The Parmeliaceae is a large and diverse family of Lecanoromycetes. With over 2700 species in 71genera, it is the largest family of lichen-forming fungi. The most speciose genera in the family are the well-known groups: Xanthoparmelia, Usnea, Parmotrema, and Hypotrachyna. Nearly all members of the family have a symbiotic association with a green alga. The majority of Parmeliaceae species have a foliose, fruticose, or subfruticose growth form. The morphological diversity and complexity exhibited by this group is enormous, and many specimens are exceedingly difficult to identify down to the species level. The family has a cosmopolitan distribution, and can be found in a wide range of habitats and climatic regions. This includes everywhere from roadside pavement to alpine rocks, from tropical rainforest trees to subshrubs in the Arctic tundra. Members of the Parmeliaceae can be found in most terrestrial environments.
Taxonomy
Based on several molecular phylogenetic studies, the Parmeliaceae as currently circumscribed has been shown to be a monophyletic group. This circumscription is inclusive of the previously described families Alectoriaceae, Anziaceae, Hypogymniaceae, and Usneaceae, which are all no longer recognised by most lichensystematists. However, despite the family being one of the most thoroughly studied groups of lichens, several relationships within the family still remain unclear. Phylogenetic analysis tentatively supports the existence of six separate clades in the family:
Alectorioid clade
Cetrarioid clade
Hypogymnioid clade
Letharioid clade
Parmelioid clade
Psiloparmelioid clade
However, this still leaves roughly 42 Parmeliaceae genera unplaced.
Characteristics
Thallus
Parmeliaceae thalli are most often foliose, fruticose or subfruticose, but can be umblicate, peltate, caespitose, crustose, or subcrustose. One genus, Nesolechia, is even a lichenicolous fungus. They can be a variety of colours, from whitish to grey, green to yellow, or brown to blackish. Many genera are lobe forming, and nearly all are heteromerous. Species are usually rhizinate on the lower surface, occasionally with holdfasts, rhizohyphae, or a hypothallus. Only a few genera have a naked lower surface. The upper surface has a pored or non-pored epicortex. is solid, but often loosely woven.
Apothecia
Apothecia are lecanorine, produced along the lamina or margin, and sessile to pedicellate. Thalline exciple is concolorous with the thallus. Asci are amyloid, and the vast majority of species have eight spores per ascus, though a few species are many-spored, and several Menegazzia species have two spores per ascus.
Spores
s are simple, hyaline, and often small. Conidia generally arise laterally from the joints of conidiogenous hyphae, but arise terminally from these joints in a small number of species. The conidia can have a broad range of shapes: cylindrical to bacilliform, bifusiform, fusiform, sublageniform, unciform, filiform, or curved. Pycnidia are immersed or rarely emergent from the upper cortex, are produced along the lamina or margins, pyriform in shape, and dark-brown to black in colour.
Chemistry
Members of the Parmeliaceae exhibit a diverse chemistry, with several types of lichenan, isolichenan and/or other polysaccharides being known from the cell walls of many species. The wide diversity in the types of chemical compounds includes depsides, depsidones, aliphatic acids, triterpenes, anthraquinones, secalonic acids, pulvinic acid derivatives, and xanthones. The compounds usnic acid and atranorin, which are found exclusively in the Parmeliaceae, are of great importance in the systematics of the family, and the presence or absence of these chemicals have been used in several instances to help define genera. Parmelia and Usnea are the best chemically characterized genera, while the species Cetraria islandica and Evernia prunastri have attracted considerable research attention for their bioactive compounds. A study of three parmelioid lichens collected from high-altitude areas of Garhwal Himalaya, showed considerable variation in the chemical content with the rising altitude. This suggests that there is a prominent role for secondary metabolites in the wider ecological distribution of Parmelioid lichens at higher altitudes.
Photobiont
The main photobiont genus that associates with Parmeliaceae species is the ChlorophyteTrebouxia. In particular, the species Trebouxia jamesii appears to be especially prominent. Some Parmeliaceae genera are also known to associate with Asterochloris, but the frequency of this association is not yet known. In general, photobiont diversity within the Parmeliaceae is a little studied subject, and much is left to discover here.
