C.orals originating in Caribbean waters may have broken a basic principle of biology. When a mutation occurs in the body cells of almost any animal on Earth, it is not passed on to offspring through the reproductive cells. But these corals carry 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 could accelerate the rate at which these organisms evolve and adapt.
“It is the first report of transmitted somatic mutations [via reproductive cells] in an animal, as far as I know, “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’ve never seen it before in the literature. “
Elkhorn coral (Acropora palmata), found in coral reefs across the Caribbean coast, grows in long, fractal-like branches reminiscent of its namesake: elk antlers. Corals with the same genetic makeup can exist in huge, mile-long colonies, indicating that they have been around for centuries. Study co-author Iliana Baums, a marine biologist at Penn State University, has been studying corals for years, trying to make sense of why and how they live so long. As corals age, they accumulate a series of somatic mutations – that is, mutations in their bodies rather than in their germ cells, which they have to live with.
Baums is also interested in the genetic diversity of corals, so she studies how they reproduce. The Elkhorn coral reproduces both sexually and asexually. During asexual reproduction, part of the parent coral germinates and attaches to the nearby seabed. Sexual reproduction is a bigger event: every August, shortly after the full moon, all corals in a reef synchronize to release their reproductive cells at once, which can then merge into the water. Eggs typically require sperm fertilization from several colonies, so they 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’s snowing, but the wrong way, from below.”
Elkhorn coral during a spawning event
It was while Baums was studying such a fertilization event at a study site in Curaçao that he accidentally made the discovery that the Elkhorn coral could transmit somatic mutations. He was looking for which of the surrounding corals had fertilized the eggs at the site and, to his surprise, found that the corals had self-fertilized.
During the process of comparing the genomes of the parent corals with the offspring born by self-fertilization and the neighboring clones that formed by budding, she and her team realized “that this really old clone had accumulated a series of somatic mutations and those mutations ended in those [offspring]”she says. Because the corals had reproduced asexually or self-fertilized, limiting the number of genetic possibilities that could occur in the offspring, she could search for somatic mutations relatively easily. She found 268 somatic mutations in the parent clone, with each nearby clone arisen. from the parent who shared between 2 and 149 of these mutations. About 50 percent of the mutations found in the parent clone also manifested in the offspring produced through self-fertilization. “It’s really unusual for an animal,” says Baums.
It turned out that self-fertilization was not necessary to transmit the somatic mutations: after unfertilized eggs from the Curaçao clone joined the sperm of a coral in Florida, they produced offspring that also shared somatic mutations with the parent of Curaçao
Previously, it was thought that in order for mutations to be transmitted to offspring in animals, they must be present in reproductive or germline cells. Mutations that develop over the course of life are thought to remain only in the cells of our body. Baums says the researchers aren’t sure how germ cells are acquiring these mutations, but speculate that somatic cells may have de-differentiated into stem cells and then redifferentiated into germ cells.
“This is an observation we made and it is truly astounding. It’s just unexpected, ”says Baums. He says somatic mutations could be a previously unrecognized source of genetic diversity for corals, which could affect how they adapt in response to stressors like ocean warming and acidification. “We really want to understand what evolutionary impact somatic mutations might have. Are they really a source of novelty and an adaptation for these corals that could be significant given the enormous stressors these corals are exposed to right now?
But Michael Lynch, an Arizona State University geneticist who was not involved in the work, says the authors’ suggested conclusions are “a bit exaggerated.” He says he’s “a little skeptical that. . . what [the authors] they are seeing here could have implications for our understanding of evolutionary rates.
He adds, “It is not clear whether the mutation rate differs between the somatic and germ lines. . . and if it doesn’t, then nothing is gained or lost ”in terms of how much variation the parents are providing to the offspring.
Much of the process by which corals transfer somatic mutations to reproductive cells remains a mystery, but Baums says his team intends to find out.