Limited accumulation of high-frequency somatic mutations in a 1700-year-old Osmanthus fragrans tree.

Lifespan varies greatly between and within species. Mutation accumulation is considered an important factor explaining this life-history trait. However, direct assessment of somatic mutations in long-lived species is still rare. In this study, we sequenced a 1700-year-old sweet olive tree and analysed the high-frequency somatic mutations accumulated in its six primary branches. We found the lowest per-year mutation accumulation rate in this oldest tree among those studied via the whole-genome sequencing approach. Investigation of mutation profiles suggests that this low rate of high-frequency mutation was unlikely to result from strong purifying selection. More intriguingly, on a per-branching scale, the high-frequency mutation accumulation rate was similar among the long-lived individuals such as oak, wild peach and sweet olive investigated here. We therefore suggest the possibility that the accumulation of high-frequency somatic mutations in very long-lived trees might have an upper boundary due to both the possible limited number of stem cell divisions and the early segregation of the stem cell lineage.