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RECORD-BREAKING: 160 Gbp GENOME OF FORK FERN SHATTERS SIZE RECORDS FOR EUKARYOTES

 

To the known nearly 20,000 eukaryotic genomes, we have come across astonishing sizes that influence cell size, physiology, morphology, life cycle, ecology, and evolution of the species. Microsporidian Encephalitozoon intestinalis with the smallest genome of 2.6 Mbp/1C (1C indicates the amount of DNA present in a single set of chromosomes) to Protopterus aethiopicus (lungfish) and Necturus lewisi (proteid salamander) with some largest genome size of 129.90 Gbp/1C and 117.47 Gbp/1C respectively. Even, some vascular plant groups like Paris japonica (148.89 Gbp/1C) and Tmesipteris obliqua (147.60 Gbp/1C) have the largest genome in eukaryotes. However, diversity doesn't correlate with the organism's complexity. This led scientists to acknowledge them as terms ‘C-value paradox' or 'C-value enigma'. 



(a) Encephalitozoon intestinalis (b) Protopterus aethiopicus (lungfish) (c) Necturus lewisi

(proteid salamander)



The term C-value paradox or C-value enigma reflects the complexity of the genome doesn't reflect the size (i.e.) complexity of the organism doesn't proportionally correlate with the size of the organism.  Despite that, researchers have identified various other factors that influence the size of the genome such as Repetitive DNA (non-coding DNA), Polyploidy (organisms with multiple sets of chromosomes), and Evolutionary Processes (duplication, transposable element activity, and the accumulation or loss of non-coding DNA). 


Recently, a plant species Tmesipteris oblanceolata subsp. Linearifolia, commonly fork fern, has been shown to have a genomic size of 160.75 Gbp/1C (i.e.) that is c. 160,000 Mbp, which is the largest genome ever found. Tmesipteris (Psilotaceae) is a small genus consisting of approximately 15 species, in which only two species have shown to have genome sizes reported as T. tannensis (1C = 73.19 Gbp) and the octoploid T. obliqua (1C = 147.29 Gbp). However,  Tmesipteris oblanceolata subsp. Linearifolia is the largest one of their species and is 7% greater than the previous record in eukaryotes. 


On analyzing the genome, it was the presence of high chromosomal numbers (octoploid species with 8 sets of chromosomes; 2n = 8x = 416), which is known as chromosome hoarding syndrome which implies numerous whole genome multiplication (WGM) that corresponded to the genome size. This chromosome number 416 majorly consists of repetitive DNA and the polyploidization process. Unlike plants, where genome size expansion is often associated with repetitive elements and noncoding DNA rather than whole genome expansion (WGM), animals with the largest genomes (eg, some salamanders and lungfish) typically do not exhibit WGM. Therefore, it is unlikely that genomes of animals exceeding the size of Tmesipteris oblanceolata will be found.


To conclude, Biodiversity Genomics is a new upcoming stream that understands the evolutionary forces behind giant genomes. The identification of the largest genome in Tmesipteris oblanceolata marks a noteworthy achievement, underscoring the species' importance for future genomic investigations aimed at understanding its ecological and evolutionary significance.


REFERENCE

Pol Fernández, Rémy Amice, Bruy D, Maarten J.M. Christenhusz, Leitch IJ, Leitch AL, et al. A 160 Gbp fork fern genome shatters size record for eukaryotes. iScience. 2024 May 1;109889–9.


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