HomeWorld's largest plant Posidonia Australis

World’s largest plant Posidonia Australis

Meet the “World’s largest plant” a single seagrass clone that stretches 180 km in Shark Bay in Western Australia.

The next time you go scuba diving or snorkeling, take a close look at those wonderfully long, bright green ribbons, which move with the ebb and flow of the water. They are seagrasses: marine plants that produce flowers, fruits and seedlings annually, like their terrestrial relatives.

Ways of growing:

These underwater seagrass meadows grow in two ways: by sexual reproduction, which helps them generate new combinations of genes and genetic diversity, and also by extension of their rhizomes, the underground stems from which roots and shoots emerge.

To find out how many different individual plants grow in a seagrass meadow, you need to analyze their DNA. We did this for seagrass beds called Posidonia australis in the sun-kissed shallows of Western Australia’s Shark Bay World Heritage Area.

The result left us speechless: everything was a single plant. A single plant has expanded over a stretch of 180 km, making it the largest known plant on Earth.

How did it evolve?

What makes this seagrass unique from the others, apart from its enormous size, is that it has twice as many chromosomes as its relatives. This makes it what scientists call a “polyploid.”

Most of the time, a seagrass seedling will inherit half of the genome from each of its parents. Polyploids, however, carry the complete genome of each of their parents.

There are many species of polyploid plants, such as potatoes, canola, and bananas. In nature they usually reside in places with extreme environmental conditions.

Polyploids are often sterile, but can continue to grow indefinitely if left undisturbed. This seagrass has done just that.

We collected shoot samples from ten Shark Bay seagrass beds, in waters where salt levels range from normal ocean salinity to almost twice as salty. Across all samples, we studied 18,000 genetic markers to show that 200 km² of weedy meadows expanded from a single colonizing seedling.

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