Tropical trees inherit low-frequency somatic mutations.

A new study by Schmitt et al. revealed that somatic mutations in tropical trees are passed on to their offspring. Furthermore, the study noted that the majority of inherited mutations were present at low allelic frequencies within the tree.

Modelling somatic mutation accumulation and expansion in a long-lived tree with hierarchical modular architecture.

In a long-lived organism with a modular architecture, such as trees, somatic mutations accumulate throughout the long lifespan and result in genetic mosaicism in each module within the same individual. In recent years, next-generation sequencing technology has provided a snapshot of such intra-organismal genetic variability. However, the dynamic processes underlying the accumulation and expansion of somatic mutations during the growth remain poorly understood. In this study, we constructed a model to describe these processes in a form that can be applied to a real tree. Given that the proliferation dynamics of meristematic cells vary across plant species, multiple possible processes for elongation and branching were comprehensively expressed in our model. Using published data from a poplar tree, we compared the prediction of the models with the observation and explained the cell lineage dynamics underlying somatic mutations accumulation that were not evident from the snapshot of the sequenced data. We showed that the somatic genetic drift during growth increases inter-meristem mosaicism, resulting in genetically distinct branches and less integrity within an individual tree. We also showed that the somatic genetic drift during branching leads to the mutation accumulation pattern that does not reflect the tree topology. Our modelling framework can help interpret and provide further insights into the empirical findings of genetic mosaicism in long-lived trees.

The genetic structure within a single tree is determined by the behavior of the stem cells in the meristem.

Genomic sequencing revealed that somatic mutations cause a genetic differentiation of the cells in a single tree. We studied a mathematical model for stem cell proliferation in the shoot apical meristem (SAM). We evaluated the phylogenetic distance between cells sampled from different portions of a shoot, indicating their genetic difference due to mutations accumulated during shoot elongation. The plant tissue has cell walls that suppress the exchange of location between cells. This leads to the genetic differentiation of cells according to the angle around the shoot and a larger genetic variance among cells in the body. The assumptions are as follows: stem cells in the SAM normally undergo asymmetric cell division, producing successor stem cells and differentiated cells. Occasionally, a stem cell fails to leave its successor stem cell and the vacancy is filled by the duplication of one of the nearest neighbor stem cells. A mathematical analysis revealed the following: the genetic diversity of cells sampled at the same position along the shoot increases with the distance from the base of the shoot. Stem cells hold a larger variation if they are replaced only by the nearest neighbors. The coalescent length between two cells increases not only with the difference in the position along the shoot but also in the angle around the shoot axis. The dynamics of stem cells at the SAM determine the genetic pattern of the entire shoot.