Editor's Picks
Suillus punctatipes, courtesy Ron Pastorino
The slippery nature of ectomycorrhizal host specificity
H
ow many times has some know- it-all remarked “Your ID is wrong
… that mushroom only grows with ___ trees, and those are not found here.”? We know that we don’t know everything about fungi. And what we do know is that fungi don’t always follow the rules. Unlike arbuscular mycorrhizal fungi, which seemingly partner up with just about any kind of plant, the ectomycorrhizal fungi (ECM)—the fungi that produce many of our favorite large and showy forest mushrooms— most often exhibit a high degree of host specificity. Tere are exceptions of course, but in general, a genus of basidiomycete fungi will have a pretty narrow host range. Te genus Suillus is ubiquitous and
well-known to us all, comprised of nearly 100 species and ranging all across the Northern Hemisphere. And everyone knows Suillus occur only with species in the family Pinaceae. Even more specific than that, Suillus partner only with members of the pinoid tribe of the Pinaceae: the genera Pinus, Pseudotsuga, and Larix. (Picea also is in this group, but is not a known host for Suillus.) Te
38 FUNGI Volume 15:1 Winter 2022
other tribe of Pinaceae is comprised of the genera Tsuga and Abies, and none from this group is thought to host these fungi. (See phylogenetic tree for land plants, below. Te image is from Uddenberg et al. [2015; Frontiers in Plant Science; doi: 10.3389/ fpls.2015.00970] and used here, courtesy of the authors.) If you’re in the forest often enough,
you’ve likely encountered a situation where you found Suillus specimens and, looking around, could find no probable host tree. Is there something I’m missing … is there a seedling that I’m not seeing, or maybe a host tree was here somewhere but cut down? What if mushrooms well-known to us simply don’t follow the rules? Tis is what a group of mycologists wanted to find out. To determine the capacity of Suillus to associate with alternative hosts in both the pinoid and abietoid clades of Pinaceae, they examined the host associations of two Suillus species (S. punctatipes and S. glandulosus)
on forest tree root tips, as well as seedling bioassays. Root tip collections underneath Suillus sporocarps were positively identified visually and using DNA sequence analysis. Te field-based results confirmed that Suillus fungi were able to associate with Abies and Tsuga hosts, representing novel host genera for this genus. In the bioassays, colonization on the primary hosts was detected in both single- and two-species treatments, but no colonization was present when Picea and Abies hosts were grown alone. Removal of a primary host had no effect on percent ECM colonization, suggesting that primary hosts are not necessary for sustaining Suillus colonization once they are successfully established on secondary hosts. Collectively, the results indicate that host specificity is more flexible in this genus than previously acknowledged and helps to explain the presence of Suillus in forests where recognized hosts are not present. Tese findings, along with further discussion of ECM-host tree interactions, were published by Eduardo Pérez-Pazos, Amanda Certano, Joe Gagne, Renée Lebeuf, Noah Siegel, Nhu Nguyen, and Peter G. Kennedy in the latest Mycologia (113[5]: 891–901; doi.or g/10.1080/00275514.2021.1921525).
Hippo gut microbes make a splash
T
he bacteria, fungi, and other microbes that live in our guts influence
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