Long lived stem cell lineages produce germ and somatic cells across decades of growth in reef building corals: a new hypothesis

In many animals, the germline differentiates early in embryogenesis, so only mutations that accumulate in germ cells over the course of an individual life are inherited by its offspring. Animal exceptions to this developmental process may point to other mechanisms that have evolved to limit the effects of deleterious mutation accumulation, for both an individual and its offspring. Stony corals are animals that can live for hundreds of years, and whether they possess embryonic germlines has been controversial. To clarify conflicting evidence about germline-soma distinction in corals and to quantify the genetic diversity in the sperm of a colony, we sequenced full genomes for parent coral (Acropora hyacinthus) branches and the sperm pools spawned by each of those branches. We identified single nucleotide variants (SNVs) unique to each parent branch, then checked whether that mutation was present in the sperm pool spawned by that branch. On average, 26% of post-embryonic SNVs were shared by the sperm and 74% were not. We also identified classical germline SNVs, those that were present in the sperm but not in the parent. Together, these three different types of SNVs suggest that corals possess a lineage of self-renewing stem cells that proliferate and differentiate into germ and soma throughout the adult life of the colony, and that there are different patterns of SNV accumulation in stem cell, soma, and germ lineages. In addition to informing the evolution of germline-soma distinction, these insights inform how corals may generate adaptive diversity necessary in the face of global climate change.