A new species of Laboulbeniales finally discovered after millions of years in the waiting

When collaborator Menno Schilthuizen, with whom I published a paper introducing a new species of Diphymyces from Borneo, mentioned Laboulbeniales on an amber piece I wasn’t sure what to expect. I sure wasn’t expecting a project that would span nearly a decade. It was December 2012 and I received this picture:

Have a close look at one of the hind legs of this beetle: there’s a weird little thing attached to the tibia. That, my friend, is a thallus of the order Laboulbeniales, microscopic fungi that are ectosymbionts of arthropods. Needless to say, I was sold upon seeing this pic; I wanted to see the piece of amber and work out the species.

Only a single fossil species of Laboulbeniales was known. Just one, on this entire planet. And here was the second, waiting for some 15–20 million years to be discovered! Isn’t that mindblowing? The first fossil “Laboul” was a species of the large genus Stigmatomyces, †S. succini, on a fossil fly (†Prosphyracephala succini) embedded in Dominican amber. [Note the use of this symbol, “†”, for fossil species.] According to the latest dating information, this material should be about 25.3–23.8 million years old.

I wrote to Michel Perreau, who had the amber piece with the fossil beetle with fungus. Not much later, the amber piece crossed the Atlantic Ocean and landed on my desk at the Farlow Herbarium. It sure was a small piece. I needed help and I was sure I would find what I needed at the Harvard Center for Biological Imaging. A successful grant application, several fancy microscopes, and lots of assistance later, I was not particularly happy with what I had been able to visualize. Here’s one of the images I took:

I suppose it was a bit of an improvement compared to the single image from before, but I was interested in the blackish area behind the leg (shown with the yellow arrows), and there was no way getting a good visual on that part of the thallus. Genus identification: still unknown. This was October 2014. We needed heavy equipment. Michel received funding to scan the specimen at the European Synchrotron Radiation Facility in Grenoble, France by phase‑contrast synchrotron X‑ray microtomography or PPC-SRμCT. This technique allows for virtual dissection of specimens down to micrometer-level; exactly what we needed. And so the amber piece went back to France.

Multiple fantastic! scans and complicated reconstructions later, we were able to formally describe this fossil fungus and give it a proper name, †Columnomyces electri (electri means “from amber”). Also the beetle represented an undescribed species, which we named †Proptomaphaginus alleni to honor Albert D. Allen, who had provided us with one of the specimens. The fossil association was finally described, but we weren’t done yet. Proptomaphaginus is a genus that has extant representatives. Michel had a few specimens of Proptomaphaginus hispaniolensis and P. puertoricensis in his collection and found both specimens of both species with Laboulbeniales thalli. So we reached out to the Canadian Museum of Nature, where the single largest collection of Proptomaphaginus beetles is preserved, with the request to screen specimens for presence of thalli. And, as you might guess at this point, more Laboulbeniales-infected specimens appeared.

All of these specimens were sent to Purdue University, where I was working as a postdoc at the time. I studied the morphology and realized that there were two more undescribed species of fungus: Columnomyces hispaniolensis on Proptomaphaginus hispaniolensis, and Columnomyces peckii on P. puertoricensis. Columnomyces hispaniolensis was named to emphasize the host specificity pattern that emerged (ever leiodid host species seems to host its own fungal species), whereas C. peckii was named in honor of our colleague Stewart S. Peck, who has made enormous contributions to the knowledge of leiodid beetles.

In the end, what started out as a simple project describing a fossil association, turned into a larger-scale project that shows the importance of both technological advancements and natural history collections in the study of these microparasitic relationships. It also shows that specimens tend to travel the globe from one researcher to the next, and often back, before they land in a collection for long-term storage. Of course, there is more to all this (parsimony-based phylogeny of Proptomaphaginus, biogeographical patterns, and a discussion of damages to specimens embedded in amber); but for those interested in more details, I refer to the actual paper.

I do want to share this 3D reconstruction movie made by Paul Tafforeau, of Columnomyces electri attached to the hind leg of Proptomaphaginus alleni. The cylinder-like structure that is artificially colored in green is the basal cell of the perithecium. While the fungus is rotating more or less around the longitudinal axis of the perithecium, note the small tooth-like structure at the tip of the perithecium, a structure that is absent in the other species in the genus.

Sources

Perreau M, Haelewaters D, Tafforeau P. 2021. A parasitic coevolution since the Miocene revealed by propagation phase-contrast synchrotron X-ray microtomography and the study of natural history collections. Scientific Reports 11: 2672. https://doi.org/10.1038/s41598-020-79481-x [pdf]

Rossi W, Kotrba M, Triebel D. 2005. A new species of Stigmatomyces from Baltic amber, the first fossil record of Laboulbeniomycetes. Mycological Research 109(3): 271-274. https://doi.org/10.1017/S0953756204001819

Tafforeau P, Boistel R, Boller E, Bravin A, Brunet M, Chaimanee Y, Cloetens P, Feist M, Hoszowska J, Jaeger JJ, Kay RF, Lazzari V, Marivaux L, Nel A, Nemoz C, Thibault X, Vignaud P, Zabler S. 2006. Applications of X-ray synchrotron microtomography for non-destructive 3D studies of paleontological specimens. Applied Physics A 83: 195-202. https://doi.org/10.1007/s00339-006-3507-2