Corals native to Caribbean waters may have broken a basic tenet of biology. When a mutation occurs in the body cells of nearly any animal on Earth, it’s not passed on to offspring via reproductive cells. But these corals do pass on such mutations, according to a new study published in Science Advances yesterday (August 31).
The finding points to a new source of genetic variation in coral that may hasten the rate at which these organisms evolve and adapt.
“It’s the first report of somatic mutations being passed [via reproductive cells] in an animal, as far as I can tell,” says Daniel Schoen, an evolutionary biologist at McGill University who was not involved in the study. “I’m not sure they make that claim, but I haven’t seen that before in the literature.”
Elkhorn coral (Acropora palmata), found in reefs throughout the Caribbean coast, grows in long, fractal-like branches that resemble its namesake—elk horns. Corals with the same genetic makeup can exist in massive mile-long colonies, indicating they’ve been around for centuries. Study coauthor Iliana Baums, a marine biologist at Penn State University, has been studying the corals for years, trying to make sense of why and how they live so long. As corals age, they accumulate a number of somatic mutations—that is, mutations in their bodies rather than their germ cells—that they must live with.
Baums is also interested in the genetic diversity of corals, so she studies how they reproduce. Elkhorn coral reproduces both sexually and asexually. During asexual reproduction, a portion of the parent coral buds off and attaches to the seabed nearby. Sexual reproduction is a bigger event: Every August, shortly after the full moon, all of the corals in a reef sync up to release their reproductive cells at once, which can then merge in the water. Eggs typically require fertilization by sperm from different colonies, then turn into larvae and swim up to hundreds of miles away to establish a new colony. “It’s an absolutely stunning experience,” says Baums. “It looks like it snows, but the wrong way around, from the bottom.”
Elkhorn coral during a spawning event
It was while Baums was investigating such a fertilization event at a study site in Curaçao that she accidentally made the discovery that elkhorn coral could pass along somatic mutations. She was looking for which of the surrounding corals had fertilized eggs at the site, and, to her surprise, found that the corals had self-fertilized.
During the process of comparing the genomes of the parent corals with the offspring that arose via self fertilization and the nearby clones that arose via budding, she and her team realized “that this really old clone had accumulated a number of somatic mutations, and those somatic mutations ended up in those [offspring],” she says. Since the corals had reproduced asexually or self-fertilized, limiting the number of genetic possibilities that could occur in the offspring, she could relatively easily search for somatic mutations. She found 268 somatic mutations in the parent clone, with each nearby clone that arose from the parent sharing between 2 and 149 of these mutations. Around 50 percent of the mutations found in the parent clone also showed up in offspring that were produced via self-fertilization. “It’s really unusual for an animal,” says Baums.
It turned out that self-fertilization wasn’t required to pass along the somatic mutations: After unfertilized eggs from the clone in Curaçao joined with sperm from a coral in Florida, they produced offspring that also shared somatic mutations with the Curaçao parent
Previously, it was thought that in order for mutations to be passed on to offspring in animals, they need to be present in the reproductive or germline cells. Mutations that develop throughout life are thought to remain only in our body cells. Baums says that the researchers aren’t sure how germ cells are acquiring these mutations, but they hypothesize that the somatic cells may have dedifferentiated into stem cells, and then redifferentiated into germ cells.
“This is an observation we made that’s just really stunning. It’s just unexpected,” Baums says. She says that somatic mutations might be a previously-unrecognized source of genetic diversity for corals, which might influence how they adapt in response to stressors such as ocean warming and acidification. “We really want to understand what evolutionary impact the somatic mutations might have. Are they really a source for novelty and an adaptation for these corals that might be significant, given the huge stressors these corals are exposed to at the moment?”
But Michael Lynch, a geneticist at Arizona State University who was not involved in the work, says that the conclusions the authors suggest are “a bit of an oversell.” He says he’s “a bit skeptical that . . . what [the authors] are seeing here could have implications for our understanding of evolutionary rates.”
He adds, “it’s not clear that the mutation rate differs between somatic vs. germline . . . and if it does not, then nothing is gained or lost” in terms of how much variation parents are providing to offspring.
Much of the process by which corals transfer somatic mutations to reproductive cells remains a mystery, but Baums says her team intends to find out.