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Cover: Australian Poseidon meadow at the bottom of Shark Bay in August 2019. Photo: Rachel Austin / University of Western Australia

This may sound like science fiction, but a mysterious supercreature is frantically expanding its territory and is now the world's largest organism, covering an area of nearly 200 square kilometers.

Located at the westernmost tip of Australia, Shark Bay (Shark Bay) has the largest seabed (about 4800 square kilometers) and the richest seaweed resources in the world. In addition, it has the world's largest population of dugongs (manatees) and the world's oldest living fossil, stromatolite (stromatolite). Stromatolite, which was formed about 3.5 billion years ago, is a hard round sediment formed by a large number of algae carbonating calcium and magnesium in seawater and bonding and precipitating clastic particles. It contains the oldest life form in the world, but most of it is found as fossils. However, there are still living algae in the stromatolites found in Shark Bay. These are the largest in the world, so Shark Bay was listed as a World Heritage site in 1991.

The dark part of the aerial picture of Shark Bay is the seaweed plain. (photo Source: Angela Rossen / University of Western Australia) among the many geographical features of Shark Bay, the largest seaweed plain undoubtedly provides an important basis for rich biodiversity in the world. There are more than a dozen species of seaweed in Shark Bay, compared with only one or two species in other waters. Elizabeth Sinclair (Elizabeth Sinclair), an evolutionary biologist at the University of Western Australia (University of Western Australia), and her colleagues have focused on the large temperate seaweed plains of Shark Bay for years.

Two years ago, they collected samples of seaweed from 10 sites in the seaweed plain to explore their genetic differences on a spatial scale. But to their surprise, samples from nine of these sites were genetically tested to be the same plant, Australian Poseidon grass (Posidonia Australis). The furthest distance between these sites is 180 km.

In June, a team led by Elizabeth published a study on Australian Poseidon grass in the Proceedings of the Royal Society B (Proceedings of the Royal Society). They tested about 18000 genetic markers in collected seaweed samples and showed that the seaweed samples were genetically identical. It also means that this vast expanse of seaweed plain may have been self-cloned and expanded from the same seedling. Martin Martin Breed of Flinders University University in Australia was also involved in the study. Recalling the scene, he said, "We were thinking, 'what on earth is going on? we were completely baffled."

The seaweed spreads about 35cm a year, so it takes about 4,500 years for the meadow to spread to its current size-spanning at least 180km and covering 200sq km (about 28000 football fields). "this is the largest known clone on earth," Sinclair said. "it's kind of the largest organism in the world."

Diploid and polyploid seaweeds generally inhabit the coastline of the ocean and the estuaries of rivers (except Antarctica). They can reproduce sexually (sexual reproduction) by flowering, and they can also clone from me by horizontal extension of the rhizome (asexual reproduction). Offspring formed by sexual reproduction usually inherit half of the genome from their parents.

The protagonist of this new study, Poseidon Grass, has a very special genome. it inherits the complete genome of each parent and is a tetraploid organism. Almost all higher animals (including humans) and more than half of higher plants are diploids (diploid) with two sets of chromosomes. Polyploids are often sterile and unable to reproduce sexually, so the only way for this seaweed to reproduce is to clone itself.

Past studies have shown that a single seaweed can be cloned indefinitely without interference. At present, four large seaweed clones have been found all over the world. in addition to the seaweed plain in Shark Bay, Australia, the other three are located in the western Mediterranean, the Baltic Sea (Baltic Sea) and the Indian River Lagoon (Indian River Lagoon) in Florida, USA. However, no polyploid seaweed was found in the three seaweed plains. Because of this, the unique polyploid characteristics of Poseidon grass in Australia have aroused strong curiosity among researchers-why this seaweed has become the largest clone in the world. Why is only the seaweed in Shark Bay polyploid? Does polyploid play a unique role in the diversification and expansion of seaweed?

Polyploidy in extreme environments long ago, Shark Bay had a wide area of sandy sediments (sandy sediments, such as sandbars) that were not affected by waves, providing a shallow and concealed habitable environment for the cloning and growth of seaweed. But since thousands of years ago, carbonate deposits (carbonate sediments) have been accumulated here due to biological processes. The coastal riverbanks began to rise and the sea became shallower, leading to more extreme ecological conditions in Shark Bay-temperature fluctuations, light levels and increased salinity. In addition, there is a lack of phosphorus, which is necessary for plant growth. The researchers realized that the unique environment of Shark Bay created extreme environmental stress, which may be the reason for the widespread polyploidy of Poseidon grass in Australia.

