Trees accumulate somatic mutations throughout their long lifespan, resulting in genetic mosaicism within an individual. Since trees derive the germline from the soma later in development, somatic mutations can be transmitted to subsequent generations and influence the evolutionary path of trees. However, how such mutations accumulate and expand to branches during tree growth remains unknown.
We estimated these dynamic processes in long-lived tropical trees, Shorea laevis and S. leprosula, which were subjected to genomic study on genetic mosaicism, using a mathematical model. The model describes the dynamics of stem cell lineages in shoot apical meristems and the mutation accumulation in them during tree growth. By model fitting, we elucidate the somatic mutation rate and the cell lineage dynamics, especially the degree of lineage replacement called somatic genetic drift. Based on this optimal prediction, we simulated genetic structure within a tropical tree and discussed the detailed dynamic mechanism beyond the snapshot sequenced data.
Tropical trees predicted having less genetic heterogeneity among stem cells within a meristem due to the strong somatic genetic drift during branch elongation. Genetic variation among branches is thus attributed to the mutation accumulation after branching of focal branches rather than the variation within a meristem. Notably, our results also shed light on cell lineage dynamics in long-lived trees which is difficult to estimate experimentally.