Cornell Mushroom Blog

This post was written by Christine Layton, a talented PhD student in Plant Pathology and Plant-Microbe Biology here at Cornell.

Almost everyone has been to a museum like The Smithsonian and seen firsthand the relics of our planet’s evolutionary past. Most of the fossils we find belonged to creatures that have long been extinct, but many of those bear a striking resemblance to organisms we share the earth with today. As with the fossilized remains of plants and animals with which most of us are familiar, fungi that existed millions of years ago have also been preserved and can be studied by paleomycologists — that special breed of mycologist who studies fungi in the fossil record.

Some of the most fantastical discoveries of ancient fungi have been in amber. Amber comes from certain species of trees whose sap was able to resist decay and weathering, and thus hardened and became fossilized over millions of years. Anything (including fungi!) that became trapped within the sap before it hardened became completely preserved– just like a time capsule. Amber deposits exist worldwide, but two of the most important are on the coast of the Baltic Sea and in the Dominican Republic. Both of these deposits differ greatly in age: Baltic amber dates around 35-55 million years old during the Eocene, or at about the time that the first modern mammals appeared, whereas Dominican amber is from the Miocene (about 15-20 million years old), making it about half the age of Baltic amber.¹ This important age difference gives us snapshots of two completely separate periods in our planet’s history, a real boon for evolutionary biologists.

Many insightful discoveries have been made about what fungi were like millions of years ago. It seems that while many of the fungi that existed back then clearly differ from the ones that exist today, the fungi of today bear a striking physical resemblance to their ancestors. And from what we can tell, it seems that ancient fungi walk the same walk and talk the same talk as their modern counterparts, too. Many fungi are parasites — of plants, of insects, and even of each other. So, it is not surprising that we should find them doing the same things in the fossil record.

One of the more well-preserved specimens of a fungus in amber comes from a piece of Baltic amber that contains a springtail (an arthropod closely related to insects) which is likely being parasitized by Aspergillus collembolorum, a previously undescribed species. The sporulating fungus is so well-preserved that the individual conidiophores (spore-bearing structures), complete with conidia (spores), can be clearly seen erupting from all over the body of the springtail. Using these physiological characters, it was placed in the modern genus Aspergillus, whose species are primarily saprophytic. However, some species are known to be facultative parasites of insects. Because A. collembolorum is the only fungus on the springtail, as well as the fact that the springtail was not decomposing when it was trapped, it is likely that the Aspergillus was acting as a parasite and not a saprophyte.² Investigating fungi trapped in amber is almost like figuring out what happened to a victim in CSI, since you have to follow the clues from what happened at the time of death to really figure what the fungus was doing, its identity, and perhaps even how it lived.

There have been some other remarkable finds of parasitic fungi of insects in amber. In Dominican amber, a mosquito was found with several types of parasitic fungi growing on its outside cuticle. What is interesting is that the fungi resemble modern day fungi in class Trichomycetes, which are common gut-inhabiting zygomycetes of insects, but they differ from Trichomycetes in that the fungi are on the outside of the insect rather than the inside. If the fungi are indeed Trichomycetes, this could be important for figuring out when the ability to live in insects’ guts was acquired in the lineage.³ Another interesting find, this time in Baltic amber, is of a parasitic fungus consisting of four club-shaped fruiting structures erupting out of the thorax of a stalk eyed fly. What’s really neat is that the fungus physiologically closely resembles a modern day Laboulbeniales, which are obligate parasites of insects and are often very host-specific. Since the fungus was found on a fly, it was able to be placed in the modern genus Stigmatomyces, which is specific to flies. This fossil, which is the oldest record of an insect-parasitic fungus, shows that these host-specific insect pathogenic fungi have existed for tens of millions of years.^4,5

Now probably one of the coolest fungal finds in amber has to be from a piece of Burmese amber dating to about 100 million years old, or around the time when the dinosaurs were in their heyday. Within this piece of amber is a fungus parasitizing a fungus that is parasitizing yet another fungus. You heard me right: three fungi eating and being eaten by one another. I should also probably mention that the piece of amber in which this was all found is itself smaller than a grain of rice! In the amber is the cap of the basidiomycete Palaeoagaracites antiquus, whose gills are covered by the hyphae of the mycoparasite Mycetophagites atrebora. And amazingly, inside of the hyphae of that mycoparasite are the hyphae of the hypermycoparasite Entropezites patricii. All three fungi were described as new genera and species from this single sample. The specimen is so well-preserved that portions of P. antiquus‘s gills appear to be liquefying from toxins released by M. atrebora. Nowadays such complex and sophisticated levels of parasitism are known amongst fungi, but the fact that they were so well-established some 100 million years ago is simply astonishing.⁶

But really, why do we even care about all of this? Knowing what kinds of fungi were out there and getting a glimpse of what they were doing millions of years ago is vital to the understanding of the evolutionary histories of the species we have around today. While we may have a good understanding of the relationships between many plants and animals, we know relatively little about the true evolutionary history and relationships of most fungi. A recent phylogenetic study using highly conserved DNA has shown that the morphological characters–primarily those of fruiting bodies–that we use to identify fungi are far from perfect at revealing the true evolutionary relationships between groups. It’s become quite clear that some traits once considered to be homologous, like the presence of gills or an enclosed sac-like fruiting body, have evolved multiple times in different phylogenetic lines, making them analogous and not homologous traits.⁷ By “filling in” the blank spaces of the past with clues from fossilized fungi, we can develop a better understanding of not only how long fungi have been filling certain ecological roles, but also when major fungal lineages diverged. Through the differences between ancient fungi and their modern counterparts, we can start to grasp when certain traits evolved and ultimately learn about the true evolutionary relationships among modern fungal taxa.

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