A tiny fern species called Tmesipteris oblanceolata, discovered in New Caledonia, a group of South Pacific islands, has set a new record for having the largest genome known to date. The genome of this fern is more than 50 times larger than the human genome and about 7 percent bigger than the genome of the previous record-holder, a Japanese flower known as Paris japonica.
It might seem surprising that such a small and simple plant could contain such a massive amount of genetic information. However, plants are unique in their genetic makeup. Not all plants have large genomes, as the smallest plant genomes can be 2,500 times smaller than the largest ones. Additionally, complexity in plants does not always correlate with genome size. For instance, the genome of an oak tree is more than 100 times smaller than that of mistletoe, a festive plant.
There are several reasons why plants can have large genomes. One major factor is polyploidy, where plants inherit multiple sets of chromosomes, leading to an increase in genome size. Another reason is the accumulation of repetitive DNA sequences, including transposable elements that can move around within the genome and create copies of themselves, expanding the genome. Whole-genome duplications throughout plant evolutionary history have also contributed to genome expansion and complexity.
Compared to animals, plants generally have larger genomes. The Australian lungfish, an endangered species that breathes air, has the largest animal genome known. However, it is still nearly three times smaller than the genome of T. oblanceolata. The large genomes in plants may provide them with a greater genetic toolkit to adapt to different environments, which is crucial since plants cannot move and must respond to environmental challenges where they grow.
Researchers led by Jaume Pellicer from the Botanical Institute of Barcelona investigated the unusually large genomes of fork ferns, including T. oblanceolata. By sequencing the genomes of six fork fern species from New Caledonia, the team discovered that T. oblanceolata has a genome with 160 billion nucleobases, making it a genetic giant among plants.
The discovery of T. oblanceolata’s massive genome raises questions about the biological limits of genome size. Maintaining such a large genome requires a significant amount of energy, especially during cell division. The reasons behind why T. oblanceolata evolved such a large genome remain unknown, but genetic bottlenecks and accumulation of deleterious mutations over time could be contributing factors.
Studying how gene copies in T. oblanceolata compare to those in smaller genomes can provide insights into the evolution of genome size extremes. Cataloging the genomes of various species, including less conspicuous ones like the fork fern, is crucial for understanding genome size diversity and evolutionary consequences. This discovery represents a significant milestone in genome evolution science and highlights the importance of further genomic analysis to comprehend the ecological and evolutionary impacts of genome size across different species, especially in the context of biodiversity loss and climate change.
