In a groundbreaking discovery, scientists have identified a small, seemingly inconspicuous fern with the largest genome of any organism on Earth. The fern, known as Tmesipteris oblanceolata, grows on the island of New Caledonia and has astounded researchers with its immense genetic material, which outstrips that of humans by a phenomenal margin.
This unassuming plant has now dethroned the previous record holder, the Japanese flowering plant Paris japonica. T. oblanceolata’s genome measures a staggering 160 gigabase pairs (Gbp), seven percent larger than the 148.89 Gbp of P. japonica. By comparison, the human genome contains only 3.1 Gbp.
To put this into perspective, if the DNA from one of T. oblanceolata’s cells were unraveled, it would stretch out to an extraordinary 106 meters (approximately 350 feet) – taller than London's famous Big Ben tower. In contrast, human DNA unraveled would cover a mere two meters.
Dr. Ilia Leitch of the UK's Royal Botanic Gardens, Kew, expressed her amazement at the discovery. “We thought we had already reached the biological limit with Paris japonica. This fern pushes the extreme boundaries of biology,” she said.
During their exploration in 2023, scientists from various institutions, including the Royal Botanic Gardens, Kew and the Institut Botànic de Barcelona in Spain, ventured to Grand Terre in New Caledonia. Their mission was to study this peculiar fern species. Led by Dr. Jaume Pellicer, the team carefully collected samples, which were then analyzed to determine the size of their genomes.
This involved isolating the nuclei of thousands of cells, staining them with a dye, and measuring the amount that bound to the DNA within each nucleus—the more dye, the bigger the genome. The analysis revealed T. oblanceolata to contain a record-breaking 160 Gbp.
Such a monumental discovery has propelled this humble fern into fame, earning it three Guinness World Records titles for the largest genome, largest plant genome, and largest fern genome. Adam Millward, managing editor of Guinness World Records, commented, "To think this innocuous-looking fern boasts 50 times more DNA than humans is a humbling reminder that there's still so much about the plant kingdom we don't know, and that record holders aren't always the showiest on the outside".
Understanding the genome size of an organism is crucial as it provides all the genetic instructions for life. A genome consists of long strands of DNA coiled up within the nucleus of each cell. While many plants possess large genomes, the reasons behind this vast amount of genetic material remain a mystery.
The concept of DNA and genomes may seem daunting to some. In simple terms, DNA comprises genetic instructions similar to a manual that guides an organism on how to develop, function, and survive. All of an organism’s DNA together is referred to as its genome.
Among the living world, there is considerable variation in genome sizes. For instance, the marbled lungfish and certain salamanders have huge genomes, while others like the carnivorous Genlisea aurea have extremely small genomes of just 0.06 Gbp.
Interestingly, having an enormous genome could be considered a disadvantage. Large genomes necessitate bigger cells to contain all the DNA, leading to larger pores in leaves and slower growth rates. They also make duplication of DNA more complex, thereby affecting reproductive capabilities.
Slow-growing, perennial plants, such as T. oblanceolata, often have massive genomes. However, researchers like Dr. Leitch argue that despite the apparent disadvantage, there is much we do not yet understand. Some scientists consider a large portion of this DNA to be "junk DNA," but Dr. Leitch believes this viewpoint reflects our ignorance rather than reality—these segments may have functions we have yet to uncover.
The discovery of T. oblanceolata’s genome opens new questions about the evolutionary causes and consequences of such extreme genome sizes. Jonathan Wendel, a botanist at Iowa State University, expressed astonishment at the fern’s genetic load, noting that it incites curiosity about how genomes expand and contract over evolutionary time.
The process of identifying genome sizes has involved estimating the sizes of over 20,000 eukaryotic organisms, revealing significant variability and its impacts on anatomy, functionality, adaptation, and survival. The study published in the journal iScience exemplifies a long journey of scientific exploration into the complexities of plant genetics.
For future research, Dr. Pellicer and his colleagues highlight the importance of continuing to investigate plant genomes. The goal is to comprehend better how these biological giants operate and adapt, arming scientists with knowledge that could inform conservation strategies and environmental policies, especially in the face of climate change.
