A South American species of lungfish, Lepidosiren paradoxa, has shattered the records for the largest animal genome ever sequenced. To give an idea, their genome size is 30 times the length of a human genome and stands at a staggering 91 billion bases, which are tiny biochemical units that build up DNA. 18 of their 19 chromosomes are larger than the entire human genome. However, like an average human, their large genome boils down to just 20,000 genes. The rest of the lungfish’s genome is filled with repetitive sequences of DNA, which can perhaps be “junk” DNA (DNA sequence with no known function). The figures don’t stop here, as scientists have found that this specific lungfish can add genomic material equivalent to the size of the human genome every 10 million years.
Why are lungfish the talk of the town
Lungfishes can breathe through their gills and, as their name suggests, have a single or pair of lungs to breathe. What makes them more interesting is that they are the closest relatives of the ancient common ancestor of all vertebrates. Take the example of the Australian lungfish, whose fin bones resemble the limb bones of various vertebrates, making their role across the evolutionary phases immense, attracting much attention from evolutionary biologists. Axel Meyer and Manfred Schartl, evolutionary biologists at the Julius Maximilians University of Würzburg, and their colleagues started sequencing lungfish genomes across Australia, Africa, and South America to better understand vertebrate evolution. Notably, after cracking the genomic code of African and Australian lungfishes, they turned their attention to South American lungfishes—they ended up with a genome size twice that of the other two species.
Why do South American lungfishes have such a large genome?
South American lungfish have large stretches of repetitive DNA called transposons or jumping genes (they move/jump from one portion of the genome to another). These transposons make copies of themselves and jump right back into the DNA. About 90% of L. paradoxa have these genes, compared to just 40% in the case of humans.
The accumulation of these repetitive strands occurs because South American lungfishes have lesser amounts of piRNA, which are known to silence the effects of transposons. Excessive quantities of DNA require a larger nucleus and cell to encase, and copying the DNA requires more energy.
Larger genome: boon or bane?
A larger genome can be of help if the organism needs to adapt to changing conditions. If a transposon lands inside or near a gene, it can alter the gene’s activity, perhaps benefiting the organism. However, the opposite can happen too, which makes the whole process random and ultimately poses the question: Why hasn’t L. paradoxa prevented the effects of these jumping genes? According to Meyer, “These are fish that evolution forgot.”
However, the largest genome across all living beings belongs to a fern with around 160 billion bases. You can learn more about it here.
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