Rebel fungus pair rewrites heredity rules and redraws tree of life

Twenty five million years ago, a copied chunk of the genome of one fungus jumped intact to the genome of another, thus rewriting heredity’s rules as we know them.

If a turtle wandered up to you and, in an uncharacteristically deft genetic maneuver, handed over a few genes that made you green, would you find that odd? Probably. It would be odd. Horizontal gene transfer isn’t for vertebrates or other complicated types. That’s the domain of asexual bacteria, right?

Aspergillus nidulans. (Wikimedia Commons)

Or, wrong. A pair of fungi – about as closely related as you and that turtle – appear to have done some serious gene swapping back in the day. That’s according to Vanderbilt University evolutionary biologists Antonis Rokas and Jason Slot, who found that two fungi, Aspergillus nidulans and Podospora anserina, have identical genetic material – 23 genes common to both species – that they both use to create a toxic compound for fending off attackers and competition.  The two scientists compared the genomes of 100 fungus species to find these 23 common genes.  What does it mean? Just that about 25 million years back in their deep evolutionary past, a copied chunk of the genome of one of the fungi jumped intact to the genome of the other.

Podospora anserina. It might look similar to Aspergillus nidulans to you and me, but it\’s about as closely related as you are to a turtle. And yet somehow these two fungi shared 23 identical genes. (Wikimedia Commons)

It’s not easy to tell some fungi apart. Sure, a puffball doesn’t look a whole lot like a wheat smut, but this pair of gene swappers aren’t that obviously different. In spite of their seeming similarities, however, surely most people would agree that there’s a substantial distinction between eating potatoes and eating plant fiber from cow poop. The pair of fungi in question have these divergent dining affinities, but they share something identical: the multi-step pathway for making that major chemical weapon.

Fungi use chemical weaponry for much the same reason people do: to kill something. In the case of fungi, the something might be other microbial invaders or just other fungi horning in on the poop-plant fiber action. While the chemical arsenal is diverse enough that we’ve ripped it off for weapons of our own against bacteria (think penicillin), the potato-loving fungus and the poop-plant fungus not only use the exact same chemical … they use the exact same set of genes in producing it.

So what? Well, that what has rewritten a basic rule of how fancy organisms like fungi, people, turtles, and squirrels hand along genes. You may be familiar with the typical process, but to review, it’s called sex. Species like us combine the genes of a willing pair to reproduce. Regardless of general preferences, this process of handing down genes is vertical, from parents to offspring. Your Aunt Edna isn’t going to come over for dinner one night and accidentally transfer a few genes to you over dessert.

Bacteria are different. In addition to being able to threaten both fungi and people, their version of Aunt Edna can show up for dinner and hand off a packet of genes to another of their sort. This kind of handoff – horizontal gene transfer – was thought to be strictly the domain of simple cells. Complex cells, like ours or those of a fungus, surely couldn’t manage this horizontal thing. It simply isn’t done.

But it is. Or at least was. Those 25 million years ago, the potato-loving fungus did fork over an entire copied chunk of 23 genes to its poop-plant-loving neighbor. These two fungi aren’t even in the same genus; they’re about as close cousins in the tree of life as you and that turtle. Yet there they were, at a moment in time, horizontally handing off genes.

That capacity opens up the door to all kinds of possibilities, not so much of the turtle-human kind or having to do with your Aunt Edna, but more of the darn, but this tree of life sure has gotten confusing kind. Scientists have until fairly recently depicted that tree as your regular tree-like image: branches, branching vertically, reaching skyward through time. Then the bacteria were exposed for the horizontally transferring rebels that they are, and suddenly, some of those branches reached across to each other and fused into a ring. Sure, everyone said, bacteria can do that kind of thing. They form bridges between each other, for Pete’s sake, and pass around genes willy nilly. But that’s for the lowly, simple bacteria, not for those complex, fascinating nucleated-cell types, like mushrooms or people.

No more. Rokas and Slot, whose diligent work exposing this dark fungal past was published in the Jan. 25 issue of the journal Current Biology, think that many more examples of horizontal gene transfer might lurk in the genomes of complex cells, waiting to be uncovered and turn life’s family tree into a confusing, twisted pretzel. It’s only fitting that the moldy, yeasty, smutty mushroomy fungi – species that deal with the detritus of life – are the ones that gave up the secret first.

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