As is the case with most titles that end in “-est,” the competition for The World’s Largest Organism is fierce. The Great Barrier Reef certainly has its proponents, most of them Australian. There's a stand of quaking aspens growing in Utah that's a single clonal organism, rather than genetic individuals. If it does not merit the title of largest organism, it can at least claim to be the most Borg-like.

The real winner does not capture the popular imagination as much as a besieged coral or a forest of yellow-leafed trees: it is somewhat less scenic and probably won’t appear as the backdrop of an inspirational poster any time soon. It kills plants and lives underground, in Oregon. It is the humongous fungus, and we now have its genome.

Armillaria species—honey mushrooms—kill all kinds of plants. Conifers, ginkgos, grasses, and shrubs, in National Forests and in your backyard; all are susceptible. The fungus attacks its prey by sending out rhizomorphs, underground structures that leach onto the plant’s roots and kill them. Then it eats the dead woody tissue of its decomposing host.

What with the agricultural implications of a fungus that attacks plants underground, scientists thought that maybe we should learn more about how and why it does what it does. So they sequenced the genomes of four Armillaria species and compared them to those of twenty-two related fungi.

Turns out that Armillaria have an outsized number of genes that encode enzymes in order to degrade plant cell walls, as well as other genes that control pathogenicity. Most of these genes arose via gene duplication, rather than through transposable elements (genes that jump from one location to another). This is surprising, since the genome expansion in other plant pathogens is driven by transposon proliferation.

The rhizomorphs are unique to Armillaria and allow them to grow so large by bridging the distances between potential host plants. However, their development, morphology, and even function are still up for debate. The genes they express were found to be unique and evolutionarily quite young, but all four Armillaria species examined had them (so they are assumed to be important).

Rhizomorphs express some proteins that degrade plant cell walls, but not as many as the branching part of the fungus, where the mushrooms come from. This supports the speculation that the rhizomorphs not only attack new hosts but may also take up and transport nutrients.

So, this is a case when the genome provides some hint of what has allowed an organism to evolve such a distinctive lifestyle. But, since all the genes are unique, they don't give us a clear picture.

Nature Ecology and Evolution, 2017. DOI: 10.1038/s41559-017-0347-8 (About DOIs).