DSE-like fungal endophytes of Jurassic gymnosperms from Poland

We document, for the first time, the occurrence of presumed dark septate fungal endophytes in leaves from the Jurassic period. Fossil endophytic fungi were identified in leaves of gymnosperms (Czekanowskia sp., Ctenis sp., and Pseudotorellia sp.) from Lower and Middle Jurassic deposits in Poland. The endophyte remains are preserved as microsclerotia, occurring either alone or in association with intracellular hyphae, and are morphologically comparable to those of extant root-inhabiting dark septate endophytic (DSE) fungi. As in modern dark septate endophytes, the microsclerotia of the Jurassic leaf endophytes most likely functioned as resting structures and propagules enabling the colonisation of new host plants. The discovery of microsclerotia in Mesozoic gymnosperms from Poland provides important new insights into the evolutionary history of plant-endophyte interactions and suggests that complex ecological relationships between dark septate endophytic fungi and terrestrial plants were already established by the Jurassic.