, 3 (4), 348-358

Somatic Mutations Substantially Increase the Per-Generation Mutation Rate in the Conifer Picea sitchensis


Somatic Mutations Substantially Increase the Per-Generation Mutation Rate in the Conifer Picea sitchensis

Vincent C T Hanlon et al. Evol Lett.


The rates and biological significance of somatic mutations have long been a subject of debate. Somatic mutations in plants are expected to accumulate with vegetative growth and time, yet rates of somatic mutations are unknown for conifers, which can reach exceptional sizes and ages. We investigated somatic mutation rates in the conifer Sitka spruce (Picea sitchensis (Bong.) Carr.) by analyzing a total of 276 Gb of nuclear DNA from the tops and bottoms of 20 old-growth trees averaging 76 m in height. We estimate a somatic base substitution rate of 2.7 × 10-8 per base pair within a generation. To date, this is one of the highest estimated per-generation rates of mutation among eukaryotes, indicating that somatic mutations contribute substantially to the total per-generation mutation rate in conifers. Nevertheless, as the sampled trees are centuries old, the per-year rate is low in comparison with nontree taxa. We argue that although somatic mutations raise genetic load in conifers, they generate important genetic variation and enable selection both among cell lineages within individual trees and among offspring.

Keywords: Base substitutions; genetic variation; mutation rate; selection within the individual; somatic mosaicism; somatic mutation.

Conflict of interest statement

The authors declare no conflict of interest.


Figure 1
Figure 1
Schematic diagrams of (A) the protocol for detecting somatic mutations in 20 trees and (B) the most recent common cellular ancestor of the bark samples. To infer the genotype of the tree when it was a young seedling, we discard tree genotypes for which the genotypes of the two bark samples differ, thus eliminating nonheritable somatic mutations in the vascular cambium. Illustration in (A) by Matt Strieby.
Figure 2
Figure 2
Mutation rates per generation (A) and per year (B) for multicellular species with sufficient data available. Per‐year rates were calculated using a reported age, an estimate of generation time, or an average of two estimates of generation time (raw data and references in Table S2). For the trees (Prunus, Picea, and Quercus), the mutation rates were estimated in exceptionally long‐lived specimens; for Picea, the lower per‐year mutation rate is our estimate for P. sitchensis and the higher per‐year mutation rate is the average neutral substitution rate for P. glauca, P. sitchensis, and P. abies (De La Torre et al. 2017). Error bars represent the Poisson confidence intervals.

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