Cetrelia

''Cetrelia'' is a genus of leafy lichens in the large family Parmeliaceae. They are commonly known as sea-storm lichens, alluding to the wavy appearance of their lobes. The name of the genus, circumscribed in 1968 by the husband and wife lichenologists William and Chicita Culberson, alludes to the former placement of these species in the genera '' Cetraria'' and '' Parmelia''.

The main characteristics of the genus are the broad, rounded lobes of the greyish-green lichen body, and the presence of tiny pores in the outer surface (the cortex) that enable gas exchange. The lower surface of the lichen is brown to black with few root-like rhizines to act as holdfasts, resulting in a fairly loose attachment to the surface the lichen is on. Sexual reproductive structures are usually not present, so the physical characteristics used to distinguish between ''Cetrelia'' species include the presence or absence of asexual reproductive structures such as isidia, soredia, and lobules. ''Cetrelia'' lichens are chemically diverse and produce several secondary chemicals: atranorin is the main compound produced in the cortex, while substances known as depsides or depsidones are compounds in the medulla (the tissue layer under the cortex) that can be used to help distinguish between species.

The number of species in ''Cetrelia'' depends on how the chemical variation in the genus is categorized: there are five distinct forms (morphotypes), and additional morphologically identical but chemically unique species (chemotypes). Although 19 ''Cetrelia'' species have been formally described, some authorities prefer to consider the morphologically similar taxa as chemotypes of the same species. Molecular phylogenetic analyses suggest that the chemical races are best treated as distinct species. Known predominantly from the Northern Hemisphere, most ''Cetrelia'' species are found in eastern or southern Asia. They prefer somewhat moist, cool habitats, and are most commonly found on tree trunks, but sometimes on rocks or on bryophytes over shaded boulders. The type species, '' Cetrelia cetrarioides'', is widely distributed, having been recorded in Asia, Europe, and North America. Most ''Cetrelia'' species that occur in Europe are considered rare or threatened, and appear on the Regional Red Lists of several countries. In Asia, some ''Cetrelia'' lichens are used in the preparation of natural dyes, for the production of material with antibiotic-like properties used in traditional medicine, or in the preparation of litmus reagent.

Systematics

Taxonomic history

The generic name ''Cetrelia'' combines ''Parmelia'' and ''Cetraria'', the two genera in which most of the species were originally classified. The genus was circumscribed by lichenologists William and Chicita Culberson in 1968. In 1960 they started a study of the genus '' Cetraria'' in the broad sense, as it had been defined by Alexander Zahlbruckner in his 1930 work ''Catalogus Lichenum Universalis''. The genus had become a wastebasket taxon, a repository for all parmeliaceous lichens with apothecia on the margins. The Culbersons started by studying the broad-lobed species that had traditionally been placed in ''Cetraria'', a group they called the "parmelioid" ''Cetrariae''. This group included ''Cetraria sanguinea'', ''C. japonica'', and ''C. collata'', as well as a group referred to as the ''Cetraria glauca'' group, including ''C. norvegica'' and ''C. chrysantha''. This latter group of species had been previously identified by Norwegian botanist Eilif Dahl, who had suggested that ''Cetraria'' was polyphyletic in a 1952 study. The Culbersons noticed that ''Cetraria sanguinea'' and ''C. japonica'' were very similar in morphology to two ''Parmelia'' species, ''Parmelia cetrarioides'' and ''P. olivetorum''. As a result of their investigations, they transferred three species to the new genus '' Asahinea'', created genus ''Platismatia'' for 10 members of the ''Cetraria glauca'' group, and ''Cetrelia'' for 14 members of the ''Parmelia cetrarioides'' group. According to a 2012 review of the family Parmeliaceae, the creation of these three new more natural genera by the Culbersons initiated "a trend of splitting that continued for more than three decades".

