Largest fern genome

The largest genome (i.e., total amount of DNA in the nucleus of a cell) for a fern – as well as for any plant or indeed any organism assessed to date using best-practice methods – is 160.45 billion base pairs (Gbp, or gigabases), for the Tmesipteris oblanceolata fork fern. It is native to tropical forests in the French overseas territory of New Caledonia and other nearby islands, such as Vanuatu, in the south-west Pacific Ocean. This fern boasts over 50 times more genetic material than that found in the nuclei of human cells (3.2 Gbp). The findings were published in the journal iScience on 31 May 2024.

The research was conducted by plant geneticists from Institut Botànic de Barcelona (Spain) and the Royal Botanic Gardens, Kew (UK), led by PhD student Pol Fernández and Dr Jaume Pellicer of Institut Botànic de Barcelona. There was also input from experts at Universitat de Barcelona, Real Jardín Botánico (both Spain), Herbier de Nouvelle-Calédonie, Université de Montpellier (both France), Curtin University (Australia) and Queen Mary University of London (UK).

To determine genome size, the most common technique that scientists employ is “flow cytometry”. This involves first isolating nuclei and staining them with a fluorescent dye that binds quantitatively to the amount of DNA present in each nucleus. The suspended nuclei are then loaded onto the flow cytometer and as they pass through a laser beam the amount of fluorescent light is measured (the bigger the genome, the more fluorescent light is detected by the flow cytometer). As well as the number of base pairs, genome size can also be expressed as a weight in picograms (one-trillionth of a gram): the total DNA of T. oblanceolata weighs in at 164 picograms.

If fully unravelled, the DNA from a cell of T. oblanceolata would span in excess of 100 m (328 ft) – greater than the height of the Statue of Liberty; a human’s total DNA in a cell, by comparison, would “only” stretch about 2 m (6 ft 6 in).

Since 2010, this record has been held by Paris japonica, a Japanese woodland plant with a genome comprising approximately 149 Gbp, as discussed in the Botanical Journal of the Linnean Society. The DNA amount of T. oblanceolata is about 7% larger, although P. japonica still retains the record among flowering plants (angiosperms). A related fork fern, T. obliqua, had already come close to taking its crown in 2017, with a 147.3-Gbp genome, proving that giant genomes are a recurring feature of this genus.

The largest plant genome to have been sequenced and assembled, which involves not just measuring the total amount of DNA but reconstructing it into its original order, is for the European mistletoe (Viscum album) for which 94 Gbp has so far been curated by scientists from the Darwin Tree of Life project, as reported in December 2022.

In the animal kingdom, the current record for the largest DNA amount belongs to the marbled lungfish (Protopterus aethiopicus), a freshwater fish from Africa with 129.9 Gbp of DNA, as documented in the Journal of Experimental Zoology in 1972. The largest animal genome to have been sequenced and assembled is that of the related Australian lungfish (Neoceratodus forsteri): 43 Gbp of its genome has been reconstructed according to a study in Nature in 2021; this surpassed the previous 32 Gbp sequenced for the axolotl salamander (Ambystoma mexicanum) as of 2018.

Photo credit: Oriane Hidalgo / RBG, Kew