Bathelium
Bathelium Ach., Methodus, Sectio prior (Stockholmiæ): 111 (1803).
Index Fungorum number: IF 517; Facesoffungi number: FoF 08789, 16 morphological species (Species Fungorum 2022), 7 species with molecular data.
Lichenized on host. Thallus corticate, yellow-brown to olive-green. Photobiont Trentepohlia. Sexual morph: Ascomata solitary to aggregated in distinct pseudostromata, prominent to sessile, brown-black and remaining fully exposed, rarely pruinose, coriaceous to carbonaceous, ostiolate, ostiole apical. Involucrellum reduced. Excipulum brown-black. Hamathecium comprising paraphysoids, hyaline, straight, branched and anastomosing, embedded in a gelatinous matrix. Asci 1–8-spored, bitunicate, fissitunicate, clavate to oblong, short pedicellate, with a non-amyloid ocular chamber. Ascospores irregularly arranged to biseriate, fusiform-ellipsoid, hyaline, septate to mostly muriform, more or less euseptate, smooth-walled, not or slightly constricted at the septa (often at the middle septum only), surrounded by a gelatinous sheath. Asexual morph: Pycnidia unknown. Chemistry: lichexanthone sometimes produced on thallus surface; yellow to orange anthraquinones often produced in the medulla of the pseudostromata (adapted from Aptroot & Lücking 2016).
Type species: Bathelium mastoideum Afz. ex Ach.
Notes: Bathelium is characterised by solitary to aggregated ascomata, brown-black excipulum, and fusiform-ellipsoid, hyaline, septate to mostly muriform ascospores surrounded by a gelatinous sheath. The asexual morph is not reported. Bathelium resembles Trypethelium s. str in having pseudostromata not covered by a thallus, internal pigment granules and thin-walled ascospores with a reduced endospore. Harris (1995) resurrected Bathelium to include species previously placed in Trypethelium and Laurera and having grouped ascomata formed in brown pseudostromata containing anthraquinones, forming an outer pseudostromatal layer of jigsaw puzzle-shaped cells and also designated a lectotype. Harris (1995) transferred Astrothelium degenerans and A. endochryseum to Bathelium and referred the hamathecium inspersion as a character of species with muriform ascospores. In the phylogenetic analysis of Nelsen et al. (2014) based on mtSSU and nuLSU, some species of Bathelium namely those characterised by small, transversely septate ascospores clustered in the “Astrothelium” clade. Nelsen et al. (2014) reported that taxa in Bathelium sensu stricto (B. lineare, B. madreporiforme, B. tuberculosum) characterised by muriform-spored produce pseudostromatal type and form a monophyletic lineage. In the nuLSU analysis of Nelsen et al. (2014), the three Bathelium species formed a monophyletic group with an undescribed species sister to it but in their mtSSU analysis, the undescribed species clustered with B. madreporiforme. The type species of Bathelium, B. mastoideum lacks sequence data. Nelsen et al. (2014) postulated that B. mastoideum is expected to group with other muriform-spored Bathelium species in the Bathelium s. str. clade based on its close similarity with B. madreporiforme. Nelsen et al. (2014) also reported important findings such as the Bathelium clade falls outside the “Astrothelium” clade in their combined analysis of mtSSU + nuLSU while members having transversely septate ascospores such as B. degenerans, B. endochryseum and B. feei clustered within or near “Astrothelium” clade. The authors confirmed the early concept of Bathelium as documented by Trevisan (1853) and Massalongo (1860), who excluded taxa with transversely septate ascospores from Bathelium. Nelsen et al. (2014) also reported that pseudostroma type, rather than ascospore type is of phylogenetic significance since the undescribed species with transversely septate ascospores clustering with Bathelium s. str. also produce pseudostromata similar to the muriform-spored species but species grouping within Astrothelium clade have smaller pseudostromata with different morphology. Aptroot and Lücking (2016) mentioned that all members of Bathelium have a clear hamathecium except for the ostiolar region, which is inspersed in many species of Trypetheliaceae. Bathelium is morphologically and phylogenetically a distinct genus in Trypetheliaceae but is heterogeneous. Molecular markers available for Bathelium include B. degenerans (LSU, SSU, RPB1, mtSSU), B. endochryseum (LSU, mtSSU), B. feei (LSU, mtSSU), B. lineare (LSU, mtSSU), B. madreporiforme (ITS, LSU, RPB1, RPB2, mtSSU), B. porinosporum (ITS, LSU, mtSSU) and B. tuberculosum (LSU, mtSSU). Bathelium needs epitypification hence, the type species of Bathelium needs to be re-collected and sequenced. More species with sequence data will help to solve the paraphyletic nature of Bathelium and resolve its taxonomy.
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