Rediscovering a common mushroom with molecular techniques: elucidating Stereum taxonomy and evolutionary history

Stereum is a genus of woodland shelflike fungus that most forest hikers will have encountered at some time or another, as it occurs worldwide. Many Stereum species have been researched for potential medical and industrial use, as they contain unique compounds and are adept wood decomposers. However, accurately identifying these species has proven to be difficult both in the past and still in the present, as they often look very similar to one another and show a wide degree of morphological variation. These seemingly overlapping features have been used in the past to combine multiple species into one, but increasingly in recent years, research has show that distinguishing fungal species on morphology alone leads to an underestimation of species diversity. By sequencing a small but informative region of fungal DNA called the ITS barcode, individuals can be genetically grouped together into species, and mapping traits on to the resulting groups helps to make clear the characteristics most important for separating them. My research shows that individuals referred to as S. ostrea in North America are in fact three previously named species: S. lobatum, S. fasciatum, and S. subtomentosum and that the real S. ostrea, originally described from Indonesia, does not occur in North America. Furthermore, these three species, while similar, can be reliably distinguished by specific morphological features. I then extend lessons from this case study to examine all publicly available Stereum ITS sequences, which includes more species from around the world, to provide a preliminary look at the global species diversity of Stereum. This research shows that not only is there considerable undescribed diversity in Stereum, but that public sequences of Stereum suffer from a high rate of misidentification. Future work using more than one barcode and sequences from undersampled regions will be critical to improving our understanding of the very common genus Stereum.