In Shark Bay, on Australia’s westernmost tip, natural grasslands of seagrass cover the ocean floor, rippling in the current and being nibbled by dugongs, cousins of manatees. A new study has revealed something unexpected about these seagrasses: Many of them are the same individual plant that has been cloning itself for some 4,500 years.
Seagrass –not to be confused with kelp– is Poseidon’s weed, or Posidonia THEusstralis. Jane Edgeloe, a doctoral candidate at the University of Western Australia and author of the article, compares its appearance to the so-called chives.
Edgeloe and her colleagues made their discovery as part of a genetic survey of Posidonia grasses in different areas of Shark Bay, where she dove into the shallows and pulled Posidonia shoots from ten different natural grasslands. On land, the researchers analyzed and compared the grasses’ DNA.
They published their results Wednesday in the Proceedings of the Royal Society B. The DNA of many of these seemingly different plants turned out to be virtually identical. Elizabeth Sinclair, also from the University of Western Australia and author of the study, recalled the emotion in the lab when she realized: “It’s just one plant.”
While some of Shark Bay’s northern natural grasslands reproduce sexually, the rest of their Posidonias clone themselves creating new shoots that branch off from their root system. Even separate grasslands were genetically identical, indicating that they were once connected by now-cut roots. Based on the age of the bay and how quickly the seagrass grows, the researchers surmise that the Shark Bay clone is around 4,500 years old.
In addition to being a clone, the herb appears to be a hybrid of two species and has two complete sets of chromosomes, a condition called polyploidy. While polyploidy can be lethal to animal embryos, it can be harmless or even useful in plants. However, it can result in sterility. Much of the clonal grass does not flower, and can only reproduce itself by continually cloning itself.
This combination of extra genes and cloning may have been the key to the grass’s survival during an ancient period of climate change. Cloning made reproduction easier because the grass didn’t have to worry about finding a mate. The extra genes could have given seagrass “the ability to handle a wide range of conditions, which is great in climate change,” Sinclair said.
Shark Bay Posidonia has not only survived this ancient climate change, it has spread. And spread. And spread more.
Today, it is arguably the largest living organism in the world. Utah’s Pando (USA), a clonal colony of 40,000 poplars connected by their roots, is the reigning “largest single plant”, covering an area greater than 80 football fields.
O humongous fungi (huge fungus) is even bigger, weaving a network of mycelial tendrils underground and under tree bark in 9 km² of Malheur National Forest in Oregon (USA). By comparison, the clonal seagrass in Shark Bay is 199 km², roughly the size of Cincinnati.
Although the Shark Bay clone has reached enormous size and age, the question remains as to whether it would be able to withstand modern climate change. Julia Harencár, a doctoral student at the University of California at Santa Cruz, who was not involved in the study, praised the project for “trying to understand in more detail why polyploidy has been advantageous at these major environmental tipping points,” which could offer lessons for climate crisis.
Seagrasses are particularly important to protect, said Marlene Jahnke, a biologist at the University of Gothenburg (Sweden), who was also not involved in the study. She added, “they are comparable to coral reefs actually in the sense that they host many other species,” as well as purifying water and storing atmospheric carbon.
While the stakes are high for seagrass, Sinclair remains hopeful that Shark Bay’s Posidonia will maintain its position as the world’s largest living plant. Although it was hampered by a heat wave in 2010 and 2011, “we’ve seen a lot more increase in buds, a lot more leaf density, so it’s recovering,” said the scientist. “I think this polyploid is probably in a pretty good state in terms of persistence.”
