Tremella cetrariellae

Tremella cetrariellae forms superficial, waxy, jelly-like basidiomata (fruiting bodies) that cause gall-like swellings on the host lichen's thallus (body). The galls are pale to dark brown or black; they start out smoothly convex to nearly spherical and usually become tuberculate (covered with small bumps) as they mature. They measure about 0.1–1.5 mm across and only rarely show a central depression.^[2] Compared with its close relative T. cetrariicola, the mature galls in T. cetrariellae are predominantly tuberculate at maturity, whereas a central depression is often present in many galls of T. cetrariicola.^[2]

Microscopically, its tissue consists of hyphae (filaments) 1.5–4.0 μm wide, often with clamp connections, and it produces frequent haustorial branches (specialized contact structures in parasitic fungi). The spore-producing layer (the hymenium) contains many probasidia (immature basidia). Mature basidia are 2–4-celled, with septa that may be transverse, longitudinal, or oblique. Transversely septate basidia measure 10–33 × 4–11 μm (including the stalk-like base but excluding epibasidia), whilst longitudinally or obliquely septate basidia are 8–13 × 11–18 μm with somewhat cylindrical epibasidia up to 28 μm long. Basidiospores are ellipsoid to roughly spherical, typically 4–8 × 4–8 μm (or occasionally 3–4 × 3–4 μm), and bear a distinct apiculus (spore attachment spot) about 1 μm wide. Chain-forming conidia (asexual spores) were also reported developing within the basidioma.^[2]

Tremella cetrariellae is known only from the thallus of Cetrariella delisei, where it produces gall-like deformations that contain the fungal tissues and its spore-producing tissue. The two sister species also differ in host and habitat. Cetrariella delisei is arctoalpine and epigeic (ground-growing), whereas T. cetrariicola is confined to epiphytic Tuckermannopsis species (growing on plants, such as bark and twigs). Cetrariella delisei contains gyrophoric and hiascic acids (lichen substances), which were reported as absent in Tuckermannopsis; host chemistry was suggested as one factor that may influence host selection in lichenicolous fungi.^[2]

The species has been reported from high-latitude sites in Finland, Greenland, Norway, Russia, Svalbard, and Sweden. The holotype was collected in Finnmark (northern Norway), in a coastal setting with siliceous rocks and dwarf-shrub vegetation (including Empetrum and Betula nana) at 21 m above sea level, and additional material was examined from northern Fennoscandia, Svalbard, Greenland (including the Disko Bay area), and the Khibiny Mountains in Murmansk Oblast (NW Russia).^[2][3]