One of the central assumptions of genetic inheritance is that only a mutation in a germline cell (e.g., a sperm or egg cell) can be inherited by an offspring and influence generational evolutionary adaptation. Mutations in somatic cells can affect an organism but not its descendants – for example, a mutation in a skin cell may cause skin cancer in an individual, but won’t cause skin cancer in that individual’s children. Once the multipotent progenitor cells of the developing organism differentiate into either somatic or germ cells, there is assumed to be no way for somatic cells to cross over and become (or have offspring that become) germ cells.
While this is well-established for humans and other similar animals, a recent paper by Kate Kuntz, Sheila Kitchen and others at Pennsylvania State University suggests that it may not hold true for other animals who may experience germline differentiation much later in development or even undergo transdifferentiation of somatic cells into germ cells. They look specifically at corals, which can reproduce both sexually and asexually, resulting in large colonies called “genets” that all share a single original genotype but which differ from each other due to their founding sexual reproduction. It has been known for a while that the coral’s asexual form of reproduction can reproduce somatic mutations, since new corals born in this way are just little buds that detach from the parent and are essentially a clone of the parent with whatever mutated alleles happened to exist in the cells that formed the bud. In this study, however, self-fertilized eggs were observed to develop, in which somatic mutations from the parent genet persisted, and in which chromosomal recombination was shown to have occurred.
In other words, these corals exhibited parthenogenesis (uniparental sexual reproduction) that enhanced the biological diversity of the offspring both by incorporating successful somatic mutations and by recombining the genes to place those mutations in new genomic settings. A lot of the support for this comes from bioinformatics work establishing the genetic distance between different parts of the original genet, unrelated corals, and the offspring investigated. The informatics of determining somatic inheritance in biparental offspring is more challenging, but this study was able to provide some evidence supporting that as well.
As the authors put it,
“Because coral genets can persist for hundreds to thousands of years, somatic mutations can rise to high frequency in modules (polyps) of a genet due to stochasticity or selection. Strongly deleterious or lethal mutations might lead to module or colony death, but not genet death, and can thus be removed from the genet’s gene pool while preserving the genet itself. Meanwhile, neutral and beneficial somatic mutations can accumulate in tissues, spread to new modules via polyp budding, and be dispersed over small spatial scales through colony fragmentation [ca. 70 m]. After these mutations are inherited by offspring, fitness variance is redistributed from the realm of within-colony to between-organism selection. Furthermore, these somatic mutations have the potential to disperse over much longer distances [hundreds of kilometers] by pelagic coral larvae that have inherited the mutations. Thus, the discovery of heritable somatic mutations in coral offspring represents a previously unconfirmed source for coral adaptation and evolution.”
In a world where rising temperatures are challenging coral ecosystems, it is good news that they may have another tool to leverage genetic diversity for increased adaptation to those environmental changes!
Inheritance of somatic mutations by animal offspring. Kate L. Vasquez Kuntz, Sheila A. Kitchen, Trinity L. Conn, Samuel A. Vohsen, Andrea N. Chan, Mark J. A. Vermeij, Christopher Page, Kristen L. Marhaver, Iliana B. Baums. Sci. Adv., 8 (35). https://doi.org/10.1126/sciadv.abn0707
Penn State. (2022, August 31). Corals pass mutations acquired during their lifetimes to offspring. ScienceDaily. Retrieved September 22, 2022 from www.sciencedaily.com/releases/2022/08/220831152728.htm