Gyalectidium

The genus was circumscribed by the Swiss lichenologist Johannes Müller Argoviensis in 1881. In the protologue, he characterised Gyalectidium as a crustose lichen with globose, green gonidia (green algae), lecanorine apothecia with a simple margin that is thalline externally, and paraphyses that are interconnected in a lattice. The spores were described as hyaline and "parenchymatous" (divided into many chambers). He distinguished the genus from Gyalecta by the structure of the paraphyses, and remarked that the species then known were foliicolous and had one-spored asci. He included 3 species: G. xantholeucum, G. dispersum, and G. filicinum;^[2] the last of these is now the type species of the genus.^[3]

In their 2001 monograph, Ferraro and colleagues proposed an infrageneric classification that organised the species treated there into sections and series, largely reflecting thallus texture (especially the distribution of calcium oxalate crystals) and the form and placement of the hyphophores. They recognised three main groupings as sections, each with a single series: sect. Placolectidium (ser. Caucasicae), comprising species with placoid to bullate thalli that are evenly encrusted with crystals and often form their hyphophores at the thallus margin; sect. Areolectidium (ser. Areolatae), including species with smoother thalli lacking crystals or with a distinctly areolate pattern, typically with well-developed hyphophore scales; and the autonymous sect. Gyalectidium (ser. Gyalectidium), centred on the G. filicinum group, characterised by a finely verrucose thallus in which crystals are aggregated into small wart-like patches and by a wide range of hyphophore forms. Two species that appeared isolated in their analysis were placed in monotypic sections and series: sect. Setolectidium (ser. Microcarpae) for G. microcarpum, which has abundant sterile setae and lacks hyphophores, and sect. Goniolectidium (ser. Yahriae) for G. yahriae, in which the hyphophores are modified into multi-part structures with a goniocystangium-like appearance.^[4]

Gyalectidium is a genus of mostly leaf-dwelling (foliicolous) crustose lichens that form small, pale greenish to whitish-grey patches, often only a few millimetres across (rarely approaching about 10 mm). The thallus is always corticate (i.e., it has a surface layer of fungal tissue), and it commonly carries colourless calcium oxalate crystals that give the surface a finely warty, coarsely verrucose, or sometimes distinctly areolate look; in a few species the thallus is smoother because crystals are sparse or absent. The photosynthetic partner is a green alga of the genus Trebouxia.^[4]

The genus is recognised by its specialised asexual structures called hyphophores: small, scale-like outgrowths that may stand upright, slant, or lie close to the thallus surface, and in some species are much reduced or missing. At the base (or centre) of each hyphophore a mass of short, branched hyphae (the diahyphal or "conidial" mass) develops; it is typically intermixed with algal cells and often breaks into smaller rounded propagules that can disperse both partners together. Sexual fruiting bodies (apothecia) are immersed in the thallus and have a thalline margin. Internally they have a colourless, non-amyloid hymenium with richly branched and interconnected paraphyses, and asci that usually contain a single, colourless ascospore. The ascospore is muriform (divided by both transverse and longitudinal septa into many chambers). Pycnidia are uncommon in the genus.^[4]