Posidonie australe, prairie à Shark Bay, Australie occidentale. Crédit : Rachel Austin, Université d’Australie occidentale.
Des scientifiques australiens pensent avoir découvert la plus grande plante du monde – et ils estiment qu’elle a au moins 4 500 ans.
Des chercheurs de l’Université d’Australie occidentale (UWA) et de l’Université Flinders ont localisé cette plante ancienne et incroyablement résistante, qui s’étend sur 180 km (112 miles).
La découverte d’une seule plante ou “clone” de l’herbe de mer. Posidonia australis dans les eaux peu profondes et ensoleillées de la zone de patrimoine mondial de Shark Bay en Australie occidentale est détaillée dans une nouvelle étude publiée dans Proceedings of the Royal Society B.
L’auteur principal, le Dr Elizabeth Sinclair, biologiste évolutionniste de l’École des sciences biologiques et de l’Institut des océans de l’UWA, explique que le projet a débuté lorsque les scientifiques ont voulu comprendre la diversité génétique des prairies sous-marines de Shark Bay et déterminer quelles plantes devaient être collectées pour la restauration des prairies sous-marines.
“On nous demande souvent combien de plantes différentes poussent dans les prairies sous-marines. Cette fois, nous avons utilisé des outils génétiques pour répondre à cette question”, explique le Dr Sinclair.
Jane Edgeloe, étudiante chercheuse à l’UWA et auteur principal de l’étude, explique que l’équipe a échantillonné des pousses d’herbes marines dans les environnements variables de Shark Bay et a généré une “empreinte digitale” à l’aide de 18 000 marqueurs génétiques.
Les eaux salées et peu profondes de Shark Bay. Crédit : Angela Rossen
“La réponse nous a époustouflés – il n’y en avait qu’un seul !” a déclaré Mme Edgeloe. “C’est ça, une seule plante s’est étendue sur 180 km dans Shark Bay, ce qui en fait la plus grande plante connue sur Terre”.
“Les 200 km existants2 de prairies d’herbes rubanées semblent s’être développées à partir d’une seule plantule colonisatrice.”
Co-auteur Flinders University ecologist Dr. Martin Breed was part of the research group. He says the study presents a real ecological conundrum.
“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,” says Dr. Breed, from the College of Science and Engineering at Flinders University.
“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 °C. 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,” Dr. Breed says.
Dr. Sinclair said 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.
“Whole genome duplication through polyploidy – doubling the number of chromosomes – occurs when diploid ‘parent’ plants hybridize. The new seedling contains 100 percent of the genome from each parent, rather than sharing the usual 50 percent,” Dr. Sinclair says.
“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 researchers have now set up a series of experiments in Shark Bay to understand how this plant survives and thrives under such variable conditions.
For more on this discovery, see World’s Largest Plant Stretches 112 Miles in Western Australia’s Shark Bay.
Reference: “Extensive polyploid clonality was a successful strategy for seagrass to expand into a newly submerged environment” by Jane M. Edgeloe, Anita A. Severn-Ellis, Philipp E. Bayer, Shaghayegh Mehravi, Martin F. Breed, Siegfried L. Krauss, Jacqueline Batley, Gary A. Kendrick and Elizabeth A. Sinclair, 1 June 2022, Proceedings of the Royal Society B.
DOI: 10.1098/rspb.2022.0538
The research was made possible through a collaboration between UWA, Flinders University and Kings Park Science (WA Department of Biodiversity Conservation and Attractions). It was funded by the Australian Government’s National Environmental Science Program Marine Biodiversity Hub and the Australian Research Council.