In the early 1970s Josef Poelt developed the concept of "species-pairs"—taxa that are morphologically and chemically similar (if not identical), but differ primarily in their reproductive modes: one taxon reproduces sexually, the other vegetatively. Using ''Cetrelia'' as an example to explain his ideas, he suggested that both members of these pairs should be assigned the rank of species. In 1976 the Culbersons introduced the idea of morphological groups, or morphotypes with the genus ''Cetrelia''. They named these groups with the Latin epithets of the most prominent species of that group, thus: ''cetrariodes'' (thallus with soredia), ''isidiata'' (thallus with isidia), ''sinensis'' (thallus with marginal dorsiventral lobules), ''collata'' (thallus without soredia, isidia, or lobules, often without apothecia and large pseudocyphellae), and ''davidiana'' (thallus without vegetative propagules and frequently with apothecia, but pseudocyphellae small). Similarly, the genus contains a number of chemotypes related to the content of the major substances in the medulla. Species with the same chemotype always have the same one or two major constituents, while the complex of minor substances may vary somewhat. Six chemotypes have been recognized in ''Cetrelia''. In a 2019 phylogenetic analysis, these chemotypes were named according to their major medullary substance(s): imbricaric, olivetoric, anziaic, perlatolic, microphyllinic, and alectoronic + α-collatolic.

An example of changing taxonomic affinities is given by the four European species of ''Cetrelia'' (''C. cetrarioides'', ''C. chicitae'', ''C. monachorum'', and ''C. olivetorum''). Several studies have suggested accepting only one species in the group with four chemotypes, separated according to the major medullary substances, which can be detected by standard thin-layer chromatography methods, while others have considered the different chemotypes as separate species.

''Cetrelia'' species are commonly known as sea-storm lichens; according to author Colin Rees, "the greenish-gray uplifted edges of its lobes are reminiscent of foam on ocean waves".

Phylogeny

Prior to the advent of molecular phylogenetics, ''Cetrelia'' was grouped together as part of the morphological group "cetrarioid lichens" – species with erect, foliose thalli with marginal apothecia and pycnidia. However, DNA evidence from several studies showed that the cetrarioid lichens were not a monophyletic group of species, and that ''Cetrelia'' clustered together outside the cetrarioid lichens, in a group called the "parmelioid Cetrariae", along with the genera ''Asahinea'' and ''Platismatia''. Morphological characteristics that unite these three genera include the presence of large, broad, rounded lobes, and thalli that are usually oriented horizontally. Similar to several other groups of parmelioid taxa, ''Cetrelia'' lichens have the polysaccharide isolichenan in their cell wall, which is absent in the cetrarioid group. In a later analysis (2010), ''Cetrelia'' is resolved as the only member of the ''Cetrelia'' clade, one of nine major groups within the parmelioid clade of Parmeliaceae that share similar morphological and chemical characteristics.

A 2019 study investigating the usefulness of secondary metabolites in molecular taxonomy of ''Cetrelia'' demonstrated a clear correlation between lichen chemistry and phylogeny, suggesting that it is useful to include information from secondary metabolites when identifying taxa. ''Cetrelia'' produces a constant set of polyphenolic compounds of still unknown function, specifically orcinol-type depsides and depsidones. Character state analyses showed that metabolites in ''Cetrelia'' seem to be evolving towards more complex substances, indicating possibly their evolutionary importance in the survival or functioning of the species. This research corroborated prior DNA-based evidence that also supported the use of chemotaxonomy to delineate species in ''Cetrelia''.

Description

''Cetrelia'' is characterized by its foliose (leafy), greyish-green thallus that is loosely attached to the substrate. The thallus has laminal pseudocyphellae – these are small pores in the cortex through which the medulla is directly visible. It has a prosoplectenchymatous upper cortex, meaning that it is made of densely agglutinated hyphae that are not swollen and have tiny lumina. The lower cortex is black, at least in parts, with sparse rhizines, and pycnidia (asexual fruiting bodies) on the margin. The ascospores made by ''Cetrelia'' lichens are colourless with an ellipsoid shape, numbering eight per ascus. Their length ranges from 11 to 22 μm, with a width of 6–12 μm. Conidia (asexual spores that are produced in the pycnidia) are rod-shaped and measure 3–6 μm long.

Atranorin is the main secondary chemical in the cortex, and is present in all species. ''Cetrelia'' makes several orcinol-type depsidones in the medulla that can be used as diagnostic substances to help identify specimens to species level. Other secondary chemicals that have been identified in this genus are: alectoronic, anziaic, collatolic, imbricaric, microphyllinic, olivetoric, perlatolic, and physodic acids.

