The parasitic ascomycete Mniaecia jungermanniae induces the formation of giant perichaetia and apogamous sporophytes in leafy liverworts

Abstract

To test the hypothesis that the apothecia of Mniaecia jungermanniae (Nees ex Fr.) Boud. growing on stems of the liverwort Cephalozia bicuspidata (L.) Dumort. might be the perfect stage of an ascomycete fungus forming a symbiotic association with swollen rhizoids in a liverwort, we grew both axenically and resynthesized the association. Contrary to our expectation, Mniaecia remained extracellular, but induced, on Cephalozia , the formation of giant perichaetia containing supernumerary archegonia followed by parthenogenetic and apogamous sporophytes. This is the first report of these latter two features in liverworts. By analogy with apogamy and apospory in mosses and pteridophytes that are induced by media supplemented with organic nutrients and growth regulators, we suggest that the abnormal structures in Cephalozia resulted from substances produced by Mniaecia. Giant perichaetia and abnormal perianths were also observed on wild colonies of Cephalozia, Diplophyllum , and Scapania heavily infected with Mniaecia. Apart from local depression of perianth formation in the wild, this parasitic fungus appears to cause no long-term damage to its hosts.