How does a Small Fern Has Genome 50x Larger Than Humans

A printed version of the entire human genome would fill 220 large books. However, printing the genome of a small fern found on a few Pacific islands would take almost 11,000 books.

This plant, called Tmesipteris oblanceolata, has the largest known genome of any organism, as discovered by Jaume Pellicer at the Botanical Institute of Barcelona in Spain and his team.

Each cell in this fern contains 321 billion letters (base pairs) of DNA in its nucleus. If laid out in a line, it would stretch about 105 meters. “As far as we know, that’s the largest,” says Pellicer.

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Compared to that, a human cell’s nucleus has about 6 billion base pairs, or roughly 2 meters, of DNA – which is about 50 times less than the fern.

Before this discovery, the largest genome belonged to a Japanese flowering plant called Paris japonica, with 298 billion base pairs in each nucleus, as reported by Pellicer in 2010. The marbled lungfish, Protopterus aethiopicus, holds the record for the largest known animal genome, with 260 billion base pairs per nucleus.

T. oblanceolata is a rare plant found only on certain islands of New Caledonia and Vanuatu in the southwest Pacific. In 2023, Pellicer and team gathered samples from New Caledonia.

To determine the fern’s genome size, they isolated the nuclei of cells from its stems, used a fluorescent dye to stain the DNA inside the nuclei, and then measured the light intensity as the nuclei moved under a light detector.

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Pellicer explains why some plants have really big genomes. Firstly, unlike animals that usually have just two sets of chromosomes, many plants have more sets. T. oblanceolata, for instance, has eight sets.

However, having more sets of chromosomes doesn’t always mean a big genome. Pellicer says the main reason for a massive genome is not controlling the growth of genetic parasites called transposons.

Transposons are bits of DNA that can duplicate themselves, causing genomes to grow quickly unless organisms can stop them or get rid of the extra DNA. Many genomes, like ours, have lots of repetitive sequences created by transposons.

Having a huge genome has downsides. It takes longer for cells to divide because they have to copy all that DNA each time. Cells also need to be bigger to hold all the DNA, and certain parts of plants, like the pores in leaves and stems, can’t react as fast to changes in the environment when they’re made of larger cells.

Pellicer thinks plants that can’t control transposons and keep their genomes small often die out. That’s why we only see them in a few plant families. T. oblanceolata might survive because it faces less competition on the small islands where it lives.

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The researchers plan to look at only a small part of the fern’s genome instead of trying to study the whole thing. Pellicer says they don’t have the computer power to handle such a big and repetitive genome for analysis.

Ryan Gregory from the University of Guelph in Canada finds it exciting that we’re still discovering new limits on how big DNA can be in cells. However, there’s a debate about how to measure genome size. Some say it should be based on the size of one set of chromosomes, not the total DNA in a cell. By that measure, the marbled lungfish would have the largest genome.

Many biologists define genome size based on the DNA in egg, pollen, or sperm cells, which is half the amount in regular cells. Using this definition, T. oblanceolata’s genome size would be around 160.45 billion base pairs.

Read the Original Article NewScientist

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