Pycnolemma
Pycnolemma is a fungal genus in the family Lichinaceae. It contains the single species Pycnolemma polycarpum, a rock-dwelling, gelatinous lichen. The genus was proposed in 2024 when molecular studies showed that the species did not belong in its former genus Lempholemma, but instead formed a distinct evolutionary lineage within the Lichinaceae. Pycnolemma polycarpum forms small, blackish rosettes on limestone rocks in tropical dry forests and is distinguished by pycnoascocarps, in which ascocarps develop from beneath pycnidia. The species has a scattered distribution across tropical regions, having been recorded from Yemen, Oman, Madagascar, Aldabra, and Puerto Rico.
Taxonomy
Pycnolemma was established by Matthias Schultz and María Prieto in 2024 to accommodate the species formerly known as Lempholemma polycarpum. The generic name refers to the production of pycnoascocarps and to the morphological resemblance to Lempholemma, which also contains Nostoc cyanobionts. The species epithet alludes to the characteristically numerous fruiting bodies.The genus was segregated from Lempholemma following phylogenetic studies that revealed the traditional concept of that genus to be polyphyletic. Pycnolemma and Peltolemma form a well-supported clade within the Lichinaceae, distinct from Lempholemma in the strict sense, which remains in the family Porocyphaceae.
Description
Pycnolemma has an to, rosette-shaped thallus composed of radiating. It resembles other or rosette-forming lichens found in dry, rocky habitats, such as Anema, Lichinella, Paulia, Peltula, Peccania, and Thyrea, but is distinguished by its use of a Nostoc photobiont. The thallus is strictly, lacking a central hyphal strand, and becomes gelatinous when wet.Thalli consist of individual, regular, umbilicate-lobate rosettes 3–6 mm across; lobes are radiating, free, tongue-shaped and convex, typically 1.5–2.5 mm long, 0.5–0.6 mm wide and 0.25–0.45 mm thick. The lobe surface is smooth but becomes when numerous small apothecia are produced. Microscopically, the forms a loose network of elongated hyphae that becomes denser towards the lobe margins. The is Nostoc, forming twisted, bead-like chains of 5–20 globose to ellipsoid cells. Apothecia are usually abundant and very small, 200 μm wide, immersed to semi-immersed with a low and a that is reddish-black when dry and dark red when moist. are, hyaline and broadly ellipsoid, c. 12.4 × 6.5 μm, with a wall that thickens with age to c. 1.5 μm.
Within Lempholemma s.l., species liable to be confused with P. polycarpum include Lempholemma socotranum, Lempholemma radiatum, and Lempholemma lingulatum ; superficially similar Collema multipartitum and C. fragile differ in apothecial development and other micromorphological traits.
The apothecia in Pycnolemma develop as pycnoascocarps, a feature shared with some species formerly placed in Lempholemma. This structure differentiates Pycnolemma from morphologically similar genera such as Lingolemma, which has a central hyphal strand, and Peltolemma, which bears typical apothecia rather than pycnoascocarps.
Habitat and distribution
Pycnolemma polycarpum occurs on inclined limestone boulders in relatively exposed habitats on rocky slopes with open forest vegetation. It is known from two localities in south-eastern Yemen: the type collection from Shah-ot, Damkaut, and a second site west of Hawf near the Omani border on south-facing, c. 45°-inclined limestone boulders among Anogeissus dhofarica and Maytenus dhofarensis.The species is found in association with other saxicolous lichens such as Paulia aldabrensis, Paulia perforata, Lempholemma botryosum, and Psorotichia species. It occupies semi-arid, subtropical environments that experience high insolation and moderate humidity from the monsoonal influence along the Arabian coast. Beyond Yemen, the species is also known from Oman, Aldabra, Madagascar, and Puerto Rico, indicating a disjunct, tropical distribution across both Afro-Indian Ocean and Caribbean regions.