Trees as mosaics of genetic variability: An assessment of the somatic mutation hypothesis

Background/Question/Methods

Although evolution is considered to be a population-level phenomenon it has been hypothesized that individuals with modular architecture could evolve through somatic mutation, serving to fine-tune an individual to changing environmental conditions.  However, the level of somatic variation within an individual has not been assessed from the gene level/whole genome perspective.  We assessed within-plant genetic variation in black cottonwood and the prospects for evolutionary change as measured by changes in amino acids.  We resequenced entire genomes from a multitude of tissues (i.e., bud, stem, and root tissues) of individual trees (parent and clonally derived offspring) using the Illumina platform. 

Results/Conclusions

On average 4,840 unique amino acid changes were found in 2,569 genes within each of 5 tissues sampled within each individual parental tree and it’s clonally derived offspring.  We also show the differential spread of approximately 1,500-1,600 of these mutations across two to three tissues within an individual tree; whether these results reflect selective spread remains to be determined.  A subset of genes, however, show positive signatures of selection (dN/dS >1) among tissues within the same parent/offspring pair, illustrating the potential for selective spread. The genes affected by somatic mutation, and unique to each tissue within a tree, were over-represented by those involved in three categories across all trees and tissues: those involved in 1) apoptosis and the immune response, 2) metabolism and protein binding, and 3) cell and pollen recognition, pollination, pollen-pistil interactions and reproductive cellular processes.  Results support the long-standing hypothesis that individuals exist as mosaics of genetic variability and can evolve.