A Novel Way to Reproduce: Pleonectria strobi

Pleonectria strobi

Surprise! Something really small! I can't help it—it's still too early here for morels or any other macrofungi so I've been wandering through the snowless, leafless woods looking for out-of-place dots. White dots on branches could be Lachnellula, yellowy orange dots on rotting logs could be one of the smaller Dacrymyces, vivid blue-green ones could be just-starting Chlorociboria. You never know what you're going to find when you're looking for dots, and two days ago I found some some very interesting ones.

Pleonectria strobi can be up to 1 mm., but are usually smaller. 

They were minute and orange, growing in clusters along the length of a twig. I thought at first they were tiny cups, but on closer inspection saw that they were actually some kind of Nectria that had become cupulate from drying in the sun. Moistened, they became much more globose. But what kind? Nectria cinnabarina, which is relatively common, is restricted to broadleaf trees. The ones I'd found, though, had erupted from pine—specifically white pine (Pinus strobus). They also seemed smaller than Nectria cinnabarina and brighter orange, the brightness due to a yellow scurfiness on their surface.  

To get a clearer idea of what they were, I squashed a few under a slide cover and stuck then under the microscope. I immediately found multiple asci, most of which were jam-packed with very tiny, slightly curved spores, so packed that, even as I watched, they were bursting out in streams of a hundred at a time. Very exciting, but also very wrong. Nectria species are supposed to produce asci containing 8 ascospores apiece, not hundreds. So what was up?

Ascoconidia streaming out of ascus.

It turns out that what I was seeing were not ascospores but ascoconidia, and what I'd found was not a simple Nectria, but a Pleonectria.

Asci stuffed with ascoconidia.

Many ascomyctes have two different ways of reproducing: sexually and asexually. Asexually they produce conidiophores that release conidiospores, which are haploid, having only one set of chromosomes instead of two. I've seen lots of conidiospores before, but none of them had been inside asci. That's because asci, as a rule, are normally reserved for the production of ascospores. This is not, however, the case for some rule-breaking Pleonectria, including the one I had found, Pleonectria strobi. As is to be expected, each Pleonectria strobi ascus first produces 8 ascospores, but then the fun starts. Ascoconidia bud off each of the filiform, multiseptate ascospores, so many, in fact that, according to Fungi of Switzerland, Vol.1, the ascospores are completely consumed. 

The multiseptate ascospores of Pleonectria strobi
look a little wormlike and vary in length.

The ascospores were certainly extremely difficult to find under the microscope. Interestingly, the ones I eventually did find were outside the asci, so perhaps not all of them are doomed to the service of generating ascoconidia. 

Reference:

Hirooka Y, Rossman AY, Samuels GJ, Lechat C, Chaverri P. (2012) A monograph of Allantonectria, Nectria, and Pleonectria (Nectriaceae, Hypocreales, Ascomycota) and their pycndial, sporodochial, and synnematous anamorphs. Studies in Mycology 71: 1–210.