A single plant or ‘clone’ of the Poseidon’s ribbon weed (Posidonia australis), a species of seagrass that occurs in the southern half of Australia, in the UNESCO World Heritage Site of Shark Bay is at least 4,500 years old and spans more than 180 km in fragmented, near-shore meadows.
Poseidon’s ribbon weed (Posidonia australis) meadow in Shark Bay, Western Australia. Image credit: Rachel Austin, University of Western Australia.
The UNESCO World Heritage Site of Shark Bay is home to one of the largest continuous seagrass meadows in the world.
Known as Gathaagudu to the traditional custodians, the Malgana Peoples, it creates habitat for a biodiverse marine environment including 12 species of seagrass.
The clear, shallow waters across most of the Bay mean that seagrass meadows are exposed to saturated light levels, experience a large annual range in temperature, typically 17-26 degrees Celsius although temperatures can exceed 30 degrees Celsius in summer.
Large, perennial, seagrass meadows of the Poseidon’s ribbon weed and the wire weed (Amphibolis antarctica) dominate much of this marine ecosystem, creating an ideal location in which to study evolutionary change and adaptation.
“Our project began when researchers wanted to understand how genetically diverse the seagrass meadows in Shark Bay were, and which plants should be collected for seagrass restoration,” said Dr. Elizabeth Sinclair, an evolutionary biologist in the School of Biological Sciences and the Oceans Institute at the University of Western Australia and the Kings Park Science.
“We often get asked how many different plants are growing in seagrass meadows and this time we used genetic tools to answer it.”
“We sampled seagrass shoots from across Shark Bay’s variable environments and generated a ‘fingerprint’ using 18,000 genetic markers,” added Jane Edgeloe, a student in the College of Science and Engineering at Flinders University.
“The answer blew us away — there was just one! That’s it, just one plant has expanded over 180 km in Shark Bay, making it the largest known plant on Earth.”
“The existing 200 km2 of Poseidon’s ribbon weed meadows appear to have expanded from a single, colonizing seedling.”
“Our research presents a real ecological conundrum,” said Dr. Martin Breed, an ecologist in the College of Science and Engineering at Flinders University.
“This single plant may in fact be sterile; it doesn’t have sex. How it’s survived and thrived for so long is really puzzling.”
“Plants that don’t have sex tend to also have reduced genetic diversity, which they normally need when dealing with environmental change.”
“Our seagrass has seen its fair share of environmental change too. Even today, it experiences a huge range of average temperatures; from 17 to 30 degrees Celsius. Salinities from normal seawater to double that. And from darkness to extreme high light conditions.”
“These conditions would typically be highly stressful for plants. Yet, it appears to keep on going.”
“How does it do it? Well, we reckon its genes are very well-suited to its local, but variable, environment and it also has subtle genetic differences across its range that help it deal with the local conditions.”
“What makes this seagrass plant unique from other large seagrass clones, other than its enormous size, is that it has twice as many chromosomes as its oceanic relatives, meaning it is a polyploid,” Dr. Sinclair said.
“Whole genome duplication through polyploidy — doubling the number of chromosomes — occurs when diploid ‘parent’ plants hybridize.”
“The new seedling contains 100% of the genome from each parent, rather than sharing the usual 50%.”
“Polyploid plants often reside in places with extreme environmental conditions, are often sterile, but can continue to grow if left undisturbed, and this giant seagrass has done just that.”
“Even without successful flowering and seed production, it appears to be really resilient, experiencing a wide range of temperatures and salinities plus extreme high light conditions, which together would typically be highly stressful for most plants.”
The team’s paper was published in the journal Proceedings of the Royal Society B.
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Jane M. Edgeloe et al. 2022. Extensive polyploid clonality was a successful strategy for seagrass to expand into a newly submerged environment. Proc. R. Soc. B 289 (1976): 20220538; doi: 10.1098/rspb.2022.0538
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