Coral Can Pass Somatic Mutations Down to Offspring

Scientists from Pennsylvania State University have published a new study in Science Advances demonstrating that coral can pass somatic mutations to future generations.

The Weissman barrier and germline mutations

Over recent decades, we have witnessed remarkable progress in the field of DNA research, owing to the completion of landmark projects such as the Human Genome Project (HGP) and advancements in modern technologies such as microarrays and next-generation sequencing. The ability to identify genomic mutations and characterize the phenotypic consequence of such mutations has furthered our understanding of genetic diversity and evolution.

A core tenet of biology was first outlined by August Weissman in 1892 – known as the “Weissman barrier”. This theory proposes that germ cells – or reproductive cells – are “fundamentally separate” from somatic cells. “For most animals, a new genetic mutation can only contribute to evolutionary change if it occurs in a germline or reproductive cell, for example in an egg or sperm cell. Mutations that occur in the rest of the body, in the somatic cells, were thought to be evolutionarily irrelevant because they do not get passed on to offspring,” says Iliana Baums, professor of Biology at Penn State.

A new discovery by Baums and colleagues challenges this principle: “Corals appear to have a way around this barrier that seems to allow them to break this evolutionary rule,” she says.

Single-parent coral provide a tool for studying somatic mutation inheritance

In most animals, the reproductive cells are isolated from somatic cells in early development. This means that only mutations occurring in the reproductive cells can be passed on to offspring, which can occur rarely and makes evolution a slow process. “However, for some organisms, like corals, the segregation of reproductive cells from all other cells may occur later in development or may never occur at all, allowing a path for genetic mutations to travel from a parent’s body to its offspring," Kate Vasquez Kuntz, a graduate student at Penn State and the co-lead author of the study explains. “This would increase genetic variation and potentially even serve as a “pre-screening” system for advantageous mutations.” Corals are not the only animals in which the germline segregates later in development – this process also occurs in flatworms, sea urchins and hydrozoans.

Elkhorn corals (Acropora palmata) are found in the Caribbean and resemble an “elk” tree due to their many branches. They can reproduce both sexually and asexually.

In 2018, the Penn State researchers observed that coral eggs collected from a single parent colony in Curaçao could spontaneously developed without the need for a second coral. “This single-parent reproduction allowed us to search more easily for potential somatic mutations from the parent coral and track them into the offspring by simplifying the total number of genetic possibilities that could occur in the offspring,” Dr. Sheila Kitchen, co-lead author of the study, a postdoctoral researcher at Penn State and the California Institute of Technology, comments.

Somatic mutations identified in offspring of asexually-reproducing coral

Taking samples from 10 different locations on a large Elkhorn colony that had produced single-parent offspring, and samples from 5 neighboring colonies, the researchers used microarray technology to conduct a genotype analysis – searching for DNA differences between the samples.

They found that the six separate colonies belonged to the same coral genotype; put simply – they were all clones derived from one original colony, produced via asexual reproduction. Any genetic variation identified between these corals would therefore have to be a product of somatic mutation, not germline.

Vasquez Kuntz and team discovered a total of 268 somatic mutations in the samples, with each of the corals possessing between 2–149 mutations individually. When conducting genetic analysis on the offspring of the single-parent Elkhorn coral colony, they found that ~50% of the somatic mutations had been inherited.

How? The molecular mechanisms that enable somatic mutation inheritance are yet to be determined. The researchers hypothesize that segregation between somatic and germline cells in corals might be incomplete – i.e., some somatic cells may be able to maintain the ability to transform into germ cells.

“Because corals grow as colonies of genetically-identical polyps, somatic mutations that arise in one coral polyp can be exposed to the environment and screened for their utility without necessarily affecting the entire colony,” said Baums.

Cells that carry harmful mutations may therefore die off, providing an evolutionary advantage to the coral colony – a characteristic that could help them defend against climate change. “If these mutations can then be passed on to offspring — as we have now demonstrated — it means that corals have an additional tool that might be able to speed up their adaptation to climate change,” Baums concludes.

Reference: Vasquez Kuntz KL, Kitchen SA, Conn TL, et al. Inheritance of somatic mutations by animal offspring. Sci Adv. 8(35):eabn0707. doi:10.1126/sciadv.abn0707.

This article is a rework of a press release issued by Pennsylvania State University of Sydney. Material has been edited for length and content.