Porcini Mushroom Evolution with a Preference for Local Adaptation – ScienceDaily


The Dentinger Lab at the Utah Museum of Natural History has published a controversial new paper in the journal New Botany describes their work with their beloved mushrooms, boletus edulis, better known by culinary experts around the world as Porcini. In the paper, Keaton Trimble and Bryn Dentinger Ph.D. present the first-ever genetic survey of porcini mushrooms across the northern hemisphere. By assessing the genetic code of these specimens from around the world, they learned that these delicious fungi evolved in surprising ways—contrary to the expectations of many who might have thought that geographic isolation would be the primary driver of species diversity. In fact, there are regions of the world in which Porcini maintain their genetic distinctiveness in local ecological niches, even if they are not geographically isolated from other genetic lineages.

The French word terroir, made famous by vintners, immediately comes to mind. Terroir describes local factors such as soil types, amount of sunshine, degree of steepness, local climate, soil microorganisms, etc. that make each plot of land produce a distinctive wine. It is a celebration of the local environment and its impact on the vines, grapes, and final product. Tremble and Dentinger’s new study of mushroom hunters presents tantalizing data for claiming that the pigs in their secret forest patch express the qualities of their land in the same way as the world’s best wines.

But that is not the aim of the study. With the advent of genetic sequencing, most genetic studies in mycology have focused on describing the unique characteristics of fungi within a small geographic area. Tremble and Dentinger wanted to do something different. Rather than simply comparing a group of mushrooms from Colorado to a group in California in order to call them different species, they wanted to better understand global trends in how the genetic code is preserved or changed in porcini. “Our study is important because it goes beyond the overly simplistic sampling method used in the past,” Dentinger says.

What they found is that porcini have evolved in different ways, but are clearly recognizable all over the world. “In North America, there is a strong categorization of genetic classes that separate into local regions, despite the fact that they are not reproductively isolated,” Trimble explains. “In Europe, however, one dynasty dominates from Spain to Georgia to Scandinavia.”

Evolutionary biologists usually believe that there is a single evolutionary strategy that governs the speciation process for a particular organism, but Triple and Dentinger have shown that porcini do in fact exhibit multiple and divergent strategies. Indeed, this is the first genetic study in any organism to show such a result on a global scale.

An important related finding is the refutation of the traditional notion that isolation is the primary way species evolve their uniqueness. Such as Encyclopedia of Ecology (2nd Edition – 2019) proudly says: “All evolutionary biologists agree that geographic isolation is a common, if not the most common, mechanism by which new species arise (Futuima, 2013).”

More than just mushroom identification

It’s an exciting time to be a mycologist. Not only has the fungi kingdom been explored and described, but DNA sequencing technology has introduced a seismic shift in how mycologists classify fungi. For thousands of years, humans have identified good edible mushrooms from poisonous ones based on their appearance or phenotype. But phenotypes can be deceiving—think of a brother and sister who have different hair color, different nose shapes, and so on. They are still more genetically similar to each other than to other populations. Thus, genetic similarities are the true marker of different species, bucking a trend of mushroom identification that stretches back to the beginning of humanity.

Further, let us remember that a fungus is just a reproductive construct of the main organism, called a fungus. Like icebergs, fungi only show us their tip, while the huge fungal body lives underground, attached to tree roots. boletus edulisIt spreads geographically thanks to the tiny spores released by the porcini mushroom, which are carried by the wind, mammals and even flies. Thus, biologists tend to believe that in any geographic area where spores can fly, the species will be determined by genetic mixing within that geographic space.

Tremble and Dentinger’s study soundly refutes this assumption.

In North America, different genetic lineages exist side by side, and despite genetic evidence of admixture, local environmental factors have played the largest role in maintaining these lineages’ distinction. “Utah just happens to be one of the areas where two distinct lineages live,” Dentinger notes. What these lineages show is that the local environment is a stronger factor in maintaining their genetic distinctiveness than gene flow from other lineages.

“This paper shows that you don’t need to be isolated because of genetic difference,” says Trimble. “The force of environmental adaptation is very strong in boletus edulis that although you can disperse spores basically anywhere, there is strong selection to adapt to certain environments.”

Dried porcini wonders

The secret to their study lies deep at the heart of natural history museums: mushroom collections. Tremble is a PhD candidate in the School of Biological Sciences, defending his dissertation in the spring of 2023 in the direction of his degree in Evolutionary Biology. He made a serendipitous choice when working with Dentinger as his advisor—as Curator of Mycology at NHMU, Dentinger set up the Genome Laboratory at NHMU to be able to analyze DNA quickly and efficiently. More importantly for this study, Dentinger’s professional contacts at natural history museums around the world helped Tremble gain access to 160 specimens that would have been nearly impossible to collect otherwise.

“You have to rely on opportunistic encounters in nature to collect a live specimen,” Dentinger explains. “This is fundamentally different than working with plants that are in every season and animals that you can hunt.” Thus, it took an incredible amount of logistics, timing, and luck to find, properly identify, and ship 160 different samples across the Northern Hemisphere to the laboratory at NHMU.

Instead, “our study was all possible thanks to fungi,” says Dentinger, referring to the name of the museums’ collections of fungi. They plumbed the fungus depths of NHMU and reached out to collaborators all over the world.

Trimble notes that “without the accumulated field work by 80 different people, this would not have been possible.” All samples of porcini mushrooms were dried, stable and ready to use Tremble for DNA extraction. Where boletus edulismycelia have a surprisingly long lifespan (estimated to be up to 45 years), they only used samples dating back to 1950 to make sure the study only lasted a few generations.

Tremble used sophisticated software to perform the statistical analyzes on these samples. He genotyped 792,923 SNPs (pronounced “snippets,” short for single nucleotide polymorphism), the individual ways in which the 160 porcini genomes differ from one another. In order to classify major lineages, he filtered out SNPs that were only present in one sample (which could be thought of as just a “family unit” or individual variant) so that instead he could only note major differences between the genomes. In the end, Trimble identified 6 major dynasties.

Feeding his data into mathematical models, Trimble unveiled an intricate web of genomic admixture, in which strains remained distinct despite evidence of other strains admixing with them. Their modeling and geographic sample data showed that this distinct survival ability was due to ecological adaptation, not physical isolation.

Genealogy or species?

Tremble and Dentinger take a decidedly neutral approach to the question of whether these six distinct breeds should be designated as “species.” They refrain from doing so in their paper because they want to focus on genetic data and the larger questions of strategy in evolutionary biology. Plus, the discussion of genres is one confusing conversation.

Trimble notes, “There is no formal process for defining a species, it’s an ongoing discussion. We didn’t want to call them species or subspecies because it automatically means that they are groups that evolve separately, which they certainly are not.” They decided to call them genealogical because that term is solvable. Genetically, that is, lineages can be quantitatively distinguished from one another using statistical genetic methods.

But this does not mean that they do not want to address the classification. “This will be a forthcoming article in a different journal,” says Dentinger. Never before had the Victorian era witnessed the explosion of species identification and naming that occurred with animals and plants. With only an estimated 5% of fungal diversity identified, nomenclature and classification need to happen, just to help mycologists talk about their subject.

However, the subspecies classification of the species boletus edulis Shake, Dentinger assures us of one thing: “Terroir is more important than people think.”



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