The researchers examined the polyploid DNA sequence of Australian Poseidon grass by genotyping method, and compared it with some existing gene sequences of Australian Poseidon grass, and evaluated the genetic diversity (genetic diversity, population genetic difference) and genetic structure (genetic structure, temporal and spatial distribution of population genetic difference) of Australian Poseidon population under different gradients in Shark Bay. The diploid ancestors of this seaweed were found in only one of the 10 sites sampled, and their growth range was limited to one area. The polyploids formed by genomic hybridization between the diploid ancestor of Poseidon and another unknown seaweed are widely distributed in the remaining 9 loci.

Sampling work was carried out on the Poseidon meadow in Australia in June 2022. (photo Source: Rachel Austin / University of Western Australia) they speculate that the rapid rise in sea level in Shark Bay before 8500 caused the seaweed habitat to be flooded, which may have a great impact on the diploid ancestors of Poseidon grass in Australia. When the diploid can not adapt to the increasingly severe environmental pressure, the polyploid has a very high ability to adapt to the environment and can break through the restriction of the environment to the diploid. It also shows that polyploidy may be a successful evolutionary strategy. Genome-wide replication not only gives polyploids better genetic diversity, but also makes them more adaptable. This polyploid seaweed can grow in new and harsher environments and is so widely distributed through continuous self-cloning. This has created the spectacular seaweed plain we see today-the largest known cloned life in the world.

Evolutionary "accelerator" organisms replicate their genomes under environmental pressure to form polyploids, thereby improving genetic diversity and adaptability. This doesn't happen only in seaweed. Many plant species on land are widely distributed in the form of polyploid around the world, and are more common in extreme environments such as high latitudes. Compared with their diploid ancestors, they can adapt to some completely different environments and habitats, which also shows the superiority of polyploidy.

In fact, some systematic genomics studies have shown that all angiosperms (Angiospermae) have experienced at least one round of polyploidy. These polyploidy events are always related to the nodes of global climate upheaval. For example, about 120 million years ago, when the temperature on the earth's surface was extremely high, some major populations of angiosperms began to polyploid independently to enhance their adaptability to the environment. Under the stimulation of many rounds of environment, angiosperms not only improve the environmental adaptability, but also promote the formation of new species and biological diversity.

A study on the flowering angiosperm Fulu. Source: Pixabay can say that the emergence of genomic polyploidy in wild plants under the stimulation of the environment is the "accelerator" of evolution. In the field of agricultural science, polyploidy of species is more like an important tool to domesticate crops and cultivate new varieties. For example, the seedless watermelon we eat today is artificially cultivated triploid.

Seeing that polyploidy events can bring many benefits to plants, I wonder if anyone will wonder whether humans can evolve by doubling their genomes. In fact, most higher animals, especially humans, because of the huge amount of genetic information in the genome, are basically precisely coupled diploids, and the fault tolerance is very poor. It also means that once chromosome doubling occurs, it will always be accompanied by serious consequences.

For example, Down's screening, which is necessary for pregnant women before birth, is designed to check for chromosomal abnormalities in the fetus, which can lead to Down syndrome (Down Syndrome). This symptom, also known as trisomy 21 syndrome, refers to an abnormality on chromosome 21, which may be an extra chromosome, or an ectopic or chimerism on the chromosome, which is usually characterized by mental retardation, low immunity and infertility. A single chromosome doubling may lead to human aberration, and if the whole genome doubles, it may disappear soon after it appears.

Reference link:

Https://www.smithsonianmag.com/smart-news/worlds-largest-plant-is-a-seagrass-that-clones-itself-180980189/

Https://phys.org/news/2022-06-world-largest-vast-seagrass-meadow.html

Https://news.sciencenet.cn/htmlnews/2022/6/480129.shtm

Https://www.researchgate.net/publication/353773037_duobeihuahexunhua

Https://www.zju.edu.cn/2020/1009/c32861a2205607/pagem.htm

Links to papers:

Https://royalsocietypublishing.org/doi/10.1098/rspb.2022.0538

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by: bad, revised: clefable

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