In Shark Bay, off the western tip of Australia, seagrass meadows line the ocean floor, rippling with the currents and nibbled by dugongs, cousins of the Florida manatee. A new study has revealed something unexpected about these sea grasses: many of them are the same individual plant that has been cloning itself for about 4,500 years.
Sea grass – not to be confused with seaweed, which is seaweed – is Poseidon’s ribbon grass, or Posidonia australis. Jane Edgeloe, PhD from the University of Western Australia. candidate and author of the article, compares its appearance to a spring onion.
Ms Edgeloe and her colleagues made their discovery while genetically surveying Posidonia grasses in different areas of Shark Bay, where she dived into the shallow waters and pulled up shoots of Posidonia from 10 different meadows. On land, researchers have analyzed and compared the DNA of grasses.
They published their findings on Wednesday in the journal Proceedings of the Royal Society B. It turned out that the DNA of many of these seemingly different plants was virtually identical. Elizabeth Sinclair, also from the University of Western Australia and author of the study, recalled the excitement in the lab when she realised, “It’s only one plant.”
While some of Shark Bay’s northern grasslands reproduce sexually, the rest of its Posidonia clones itself by creating new shoots that branch out from its root system. Even separate grasslands were genetically identical, indicating that they were once connected by now severed roots. Based on the age of the bay and how quickly the sea grass grows, the researchers guess the Shark Bay clone is around 4,500 years old.
Besides being a clone, grass 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 helpful to plants. However, this can lead to sterility: much clonal grass does not flower and can only reproduce by continuing to clone itself.
This combination of extra genes and cloning could have been the key to the weed’s survival during a time of ancient climate change: the cloning made it easier to reproduce because the weed didn’t have to worry about finding a mate . The extra genes could have given seagrass “the ability to cope with a wide range of conditions, which is a good thing in climate change,” Dr Sinclair said.
Shark Bay’s Posidonia not only survived this ancient climate change, it spread. And spread. And spread some more.
Today, it is arguably the largest living organism in the world. The Utah Pando, a clonal colony of 40,000 root-bound aspens, is the reigning “largest individual plant”, covering an area more than 80 football fields. The Humongous mushroom is even larger, weaving a web of mycelial tendrils underground and under tree bark across 3.5 square miles of Oregon’s Malheur National Forest. By comparison, the clonal seagrass in Shark Bay is 77 square miles, about the size of Cincinnati.
As the Shark Bay clone has grown to enormous size and age, the question remains whether it would be able to withstand modern climate change. Julia Harenčár, Ph.D. candidate at the University of California, Santa Cruz, who was not involved in the study, praised the project for “trying to understand in more detail why polyploidy has been beneficial to these large environmental tipping points,” which could offer lessons for the climate crisis.
Seagrass beds are particularly important to protect, says Marlene Jahnke, a biologist at the University of Gothenburg in Sweden, who was also not involved in the study. She added, “They’re comparable to coral reefs, really, in that they host a lot of other species,” in addition to purifying water and storing atmospheric carbon.
Although the stakes are high for sea grasses, Dr Sinclair remains hopeful the Shark’s Bay Posidonia will retain its status as the world’s largest living plant: despite being damaged in a 2010 heat wave in 2011, “we saw a lot more shoot increase, a lot more leaf density, so it’s recovering,” she said. “I think this polyploid is actually probably in a pretty good state in terms of persistence.”