This fern has the largest genome in the world, and scientists can’t explain why
The human genome consists of 3 billion base pairs of DNA. But this is nothing compared to the variety of New Caledonian fern (Tmesipteris lanceolate), a leafy plant with tendrils native to several Pacific islands, whose genome … It contains a staggering 160 billion base pairs, 53 times more than ours, making it the largest ever observed. This discovery could help scientists understand how and why some genomes become so large and how these huge sets of genes affect the adaptability and survival of species.
DNA is made up of base pairs—two molecules connected by hydrogen bonds. The smallest genome discovered so far belongs to a mammalian parasite called Encephalitozoon intestinalis, measuring a measly 2.25 million base pairs in length. However, plant genomes tend to be heavier, and scientists know that plants with relatively large genomes tend to live longer, reproduce more slowly, and are also more vulnerable to environmental stress. But this is only indirect evidence. In fact, there is no clear relationship between the size of an organism’s genome and its physical or physiological complexity. That is, as far as we know, the size of the genome does not affect how complex a living thing can become.
Record genome
According to Jaume Pellicer of the Botanical Institute of Barcelona and lead author of a paper recently published in the journal iSience, their intention was not to enter the record books, but rather they made the discovery by investigating the role played by repeating sequences. DNA in the evolution of plants with very large genomes. And it has long been known that ferns have huge ones.
So, realizing that ferns often have surprisingly long stretches of repetitive DNA, the team decided to study them further. Tmesipteris oblanceolate. The plant is not found in the tropical forests of the few Pacific islands where it is native, including New Zealand and New Caledonia. “It’s not a flowering plant,” notes Pellicer, “so it doesn’t get a lot of attention. “I think beauty is on the inside.”
To count the fern’s 160 billion base pairs, the researchers used a technique called flow cytometry, which analyzes cellular characteristics using a laser. The truly impressive figure is 11 billion more base pairs than the previous genome record holder, a Japanese flowering plant called Paris japonica. But why does a simple fern have such a huge genome? For Pellicer, this is precisely the “million dollar question.”
For no apparent reason
As far as we know, there are two main ways for plants to produce very large genomes. One is the massive copying of the genome within the nucleus, a process known as polyploidy. Another is to have a huge set of so-called repetitive non-coding DNA sequences. “We used to call these sequences junk DNA,” Pellicer says. “But we now know that they have the ability to insert themselves into nearby genes and give them new functions or silence them.”
However, it is very difficult for scientists to imagine what the advantage of having such disproportionate amounts of non-coding DNA might be, since “creating” all this DNA for each cell, then packaging it and protecting it, is a very heavy burden. on any organism.
Is it possible that there are genomes even larger than those of Tmesipteris lanceolate? The study’s authors doubt this, so we may be faced with one of the living creatures with the largest genome. “We have a good understanding of the genomes of all families of organisms,” Pellicer concludes, “and it is very likely that this fern is close to the upper limit. And if we are not at the top of the scale, we are certainly close to it.