In a study of the carotenoids present in six ''Cetrelia'' species found in Eurasia, only violaxanthin was found to be present in all samples of all species collected in a range of environments. Of the twenty different carotenoids identified in this study, most are common, although a few are rare in lichens. For example, flavoxanthin, detected in the thalli of ''C. japonica'', had previously only been found in three other instances, although it is not uncommon in the blossom and fruit of higher plants. The presence of a single carotenoid common to all species suggests that in these species, carotenoid biosynthesis is to a large extent dependent upon the conditions of the environment.

The presence or absence of reproductive propagules such as isidia, soredia, and lobules are important physical characters in the taxonomy of ''Cetrelia''. Isidia are rare in the genus, being associated only with ''C. braunsiana'' and ''C. isidiata''. Soredia are common; with the exception of ''C. alaskana'', all species are sorediate. The soredia are fine and powdery, and produced on the edge of lobes as continual marginal soralia, which often need magnification to be seen. Lobulae are tiny lobes on the upper surface and margins of the thallus. They are found in ''C. japonica'', ''C. sinensis'', and ''C. pseudolivetorum''.

Morphological characters or chemical characters alone are not enough to distinguish between species in ''Cetrelia'' and they should be considered together. For example, ''C. pseudolivetorum'' and ''C. japonica'' are difficult to distinguish with only a brief physical examination, and are often incorrectly labeled in collections in locations where both species occur. However, they can be identified by checking the colour reaction of C (sodium hypochlorite): a positive reaction (C+), indicates the presence of olivetoric acid, which only occurs in ''C. pseudolivetorum''. In contrast, ''C. chicitae'' and ''C. braunsiana'' produce the same secondary compounds, and so cannot be distinguished by spot tests or thin-layer chromatography. However, they are readily distinguishable through examination: ''C. chicitae'' has soredia on its margin, while ''C. braunsiana'' has isidia.

''Cetrelia'' lichens resemble somewhat those in genus ''Parmotrema'', but species of the latter never have pseudocyphellae and usually have cilia on the margin. ''Platismatia'' is another morphologically similar genus, but it differs from ''Cetrelia'' in always lacking caperatic acid. '' Cetreliopsis'' is a genus segregated from ''Cetreliopsis'' in 1980. Although superficially similar, ''Cetreliopsis'' lichens have a distinct secondary chemistry: they contain usnic acid in the upper cortex, and orcinol depsidones in the medulla.

Species

Altogether, 19 species have been included in ''Cetrelia'', either described as new species, or transferred from other genera. Morphotypes are distinguished primarily by the vegetative propagules: presence or absence of soredia, isidia and lobulae. Although there are only five distinct morphologies (morphotypes) in ''Cetrelia'', 18 species are generally recognized based on differences in secondary compounds (chemotypes), in combination with morphology. Based on the distribution of the majority of the chemotype and morphotype combinations, the centre of speciation in ''Cetrelia'' is thought to be southern and eastern Asia.

*'' Cetrelia alaskana'' – Alaska
*'' Cetrelia braunsiana'' – south Asia
*'' Cetrelia cetrarioides'' – eastern North America; Asia; western Europe
*'' Cetrelia chicitae'' – North America; south Asia; Europe
*'' Cetrelia collata'' – China; Nepal
*'' Cetrelia davidiana'' – China
*'' Cetrelia delavayana'' – China
*'' Cetrelia isidiata'' – Japan
*'' Cetrelia japonica'' – Japan; South Korea; Indonesia; Taiwan
*'' Cetrelia monachorum'' – Asia; Europe; North America
*'' Cetrelia nuda'' – Japan; China; Taiwan
*'' Cetrelia olivetorum'' – Asia; Europe; North America
*'' Cetrelia orientalis'' – Russia
*'' Cetrelia pseudocollata'' – China
*'' Cetrelia pseudolivetorum'' – south Asia
*'' Cetrelia sanguinea'' – Asia
*'' Cetrelia sayanensis'' – Siberia
*'' Cetrelia sinensis'' – China; Taiwan

''Cetrelia rhytidocarpa'' , transferred from ''Cetraria'' to ''Cetrelia'' by H. Thorsten Lumbsch in 1988, is now classified as the type species of '' Cetreliopsis''.

