The Quest for Phaeocollybia

part 2

 

The Investigation Proceeds

Leon Shernoff: Now, you’ve said that Phaeocollybia is mycorrhizal?

Lorelei Norvell: It was a really nice result. Brandon Matheny agrees with me on this, and Steve Trudell has done some studies on its nitrogen isotopes, which show very high N-15 readings3 – it’s actually higher than you get with Cortinarius. So there’s some supporting evidence. But the way I went about it was purely mechanical. I didn’t do too good in my generals,4 because Jim Edmonds who was on my committee, asked how would I go about proving that a mushroom was mycorrhizal or saprophytic. He was thinking the traditional method, which is to try to get it in culture and then see if you could get it to join with a seedling. Well, I knew you couldn’t even get the stupid thing to germinate, so I never gave that answer. Here he gave me a nice soft balloon of a question and I totally blew it.

A pseudorhiza is not (always) just a simple extension of the stem. Here are the complicated pseudorhizae of several fruiting bodies of Phaeocollybia scatesiae arising from a mother pseudorhiza (the long ropy thing at the bottom).

A pseudorhiza is not (always) just a simple extension of the stem. Here are the complicated pseudorhizae
of several fruiting bodies of Phaeocollybia scatesiae arising from a mother pseudorhiza (the long ropy thing at the bottom).

I kept saying, “Well, basically you would dig, because these things have radicating stipes.”5

3 This is a line of inquiry that has gained many more adherents over the past decade. It turns out that mycorrhizal and saprotrophic species of fungi generally have a different profile of nitrogen and carbon isotopes from each other. Phaeocollybia’s high level of N-15 is consistent with it being a mycorrhizal fungus.
4 General exams, where the faculty members quiz you on your doctoral project and knowledge of the field.
5 Radix is the Latin word for a root, so a radicating stipe is a stem that goes down into the ground like a taproot. The word survives in general English as a negative: eradicating something means destroying its roots.

This we had actually done. I dug down – and we’re talking way down, sometimes close to a meter – to the very very base, and you would find it connected to a rootlet with an ectomycorrhizal mantle. The neat thing about Phaeocollybia is that its mycelium has these tibiiform diverticula6 on it. They’re very characteristic. And the mantle surrounding the root also had these very characteristic diverticula on it.

6 Tibiiform means something that’s shaped like a tibia – that is, a stereotypical straight bone with knobs on each end. A diverticulum is just something that sticks out that doesn’t have any other name.

This was an ‘edge up’ that I could demonstrate. But my mycorrhizal evidence was all circumstantial. I did try to get some root DNA to see if Phaeocollybia DNA is in the rootlet itself, but that was inconclusive: at the time I was still doing RFLPs;7 we didn’t have funds for sequencing, so I couldn’t really prove anything there.

I knew that Scott had been the first to actually excavate the pseudorhiza (a sort of thread-like root extension) joined to a rootlet, and ….

7 Restriction fragment length polymorphism, an early technique for comparing DNA between species. Now replaced by PCR sequencing.

LS: He did that with Xerula, right?

LN: No. Although he excavated Xerula, he was not the first to demonstrate its root-pseudorhiza connection. But he was the first with Phaeocollybia: he excavated P. jennyae in northern Canada, by digging up a large block of soil surrounding the mushroom, and separating the mass under the dissecting scope in his lab. And he gave me his slide demonstrating that P. jennyae was possibly parasitic, given that he found the pseudorhizal thread on a senescent rootlet of spruce. It definitely looks like the fungus had invaded the rootlet and was destroying it. I later asked him “Yes, but at what time of year did you get this?” If it was in the fall, well, these little rootlets, these ectomycorrhizal tips, usually die off on their own at that point.