A small fern species, Tmesipteris oblanceolata, found in the South Pacific islands of New Caledonia, now holds the record for the largest known genome. New research reveals that the fern’s genome is over 50 times the size of the human genome and about 7 percent larger than that of the previous record holder, the Japanese flower Paris japonica.
A Giant Among Genomes
It seems extremely counterintuitive that such a small and simple plant could store such a mind-boggling amount of genetic information. However, plants are just odd when it comes to genetics. First of all, not all plants have large genomes. The smallest plant genomes can be 2,500 times smaller than the largest. Secondly, more complex plants don’t necessarily have bigger genomes. For example, the oak tree genome is surprisingly over 100 times smaller than the genome of the famous festive plant, mistletoe.
Generally, plants can have large genomes through a few key processes. One major contributor is polyploidy, where plants inherit multiple sets of chromosomes, as opposed to a single pair of homologous chromosomes, thereby increasing genome size. Another factor is the accumulation of repetitive DNA sequences. These sequences can include transposable elements, which are DNA segments that can move around within the genome, creating copies of themselves and expanding the genome. Additionally, the large genome size can be a result of whole-genome duplications, which have occurred multiple times in plant evolutionary history, leading to genome expansion and complexity.
In general, plants seem to have larger genomes than animals. The largest animal genome belongs to the Australian lungfish, an endangered air-breathing species. It is a relative of the first “land fish” that evolved into the first terrestrial animals more than 380 million years ago. The Australian lungfish has 43 billion base pairs, which is 14 times more than the human genome, but still nearly three times less than the that of T. oblanceolata. The large genomes in plants may provide them with a greater genetic toolkit to adapt to diverse environments. This may be critical, since plants cannot move and must respond to environmental stresses on site.
The Mystery of Genome Size
Previous studies suggested that fork ferns (Tmesipteris) have unnaturally large genomes, which prompted the team led by Jaume Pellicer, an evolutionary biologist at the Botanical Institute of Barcelona, to investigate further. To this aim, the researchers sequenced the genomes of six fork fern species from New Caledonia. They isolated the nuclei from the ferns’ cells and stained their DNA with fluorescent dye to measure genome size.
Their efforts revealed that T. oblanceolata has a genome consisting of 160 billion nucleobases. This fern typically grows on larger forest plants and reaches only 15 centimeters in length.
The discovery of T. oblanceolata‘s enormous genome prompts intriguing questions about the biological limits of genome size. Maining such a huge genome is not without costs. Every time a cell divides, it needs to replicate its DNA. The larger the genome, the more energy the organism needs to spend. In the case of this fork fern, if you were to unpack its DNA, the plant would need to replicate nearly 100 meters (328 feet) of DNA during every cellular division.
This begs the question: why would T. oblanceolata evolve such a stupendously large genome? We don’t really know, but repeated genetic bottlenecks might have played a role. Plants that have gone through many cycles of near-extinction might have accumulated many deleterious mutations and “junk DNA” that might have been useful millions of years ago but today have no useful function.
A Milestone in Genome Evolution Science
Understanding how the gene copies in T. oblanceolata compare to those in smaller genomes, like some aquatic ferns, may help explain the evolution of genome size extremes. This is why cataloging the genomes of as many species as possible is important, particularly for less conspicuous species. The fork fern’s deceptively modest appearance hid a genetic marvel all this time. Who knows what scientists might discover next?
“Our discovery of the largest genome in T. oblanceolata represents a unique milestone in the field, identifying this species as a critical asset for future detailed genomic analysis so we can fully understand the ecological and evolutionary consequences of genome size diversity and architecture across eukaryotes, especially in the face of ongoing biodiversity loss and climate change,” the authors of the new study wrote.
The findings were reported in the journal iScience.
