Scleromitrula capillipes comb. nov., a stipitate-cupulate species on decayed Alnus leaves, with critical notes on its remarkable spore number variation and the circumscription of Ciborinia and the family Sclerotiniaceae

Phialea capillipes was described by Quélet in 1877 from dept. Haute-Saône, France, and was transferred by Saccardo to the genus Sclerotinia because of its apothecia emerging from a seminiform sclerotium. Remnants of host vessel elements (vascular bundles, tracheids) within the sclerotial medulla, however, visible in microscopic sections as spiral thickenings, exclude Sclerotinia but suggest, together with the absence of vacuolar bodies (VBs) in the paraphyses and a macroconidial state, a relationship with Ciborinia and Scleromitrula (= Verpatinia). The species is reported in the present study from over 30 collection sites in different parts of Europe. It is characterised by discoid apothecia emerging singly from the often banana-shaped sclerotium by a usually very long and thin, often flexuous stipe. Another characteristic is the asci being either almost uniformly 4-spored within an apothecium by forming binucleate ascospores, or 8-spored by forming uninucleate ascospores. The sclerotia develop in veins of Alnus leaves, on which the fungus dwells as a possible parasite. The obvious preference of the fungus for alder leaves was impossible to determine with certainty in many of the collections because the substrate tends to be very decayed and fragmentated when the apothecia appear, with often only small pieces remaining attached to the sclerotia. While the type of P. capillipes was 8-spored, a majority of collections had 4-spored asci. Because of the occasional mixed occurrence of 4- and 8-spored apothecia at the same collection site, it is assumed that the two variants belong to a single species. Two generated ITS sequences, one from a 4-spored apothecium and one from an 8-spored apothecium, confirm conspecificity of the two variants. Phylogenetic analysis of this barcode region refers P. capillipes to the genus Scleromitrula, into which the species is consequently transferred. After the very similar but consistently 8-spored S. candolleana, which is adapted to Quercus leaves, P. capillipes is the second species combined in Scleromitrula having stipitate-cupulate (discoid) instead of stipitate-capitate (campanulate, mitruloid, verpoid) apothecia as in the typical species of the genus. Despite infraspecific variation in some of the Scleromitrula spp., the ITS region proved to be a valuable barcode in this genus, with interspecific distances of 1.1–5.5%, although ITS2 alone was partly insufficient. S. shiraiana is shown to represent two different species, based on an ITS distance of 4.1–4.4%. A third species with cup-shaped apothecia, Ciborinia bresadolae, is transferred to Scleromitrula because of its high morphological similarity with S. candolleana, whereas Ciborinia s.str. (C. foliicola and C. whetzelii, perhaps also C. violae and C. wisconsinensis) is genetically distinct and differs from Scleromitrula by ascospores with two medium-sized lipid bodies. Several “Ciborinia” spp., such as C. erythronii, C. gracilis, and C. trillii, are misplaced in Ciborinia because of a prosenchymatic ectal excipulum reminiscent of Monilinia, but molecular data are lacking in some of them. When established, Scleromitrula was included in Geoglossaceae because of its mitruloid or verpoid ascomata by overlooking the polyphyletic origin of this feature. Morphological distinction between S. candolleana and 8-spored apothecia of S. capillipes, the former with slightly shorter and broader ascospores, is problematic, but the apparent host specificity of the two species, the consistent absence of 4-spored asci in collections on Quercus, and molecular data support their independence.