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Certain Mycena strains living in the Arctic have some of the largest mushroom genomes ever described.
Mycena mushrooms exhibit unexpectedly large genomes, particularly in Arctic
Implications of Genome Size for Adaptation
“Evolution tells us that non-advantageous traits tend to disappear over time, so an obvious implication is that adaptability and generalism in those large genome structures must be an advantage for these fungi,” added Francis Martin of the INRAE (French National Research Institute for Agriculture, Food and Environment) and the University of Lorraine in Champenoux, France. “This is despite the costs of having a large genome with lots of possibly unnecessary features that must be replicated in each cell division. This may be particularly true in an extreme environment like the Arctic, as also seen in plants.”
Study Motivations and Methodology
The researchers set out to study Mycena based on their role as a main mushroom decomposer of litter and leaves in forest ecosystems. Despite their tiny fruiting bodies, Mycena have an important role in the global carbon cycle. This group of mushrooms had long been thought to live purely on dead organic material, but more recently it was found that some species also make a living through cooperative or parasitic interactions with living plants.
Mycenas are also bioluminescent – i.e. they glow in the dark – and earlier work describing the genomes of five Mycena species had investigated this phenomenon. To learn more about their direct lifestyle habits, the researchers now wanted to study a broad palette of Mycena species with different preferences for substrates.
Extensive Genome Studies Reveal New Insights
In the new study, they generated new genome sequences for 24 additional Mycena species and a related species now known as Atheniella floridula. The species included represent six decayer categories: wood generalists, broadleaf wood decayers, grass litter generalists, broadleaf litter decayers, coniferous litter decayers, and overall litter generalists. It also included three Arctic species.
They added their new genomes to 33 additional genomes from non-Mycena species. They wanted to understand how the genomes had evolved and expanded over evolutionary time and how species might differ in plant cell wall-degrading enzymes based on their lifestyle habits.
Unprecedented Genome Sizes in Arctic Species
They were surprised to find that Mycena showed massive genome expansions overall, affecting all gene families regardless of their expected habits. The expansion appeared to be driven by the emergence of novel genes as well as gene duplications, enlarged collections of genes that produce enzymes for degrading polysaccharides, the proliferation of transposable elements, and horizontal gene transfers from other fungal species. They also found that two species collected in the Arctic had the largest genomes by far, at a size that is two to eight times bigger than Mycena living in temperate zones.
The researchers were particularly surprised to find that the genomes of the Arctic species expanded significantly beyond the general Mycena expansion. Additionally, they discovered that Mycena fungi had acquired genes from Ascomycetes through horizontal gene transfer. These species are also found in temperate regions, but it remains unclear whether their large size is due to specific species characteristics or an effect related to the Arctic environment.
The Role of Environmental Factors in Genomic Variations
However, some Arctic plants have been shown to inflate their genomes with transposable elements, or simply duplicate their entire genomes altogether compared to their close relatives in temperate areas, and it is, of course, tempting to suggest that a similar parallel evolution could be happening in Arctic mushrooms.
Ongoing Observations of Mycena’s Evolutionary Adaptations
“The evolutionary transition from decomposer to symbiotic fungi is generally believed to have happened in parallel in several fungal groups throughout the course of evolution millions of years ago,” says Håvard Kauserud of the DOI: 10.1016/j.xgen.2024.100586