Excipulum Structure of Opegrapha, Alyxoria and Zwackhia

On the basis of molecular results, several Opegrapha species have been assigned to other genera. The aim of this study was to investigate if there are differences in excipulum structure between Alyxoria, Opegrapha s. str. and Zwackhia. Microscopic examination of the excipulum of Alyxoria culmigena, A. ochrocheila, A. varia, Opegrapha vulgata (type species of Opegrapha), Zwackhia sorediifera and Z. viridis was performed on 3-5 μm thick frozen sections. Small portions of the ascomata were frozen in water and cut with a freezing microtome. The sections were incubated with NaOCl (Klorin 1:10), and the progressive dissolution of the dark excipulum pigmentation was photographed continuously using a light microscope. Maximum dissolution time was about 50 minutes. A computer generated contrast enhancement of the hyphal walls was made with Adobe Photoshop. The molecular-based distinction between the genera Alyxoria and Zwackhia is supported by morphological differences. The excipulum hyphae of Alyxoria originate from the hypothecium while in Zwackhia they apparently originate from a layer below the hypothecium. Thus, our results indicate that excipulum structures can be used to distinguish these genera. For the morphological differentiation of Opegrapha vulgata and Alyxoria spp. more investigation is required.