Mosaic Accumulation of Somatic Genetic Variation and Estimates of Age in the Long-Lived Reef-Building Coral Acropora palmata.

Somatic genetic variation (SoGV) accumulates during an organism's lifetime and is traditionally viewed as detrimental rather than adaptive due to links with cancer and senescence. However, in modular organisms like corals, deleterious mutations can be purged at the cellular or polyp level, while adaptive mutations may rise in frequency as polyps create genetically distinct modules. Quantifying the somatic genetic landscape in corals is necessary to understand the role these mutations may have in coral and clonal animal development and evolution. Here, we catalogue somatic genetic variation in eight Acropora palmata colonies from Curaçao. Whole genomes were sequenced (70-100× depth) to document mutation variant allele frequency shifts as genets age. Large numbers of SoGVs were observed in six- to ten-year-old colonies, and inferred mutation rates were used to age a genet of uncertain age to almost a century old. Mutations were not fixed at the polyp or branch levels, i.e., they always displayed frequencies < 0.5 as expected at mutating homozygous sites. However, their allele frequencies followed a power-law distribution, similar to aging human tissues. No signs of positive selection were found; instead SoGVs were under purifying selection. In one colony, mutations in 28 samples from along a branch were analysed using a SNP microarray. Contrary to expectations, genetic and physical distances were unrelated. Along with an observed lack of fixation this pattern may be explained by a large stem cell population, the de-differentiation or dormancy of stem cells, the contribution of strong purifying selection, or a combination of the previously mentioned. Our findings provide a neutral framework against which to test for module-level selection of genetic variation in corals, explore the relationship between physical and genetic distance within a colony, and apply a somatic genetic clock to colonies of Acropora palmata. This work provides fundamental insights into the landscape of somatic mutations in reef-building coral, highlighting the importance of studying somatic genetic variation as they may contribute to genetic diversity and adaptability in colonial animals.