A team of researchers has identified the world's largest known genome in the cells of a small, unassuming fern. The fork fern Tmesipteris oblanceolata grows on the ground or atop fallen tree trunks and is typically just a few inches tall. As reported last month in the journal iScience, the specimens were found on Grande Terre Island in New Caledonia, a French territory in the southwest Pacific Ocean.
A genome encompasses all the genetic material of an organism, contained as DNA within a cell's nucleus. The double helix structure of DNA is composed of base pairs, so a genome's size is measured by the total number of base pairs. The New Caledonian fork fern's genome comprises 160 billion base pairs, more than 50 times the size of the human genome (approximately 3.1 billion base pairs). To put this into perspective, if the fern’s DNA were unraveled and stretched upright, it would surpass the height of the Statue of Liberty (305 feet and 1 inch, or 93 m). Previously, the largest known genome belonged to Paris japonica, a white, star-like flowering plant from Japan with a genome of 149 billion base pairs.
Surprisingly, scientists still can’t explain why some organisms have large genomes and others don't. Nor does genome size appear to correlate with an organism’s complexity, as illustrated by the example of the fern vs. human genome. In fact, research suggests that having a large genome is not advantageous for an organism, as it requires more resources for reproduction. Plants with extensive DNA grow slowly and are less efficient at photosynthesis. Evidence suggests that organisms with larger genomes are less capable of adapting to climate change and are more vulnerable to extinction.
However, despite the apparent massive inefficiency of copying 330 feet of genetic material every time a Tmesipteris oblanceolata divides, perhaps this hasn’t impacted the fern’s survival, which is why it keeps accumulating repeating DNA sequences.
Big genome, tiny package:
- The New Caledonia fork fern now boasts three Guinness World Records: “Largest Genome,” “Largest Plant Genome,” and “Largest Fern Genome.” The study authors believe this is close to the upper limit of genome size, though it’s possible that a plant with an even larger genome is out there, undiscovered.
- Like many ferns, the New Caledonian fork fern has long, nonfunctional sequences and a huge amount of repetitive DNA. Only around 1% of the genome consists of genes that encode for proteins. This phenomenon is seen in the genomes of many other organisms, including humans (98% of our genome is non-coding DNA).
- Despite the existence of outliers like Paris japonica and Tmesipteris oblanceolata, massive plant genomes are rare, according to Jaume Pellicer, a biologist at the Botanical Institute of Barcelona and a co-author of the study. Pellicer was a member of the teams that discovered both Tmesipteris oblanceolata and Paris japonica.
