Hypogymnia Schizidiata

''Hypogymnia'' is a genus of foliose lichens in the family Parmeliaceae. They are commonly known as tube lichens, bone lichens, or pillow lichens. Most species lack rhizines (root-like attachment organs on the lower surface) that are otherwise common in members of the Parmeliaceae, and have swollen lobes that are usually hollow. Other common characteristics are relatively small spores and the presence of physodic acid and related lichen products. The lichens usually grow on the bark and wood of coniferous trees.

''Hypogymnia'' was proposed by lichenologist William Nylander, first as a subgenus of '' Parmelia'' in 1881, and 15 years later as a distinct genus of two species, including the widespread and common type species, '' Hypogymnia physodes''. It has since grown to about 90 recognized species. ''Hypogymnia'' has a centre of biodiversity in China, where many of its species are found.

Taxonomy

''Hypogymnia'' was first created as subgenus of '' Parmelia'' by Finnish lichenologist William Nylander in 1881. He associated it with the species ''Parmelia physodem'' (which ultimately became the type species, '' Hypogymnia physodes''), noting the lack of rhizines as the characteristic distinguishing it from ''Parmelia''. Nylander later promoted it to generic status in 1896. At this time, the genus contained only two species: ''Hypogymnia pertusa'' (currently named '' Menegazzia terebrata'') as well as the type species. For many decades afterwards, the genus did not have widespread recognition, as most lichenologists preferred to include the "hypogymnioid" lichens in the classic form genus ''Parmelia''. In 1951 Hildur Krog considered the morphology and chemistry of this group of species to be distinctive and reinstated the genus ''Hypogymnia''. Krog included four subgenera, including ''H''. subg. ''Cavernularia'' and ''H''. subg.''Everniiformes''. These later became accepted as distinct genera (the former only temporarily; see synonymy below), the latter under the name '' Pseudevernia''.

In 1974, Krog published an account of three Northern Hemisphere ''Hypogymnia'' species that grow on acid rock in arctic and alpine habitats. These species, namely ''H.  atrofusca'', ''H. intestiniformis'', and ''H. oroarctica'', make up the ''H. intestiniformis'' group. This biologically discontinuous assemblage of species was segregated from ''Hypogymnia'' by Trevor Goward under the genus name ''Brodoa'' in 1986. It differs from ''Hypogymnia'' in its compact medulla, larger spores and different cortical structure.

''Hypogymnia'' is classified in the Parmeliaceae. The family Hypogymniaceae has been proposed in the past to contain the genus and other similar hypogymnioid lichens, but this taxonomic arrangement has not been widely accepted by other taxonomists. For example, Krog argued that no critical characters had been suggested that could be used as a defining familial characteristic. In the Parmeliaceae, ''Hypogymnia'' belongs to the hypogymnioid clade along with the genera '' Arctoparmelia'', ''Brodoa'', and '' Pseudevernia''. All of these genera share the common characteristic of having a loosely compact medulla.

''Hypogymnia'' lichens are commonly known as "tube lichens", "bone lichens", or "pillow lichens". The name ''Hypogymnia'', derived from the Ancient Greek - (''hupó'', meaning "under") and (''gumnós'', meaning "naked"), refers to the bare lower surface of the thallus.

Synonymy

Synonyms of ''Hypogymnia'' are ''Cavernularia'', created by Gunnar Degelius in 1937, and ''Ceratophyllum'', created by Maurice Choisy in 1951. ''Cavernularia'' contained two hypogymnoid species, ''C. lophyrea'' and ''C. hultenii''. This species pair has an array of pronounced but small depressions in the lower surface, instead of the smooth or irregularly wrinkled surface typical of ''Hypogymnia''; Degelius called these minute cavities (about 0.1 mm diameter) "cavernulae". The two ''Cavernularia'' species are otherwise similar to ''Hypogymnia'' in terms of overall morphology, microstructure of the apothecia, and chemistry. Molecular analysis showed that ''Cavernularia'' needed to be subsumed into ''Hypogymnia'' in order for the latter genus to be monophyletic. This suggested synonymy had already been proposed several decades earlier by Veli Räsänen in 1943 and Hildur Krog in 1952, but not adopted by later authors, including Krog herself.

Description

''Hypogymnia'' is a genus of medium to large foliose lichens. They are typically greenish grey to brownish grey in colour; some species are yellowish (from usnic acid). The thallus comprises more or less inflated but hollow (tube-like) lobes. These lobes often have a perforation at the tip. The colour of the ceiling of the tubes (the medullary surface) is dark brown or white, and is often used as a characteristic to distinguish between species. The lower surface of the tube is black with a smooth or wrinkled texture.

Rhizines are root-like attachment organs on the lower surface of a lichen thallus, made of elongated strands of hyphae; a shorter version of this attachment organ is called a ''hapter''. Although many recent lichen floras and manuals describe ''Hypogymnia'' as lacking rhizines, a 2015 study challenges the universality of this assertion. In the study, researchers studied a large number of ''Hypogymnia'' collections, representing 72 species as well as 64 type specimens. They found that rhizines and hapters were occasionally present on the lower surface of 35 species. Two types of attachment organs are found: slender rhizines with fine and tapering tips (found in ''H. krogiae'' and ''H. subfarinacea''), and the more common hapters, which are thick with broken tips. Both types are dark brown to black and usually the same colour as the lower surface. In all cases where these attachment organs are found, however, they are few in number and are sparsely distributed on the lower thallus surface.

The apothecia of ''Hypogymnia'' are lecanorine in form with a constricted base and are often raised or shaped like an urn. The apothecial discs are red-brown and typically concave. Ascospores are colourless, ellipsoid, and number eight per ascus. They are relatively small, less than 9 μm long. Pycnidia are black and appear as small dots on the surface of the lobes. The photobiont partner is trebouxioid–a green alga from the genus '' Trebouxia''.

The cortex contains atranorin (responsible for the grey colour), while the medullae of most species have physodic acid, and some species contain other orcinol and beta-orcinol depsidones, including protocetraric acid and physodalic acid.

The genus ''Menegazzia'' contains species that could be confused with ''Hypogymnia''; ''Menegazzia'', however, has perforations on the upper lobe surface, unlike ''Hypogymnia''. Other superficially similar genera, such as ''Brodoa'' and ''Allantoparmelia

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