Polychidium
Polychidium is a genus of lichen-forming fungi in the family Massalongiaceae. These lichens form tiny, shrub-like tufts with branching filaments that create woolly cushions on moss-covered rocks and tree twigs. The genus was traditionally thought to include four similar-looking species, but molecular studies revealed that three of these actually belong to a different genus called Leptogidium. Today, Polychidium in the strict sense contains only P. muscicola and three closely related species, found from tropical to subarctic regions worldwide.
Taxonomy
Polychidium has historically been regarded as a single genus that unites four morphologically similar cyanolichen species, one ground-dwelling taxon containing Nostoc and three epiphytes housing Scytonema.The modern concept of the genus was framed by Aino Henssen in 1963, who placed these species together because of their distinctive dendroid thallus and apothecia, explicitly down-playing differences in choice.
Molecular studies have since overturned that pragmatic morphology-based arrangement. A multilocus phylogeny demonstrated that the terricolous P. muscicola nests in the family Massalongiaceae, whereas the three Scytonema-bearing epiphytes form a well-supported clade inside the Pannariaceae. Because that clade is not in the same genus as P. muscicola, it was resurrected under its oldest available name, Leptogidium.
These results show that the genus as traditionally circumscribed is polyphyletic and that its dendroid body plan evolved at least twice, an example of convergent evolution in the Peltigerales. Following these molecular findings, Polychidium in the strict sense is now restricted to the Massalongiaceae and includes P. muscicola and three closely related species, while the former epiphytic species have been transferred to Leptogidium within Pannariaceae.
Description
Polychidium forms minute, shrub-like tufts made of slender filaments that branch almost dichotomously and weave into domed or straggling cushions. Each filament is round in cross-section and wrapped in a distinct one to several cells thick, enclosing a central bundle of hyphae with rounded walls. The lichen's photosynthetic partner is the cyanobacterium Nostoc, which gives the thallus its ability to fix atmospheric nitrogen. Each "woolly‑bear" tuft is engineered like a tiny brush: stout fungal hyphae run perpendicular to the branch axis, creating a stable core in which chains of Nostoc lie twisted or straight. The filament is wrapped in a distinctive brick‑work cortex of rounded fungal cells, contrasting with the interlocking‑puzzle cortex seen in the related genus Leptogidium.The sexual fruiting bodies are mainly lateral apothecia whose brown sit slightly sunken below a low rim. No true surrounds them, but a of fungal tissue is present. Threads called paraphyses run through the hymenium; they are unbranched, segmented by cross-walls, and carry tiny swollen tips. Asci are broadly cylindrical, eight-spored and display a blue reaction in their thickened apices. The ascospores are one- or two-celled, colourless, ellipsoidal to spindle-shaped, and may have either thin or relatively thick walls.
Asexual reproduction occurs in small, brown pycnidia that also develop on the sides of the filaments. These structures contain short conidiogenous cells that bud off rod-shaped conidia. Thin-layer chromatography has revealed no secondary metabolites in the genus.