A little fish with a whopping genome

Lungfish are strange animals. Air-breathing fish that can hop onto land using weird limb-like fins, they are ancient animals, largely unchanged in 300-400 million years, earning them the label "living fossils". They may also be the closest living relative to the original tetrapods, the ancestors of all vertebrates, including us.

But it turns out they are stranger than we ever thought. Scientists have been sequencing the genomes these strange beasts, and this has yielded some unexpected results. Starting with the Australian lungfish and then the African lungfish, researchers have been surprised at the size of the genomes. But the recently sequenced South American lungfish (Lepidosirem paradoxa) has blown them all away.

This ancient fish has a genome with about 91 billion base-pairs in its DNA, about 30 times the size of the human gemome. However, only about 20,000 of these genes actually code for proteins. About 90% of them are "transposable elements" (TEs), highly repetitive "jumping genes" copied from elsewhere in the genome, possibly as a result of interaction with so many viruses over the eons.

Carrying around so much genetic material has its drawbacks. For one thing, it takes a lot of energy to repeatedly copy all that DNA into new cells, and the cell nuclei and the cells themselves have to be physically bigger. That said, the extra DNA may come in handy if the fish needs to adapt to changing environments, because the TEs can ramp up or down expressions of certain genes, allowing for more rapid adaptation, which maybe why lungfsh have survived quite so successfully thus far.

And it may not end here. The marbled lungfish is thought to have a genome 50% bigger still, which would make it far and away the largest genome of any animal, although the full sequencing remains to be done. 

And even that pales at the side of Tmesipteris oblanceolata, a tiny fern from Eastern Australia, commonly called a fork fern. This simple little plant has a humungous 160 billion base-pairs in its DNA, more than 50 times more than human beings, thus proving definitively that an organism's complexity is completely unrelated to the size of its genome.