International study suggests that flexibility of dispersal strategies is critical to the successful invasion of California by European mushroom species

Over the past 15 years, a team of scientists spanning two continents (Europe and the USA), including CFE researcher Susana C. Gonçalves, has sampled the fungus Amanita phalloides to reveal that this European mushroom often establishes itself from sexual spores, but that in California forests, where it is invasive, individuals can live for at least a decade and grow up to 10 m in diameter.

The new study "Death caps (Amanita phalloides) frequently establish from sexual spores, but individuals can grow large and live for more than a decade in invaded forests", coordinated by Anne Pringle from the University of Wisconsin-Madison, USA, has just been published in a special issue of the prestigious journal New Phytologist dedicated to the investigation of mycorrhizal symbioses.

Biological invasions are reshaping Earth's biodiversity, but changes in the distribution of nonpathogenic fungi remain largely undocumented, as do the mechanisms that enable the invasions. Although it has been known for several years that the ectomycorrhizal mushroom Amanita phalloides is native to Europe and invasive on the west coast of North America, there was a lack of information about its mode of reproduction and the persistence of individual "genets", that is, genetically distinct individuals.

Populations from the invaded area in California had also never been compared with populations from the European native range. In this work, through sampling on both continents and population genetic and genomic analyses on mapped mushrooms, the researchers tested whether the species disperses preferentially through mycelium growth or from sexual spores. Then, they tested whether genets are temporary or persistent by analysing the same populations over time.

The researchers found that most Amanita phalloides genets in Europe and California are small and that most individuals correspond to a single mushroom, suggesting frequent establishment from sexual spores. However, in the area where it is invasive, the species appears to hedge its bets: At two sampling sites in California, 17 mushrooms sampled in 2004, 2014, and 2015 represented single genets and, in one particular case, a nearly 30-foot individual width. The authors suggest that "a flexible life history strategy may be part of the reason for the success of the invasion on the US West Coast."

"This study elucidates the strategies used by this fungus to colonize new areas and compete with native species, but the implications for the functioning of ecosystems are not clear", notes Susana C. Gonçalves. "Larger individuals have the potential to establish connections between several trees in the forest in a common mycelium network. But more studies are needed to characterize the nature and implications of this interaction in their distribution areas, comparatively", he reinforces.

"A practical aspect of this study's results is the conclusion that mushroom picking will not allow control because the persistent mycelia will continue to inhabit the soil and reproduce!" concludes the researcher.