, ''Species Fungorum'' accepts five species of ''Cetrelia'': ''C. cetrarioides'', ''C. chicitae'', ''C. monachorum'', ''C. olivetorum'', and ''C. sayanensis''.

Habitat and distribution

''Cetrelia'' lichens usually grow on bark, but sometimes they are found on rocks and bryophytes. They have been found at a wide altitudinal range, from . In general, the genus prefers somewhat moist, cool habitats, and is often found on tree trunks or bryophytes over shaded boulders. ''Cetrelia'' species found in Europe are largely epiphytic (growing on plants), usually encountered in old natural or seminatural forests on tree bark, and sometimes on mossy rocks. They are usually in areas of high humidity, such as swamp forests, or forests that are surrounded by a lake or river or on hillsides near lakes or streams. The observation that ''Cetrelia'' lichens are commonly found among bryophytes or on bark-growing bryophytes suggests that this association provides better moisture conditions for the lichens. ''Cetrelia'' usually prefers deciduous trees, although on rare occasions it is recorded growing on conifers. A study of several hundred collections of the genus from locations largely in the eastern European Alps revealed the trees most preferred as substrates: sycamore (34%), European beech (14%), alder (mostly grey alder, 13%), oak sp. (8%), willow sp. (6%), and European ash (6%). The substrate selectivity of ''Cetrelia'' is not always consistent through broad geographical regions such as Europe, because the range of the substrate determines whether it will be present in a certain area.

Most ''Cetrelia'' species are found in eastern or southeastern Asia. None are found in Africa, Australia, New Zealand, or South America. Four species have been recorded from Europe: ''Cetrelia cetrarioides'', ''C. chicitae'', ''C. monachorum'', and ''C. olivetorum''. In a 2021 study of ''Cetrelia'' species in Hungary, the use of chemical analysis to determine the lichen secondary metabolites revealed that all four of those species are found in that country, double the number previously thought to be part of the Hungarian lichen flora. Ten species are found in India, nine species from northeast China, and four from South Korea. Five species are known from North America, while eight species have been recorded in the expansive geographic area formerly known as the Soviet Union. Of the five morphotypes, only ''cetrarioides'' is widely distributed in the world. ''Cetrelia cetrarioides'' is found throughout the Northern Hemisphere, with a few records from the Southern Hemisphere.

Conservation

Some of the ''Cetrelia'' species found in Europe are on various Regional Red Lists. In the Baltic countries, the ''Cetrelia'' species are rare and threatened. In Estonia, ''C. olivetorum'' and ''C. cetrarioides'' are considered as Endangered and ''C. monachorum'' as Critically Endangered. Factors threatening these lichens include forest cutting and overgrowing of dunes and alvars due to the cessation of traditional forest management practices. ''Cetrelia olivetorum'' has been included in the Legislative List of Legally Protected Lichens of Latvia, a status that allows for the establishment of small nature reserves (up to ) to aid in its conservation. In Lithuania, all species of ''Cetrelia'' are considered as Endangered. They also appear on the Red Lists of Fennoscandian countries; for example, in Sweden, ''C. olivetorum'' ''sensu lato'' is treated as Critically Endangered, and in Finland, ''C. cetrarioides'' and ''C. olivetorum'' as Endangered. In Belarus, ''C. olivetorum'' and ''C. monachorum'' are listed as Endangered, and ''C. cetrarioides'' as Critically Endangered.

Uses

In India, ''Cetrelia braunsiana'' is used as a source of a natural dye to colour different fibres such as silk, tussar silk, and cotton. Depending on the type of method used to extract the lichen dye, it produces a final colour of ivory yellow or light yellowish olive. Atranorin is one dye-producing substance found in this species; alectoronic and α-collatolic acid are additional secondary chemicals that occur in ''C. braunsiana''.

In China, ''Cetrelia cetrarioides'', ''C. olivetorum'', and ''C. pseudolivetorum'' are collected in bulk for the preparation of material with antibiotic-like properties used in traditional medicine. These three species are also used as a raw material in the production of litmus reagent.

References

Cited literature

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{{Authority control

Parmeliaceae
Lichen genera
Lecanorales genera
Taxa described in 1968
Taxa named by William Louis Culberson
Taxa named by Chicita F. Culberson