Rolueckia

Rolueckia was circumscribed in 2008 by the Thai lichenologists Khwanruan Papong, Achra Thammathaworn, and Kansri Boonpragob to accommodate the Calenia conspersa species complex, a group that had most recently been placed in the genus Caleniopsis but showed a distinctive asexual anatomy not seen elsewhere in the family Gomphillaceae, order Ostropales. The authors drew attention to the setiform hyphophores: stiff, reddish-brown bristles that widen into a minute club and shed countless rod-shaped propagules (diahyphae). Because these structures are much larger and morphologically simpler than the branched, prothallus-borne hyphophores of Caleniopsis, they considered the conspersa group generically distinct. Their diagnosis was further supported by a phenotype-based phylogenetic analysis that had already recovered the group as a separate, monophyletic lineage sister to Caleniopsis within Gomphillaceae.^[1]

The genus name honours the German lichenologist Robert Lücking for his extensive work on foliicolous (leaf-dwelling) lichens. Papong and co-workers designated Rolueckia conspersa as the type species, tracing its nomenclatural journey from Thelotrema conspersa (1878)^[2] through Calenia (1952) and Caleniopsis (2005) before settling in its present combination. Two additional taxa were assigned at the same time: R. aggregata, transferred from Calenia, and R. siamensis, described as new. All three species share the same immersed-erumpent, zeorine apothecia and a small, few-segmented ascospore design, yet differ in apothecial aggregation, septation pattern and the presence or absence of sterile setae.^[1]

Rolueckia forms a thin, crust-like film (the thallus) that grows directly on living leaves in moist tropical forests. This film is smooth, pale green-grey to whitish, and often edged by a narrow reddish-brown strip known as the prothallus, which marks where the lichen meets the leaf surface. Inside the thallus live microscopic green algae (Trebouxia-type photobionts) that supply food through photosynthesis. Sexual reproduction occurs in minute, round fruit-bodies (apothecia) that begin buried in the thallus and later break through it. Each apothecium is less than 0.2 mm across and encircled by a mixed fungal-and-thallus rim (described as zeorine). The spore-producing layer (hymenium) is colourless, as is the supporting tissue beneath (hypothecium). Within every club-shaped ascus eight colourless spores develop; these spores are exceptionally small (about 10–14 μm long) and are divided by one to three cross-walls (septa), a feature that helps distinguish species in the genus.^[1]

A conspicuous trait that separates Rolueckia from other members of the family Gomphillaceae is the presence of tall, hair-like asexual organs called hyphophores. These rise 0.3–0.5 mm above the thallus as stiff reddish-brown bristles that broaden into a tiny club at the tip. The club produces countless rod-shaped propagules (diahyphae) only 5–6 μm long; unlike ordinary fungal hyphae, these lack cross-walls and behave much like single-celled spores, allowing the lichen to spread without forming new apothecia. The internal tissues of the thallus and apothecia consist of tightly packed, lengthwise-oriented fungal cells (prosoplectenchyma), while delicate supporting filaments (paraphyses) branch and knit together above the asci. In some species, additional sterile bristles (setae) dot the surface, giving it a slightly fuzzy appearance. The combination of a leaf-dwelling crustose thallus, zeorine apothecia, club-tipped hyphophores with bacilliform diahyphae, and small, few-segmented spores defines Rolueckia and sets it apart from superficially similar genera.^[1]