Xerotrema
Xerotrema is a small genus of fungi in the family Odontotremataceae. It comprises two species. The genus was circumscribed by Martha Allen Sherwood and Brian John Coppins in 1980. The type species, Xerotrema megalospora, is found in the United States and Canada. X. quercicola was added to the genus in 2008. These fungi produce tiny, deeply cup-shaped fruiting bodies on weathered, barkless wood of conifers and hardwoods in temperate regions. While the genus was originally thought to be non-lichenized, at least one species appears to form facultative partnerships with green algae, meaning it can survive with or without an algal partner.
Taxonomy
Xerotrema was erected by Martha Allen Sherwood and Brian John Coppins in 1980 for a distinctive disc-shaped fungus. Unlike most lichens, which have eight spores per spore sac, this genus has just one spore per sac. That single spore is unusually large and divided into many cells by walls running in multiple directions. The type species is X. megalospora, described from material collected in Scotland and the western United States. Sherwood and Coppins separated the genus from Odontotrema on the spore together with additional microscopic differences from the Odontotrema type. Sherwood and Coppins also argued that odontotremoid fungi are better treated in the Ostropales rather than the Helotiales, and they presented Xerotrema, with its muriform spore, as further evidence for that placement.
A second species, X. quercicola, was described by Coppins and Aptroot in 2008, who noted that the genus was then being placed in the Odontotremataceae. In the same work they pointed out that X. quercicola has amyloid muriform ascospores and a green-tinged, features they considered suggestive of an affinity with the Thelotremataceae and therefore reason to re-evaluate the family placement of the genus. The new species is otherwise very similar in external appearance to X. megalospora, and the two were chiefly separated by the iodine reaction of the spores alongside substrate differences. Sherwood and Coppins discussed occasional associations of Xerotrema with algae but, for bibliographic and nomenclatural reasons, treated such cases as non-lichenised, whereas Coppins and Aptroot later recorded X. megalospora as usually associated with Trentepohlia, i.e. at least facultatively lichenised.
Description
The thallus of Xerotrema species is poorly developed or absent; in some collections it is effectively invisible and the lichenisation is doubtful. By contrast, X. megalospora is usually associated with the green algal genus Trentepohlia, so the genus can be at least facultatively lichenised. The fruiting bodies are tiny, about 0.3–0.6 mm across, and develop within the wood; they begin closed and immersed, then break through and open by a minute, often toothed pore to reveal a deeply cup-shaped disc. The margin is thick and blackened with a rough, crumbly surface, and its inner face bears fine hyphae set in a gelatinous matrix. In X. quercicola the margin shows a distinct green tinge in water mounts, and the lower part of the margin gives a positive K/I reaction. Microscopically, the hymenium has a brown formed by the tips of slender, septate paraphyses cemented in gel, and the gel does not stain with iodine. Asci are functionally, initially thick-walled but lacking a distinct apical apparatus, and typically contain a single spore; they measure about 140–180 × 35–40 μm. The spores are colourless and muriform—that is, divided by both cross and longitudinal walls into many small chambers—and are unusually large for this group. In X. megalospora they are roughly 90–140 × 22–40 μm. In X. quercicola they are smaller at about 52–98 × 16–28 μm and are amyloid. In X. megalospora the spores remain I−. A conidial state has not been seen in X. quercicola. When mature spores are crushed, their internal chambers can separate, which is consistent with late-formed septa that make the spore appear "brick-walled" under the microscope.
Habitat and distribution
Xerotrema is lignicolous, producing its fruiting bodies on weathered, hard, barkless wood, typically on standing trunks rather than on well-rotted timber. X. megalospora is recorded from Scotland and Oregon, where it grows on decorticated wood of pine and of shrubby Arctostaphylos. In Britain it occurs in the native pinewoods of the Scottish Highlands, on dry, standing, barkless pine trunks. Sherwood and Coppins considered it uncommon and noted that it tends to fruit in late winter, when it is easily overlooked. X. quercicola grows on dry, firm lignum of standing, decorticated oak trunks, with records from Wales, western Scotland and southern England. It characterises former coppiced oak woodland undergoing self-thinning, where scattered whitish, barkless snags provide the required substrate.