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A lush, rhizomatous seagrass meadow in the Mediterranean. Creative Commons Arnaud-Haond et al. 2012, PLoS One. Click image for link.
In the world of gigantic plant and fungus clones, there is no lack of contenders for the title Oldest, Heaviest, and Most Ginormous. The humongous fungus is a mushroom-producing parasitic fungus called Armillaria bulbosa that spans 635 m, is estimated to weigh 21,000 pounds, and is over 1,500 years old. Pando, (aka "The Trembling Giant") is a half-kilometer wide grove of genetically identical aspen trees estimated to be over 10,000 years old and perhaps much, much older. And "King Clone" is a 11,700 year old creosote clone in California's Mojave Desert. But it's time for them to make room for MegaGrass.
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Posidonia oceanica is a seagrass -- a flowering plant that followed the path of (or blazed the trail for) whales, sea turtles, sea snakes, and any number of other creatures who sampled terrestrial life, decided it was just not for them, and headed back to the ocean. There aren't many land plants who have done this, but there are several other sea grasses related to P. oceanica. It lives in the Mediterranean Sea. Balls of fiber from the plant sometimes wash up on shore like giant marine hairballs. When it's feeling up to sex it makes floating fruits the Italians have dubbed "the olive of the sea". But it can also grow clonally, or asexually, as many plants can, propagating itself slowly but steadily by above or below ground stems called rhizomes.
Scientists have found that mild-mannered Mediterranean sea grass is far older than anyone guessed, and published their results Feb. 1 in the journal PLoS ONE. The scientists began their investigation because they suspected that the age and size of plant clones were being grossly underestimated, because, simply put, the largest clonal size for many plants tended to be the largest sample size of the researchers. In other words, when the scientists took samples a maximum of 1000 m apart, the largest clones they found tended to be ... wait for it .... 1000 m wide. As a result, the authors suggest the clonal capability, size and age of plants has been grossly underestimated.
So, as a test case, they looked at the sea grass P. oceanica. Preveiously, the largest clonal size measured was 80 m . . . but so was the largest sample size. Using features of DNA called microsatellites as markers of genetic identity, they sampled 40 locations across 3500 km of the Mediterranean. They found very large clones in about 3 to 9% of samples, ranging from one to 15 kilometers in size. And remember -- the previous measured maximum clone size was 80 m. Based on known rates of P. oceanica growth (4 cm/year was the number they used), they estimate their new clones are up to tens of thousands of years old, and could theoretically be as much as 200,000 years old for the largest clones, though the authors consider an age that great unlikely for a variety of reasons. Still, if that were the case, that puts the sprouting of the parents of these clones in the late Pleistocene.
How do cloned organisms escape the problems of asexuality -- namely, the accumulation of deleterious mutations over time that for which sexual recombination normally acts as a pressure release valve? The authors conclude that for P. oceanica, there are several answers: the clonal population of plants is so large that mutations are unlikely to affect a great portion of them, the mutation rate is extremely low in this plant, and the genotype of this organism is supremely adapted to its environment and very plastic; that is, the genes of this plant allow it to engage in a certain amount of adaptation to changing environments (different size; different growth habit, etc.) without having to shuffle its genes through sex.
Prior to this study, botanists knew that P. oceanica is among the slowest-growing and longest-lived plants on Earth and dominates Mediterranean underwater meadows. It lacks major predators or competitors. But humans are changing that. In spite of their incredibly long lives and steady growth, the authors of the study noted that ancient Mediterranean seagrass meadows are shrinking several hundred times faster (at a rate of about 5% per year) than the rate at which they achieved their present size. Given how slowly seagrass grows, this isn't good news.
In any case, the authors made their point about the possibility of massive and ancient plant and fungus clones as yet undetected all around us. Who will be next? MegaGrass, meet Mossra.