Corals have an evolutionary superpower. Adult corals can pass聽on mutations they have acquired during their lives to their聽offspring, overturning a聽long-standing belief that no聽animals can hand down such mutations 鈥� although most can鈥檛.
鈥淛uvenile corals inherited mutations that were acquired during the parents鈥� lifespan,鈥� says聽Iliana Baums at Pennsylvania State University. 鈥淚t has not been observed before in animals, but it聽has been observed in plants.鈥�
Corals belong to one of the oldest animal groups. They are similar to plants in many ways, such as spending most of their lives fixed in one place, in their case on reefs, says Baums. One way聽that corals and their relatives differ from mammals or birds is in聽their germ line, the cells in their bodies that form eggs or sperm.
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In most animals, including humans, the germ-line cells are strictly separated from the rest of the body. This limits which genetic mutations can be passed on. For example, a gene might mutate in one cell of a person鈥檚 body and change that cell鈥檚 behaviour聽鈥� perhaps turning it cancerous聽鈥� but the mutation won鈥檛 be passed to their children. Only mutations in germ-line cells can be inherited.
Biologists already knew that coral germ lines aren鈥檛 like this. Adult corals have groups of primordial stem cells that can give rise to both germ-line cells and body cells. Body cells sometimes change back into stem cells, and聽then into germ-line cells. This聽blurs the line between the germ line and the rest of the body.
Baums and her colleagues have聽now found evidence that mutations that arise during a聽coral鈥檚 lifespan can enter the germ聽line and be passed on.
They studied elkhorn corals (Acropora palmata) from Florida and Cura莽ao. These live in colonies of genetically identical polyps that聽divide asexually, allowing the聽colony to grow. They also release sperm and eggs into the water that were thought to need to聽encounter sperm or eggs from another colony to develop.
The study began with a peculiar observation: some eggs developed into larvae without being fertilised. To confirm this, the team collected more larvae and compared their genes with the parent colony. The聽larvae only contained genes from the colony, albeit reshuffled. 鈥淭here was no input of foreign sperm,鈥� says Baums. The team still isn鈥檛 quite sure what happened.
However, there was an even bigger surprise lurking. The team knew that individual polyps in the colony weren鈥檛 quite genetically identical. It has been there for many years, and some of the polyps had acquired mutations during their lives that weren鈥檛 there in the founding individual.
The analysis revealed that some of these mutations were present in the larvae. The finding indicates that corals can pass on new genetic variants, and evolve, in a way that no other animal is known to do so.
鈥淭hey ran all thinkable controls,聽therefore I think technically it鈥檚 absolutely sound,鈥� says Thorsten Reusch at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany.
Baums and Reusch both say it is important not to misinterpret the finding. One possible misreading concerns the long-disproved idea聽that acquired traits can be inherited and that this explains how new species evolve. For example, the long necks of giraffes were imagined to have arisen because early giraffes stretched to reach tall trees, making their necks longer, and they passed this to their offspring. The idea is sometimes known as Lamarckism, after biologist Jean-Baptiste Lamarck.
However, in Lamarckism, mutations are driven by an animal鈥檚 actions, so the creature has some control over which genes it passes on. In reality, mutations arise randomly and any that benefit an animal may help it survive and produce offspring 鈥� and that seems to be just as true of the corals. 鈥淚t doesn鈥檛 reintroduce Lamarck,鈥� says Reusch.
But it means corals have a way of creating genetic diversity even when reproducing asexually. In this, they again resemble plants.
Reusch and his colleagues showed in that colonies of seagrass can undergo a聽similar process, in which clones pass on acquired mutations. Beneficial ones can spread to dominate entire seagrass colonies.
Baums and her team have yet to find evidence that any of the coral mutations are beneficial, but they plan to investigate this next.
Reference: bioRxiv, DOI:
