# Polychidium

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{{Short description|Genus of lichen-forming fungi}}
{{Use dmy dates|date=July 2025}}
{{Use Oxford spelling|date=July 2025}}
{{Automatic taxobox
| image = Polychidium muscicola 1544806604.jpg
| image_caption = ''Polychidium muscicola''
| taxon = Polychidium
| authority = ([Ach.](/source/Ach.)) [Gray](/source/Samuel_Frederick_Gray) (1821)
| type_species = Polychidium muscicola
| type_species_authority = ([Sw.](/source/Sw.)) Gray
| synonyms_ref = <ref name="Species Fungorum synonymy"/>
| synonyms = {{Collapsible list|bullets=on
|''Collema'' †††††††† ''Polychidium'' {{au|Ach. (1810)}}
|''Collema'' sect. ''Polychidium'' {{au|(Ach.) [Bory](/source/Jean_Baptiste_Bory_de_Saint-Vincent) (1823)}}
|''Garovaglia'' {{au|[Trevis.](/source/Trevis.) (1853)}}
|''Pilonema'' {{au|[Nyl.](/source/Nyl.) (1858)}}
|''Garovaglina'' {{au|Trevis. (1880)}}
|''Leptodendriscum'' {{au|[Vain.](/source/Vain.) (1890)}}
|''Pseudoleptogium'' {{au|[Jatta](/source/Antonio_Jatta) (1900)}}
|''Wilsonia'' {{au|[Cheel](/source/Edwin_Cheel) & Dughi (1944)}}
|''Leptogidiomyces'' {{au|[Cif.](/source/Cif.) & [Tomas.](/source/Tomas.) (1953)}}
|''Colleptogium'' sect. ''Polychidiopsis'' {{au|[M.Choisy](/source/M.Choisy) (1962)}}
}}
| subdivision_ranks = Species
| subdivision = ''[P. cetrarioides](/source/Polychidium_cetrarioides)''<br />
''[P. dactylinum](/source/Polychidium_dactylinum)''<br />
''[P. muscicola](/source/Polychidium_muscicola)''<br />
''[P. pulvinatum](/source/Polychidium_pulvinatum)''
}}

'''''Polychidium''''' is a [genus](/source/genus) of [lichen](/source/lichen)-forming [fungi](/source/fungi) in the family [Massalongiaceae](/source/Massalongiaceae).<ref name="CoL_CDW33"/> These lichens form tiny, shrub-like tufts with branching filaments that create woolly cushions on [moss](/source/moss)-covered rocks and tree twigs. The genus was traditionally thought to include four similar-looking species, but [molecular](/source/molecular_phylogenetics) studies revealed that three of these actually belong to a different genus called ''[Leptogidium](/source/Leptogidium)''. Today, ''Polychidium'' in the strict sense contains only ''[P.&nbsp;muscicola](/source/Polychidium_muscicola)'' and three closely related species, found from [tropical](/source/tropical) to [subarctic](/source/subarctic) regions worldwide.

==Taxonomy==

''Polychidium'' has historically been regarded as a single genus that unites four [morphologically](/source/morphology_(biology)) similar [cyanolichen](/source/cyanolichen) species, one ground-dwelling [taxon](/source/taxon) containing ''[Nostoc](/source/Nostoc)'' and three [epiphyte](/source/epiphyte)s housing ''[Scytonema](/source/Scytonema)''.
The modern concept of the genus was framed by [Aino Henssen](/source/Aino_Henssen) in 1963, who placed these species together because of their distinctive dendroid ("woolly-bear") thallus and {{lichengloss|hemiangiocarpic}} apothecia, explicitly down-playing differences in {{lichengloss|photobiont}} choice.<ref name="Muggia et al. 2011"/>

[Molecular](/source/molecular_phylogenetics) studies have since overturned that pragmatic morphology-based arrangement. A multilocus phylogeny demonstrated that the [terricolous](/source/terricolous_lichen) ''P.&nbsp;muscicola'' nests in the family Massalongiaceae (suborder Peltigerineae), whereas the three ''Scytonema''-bearing epiphytes (''P.&nbsp;contortum'', ''P.&nbsp;dendriscum'' and ''P.&nbsp;stipitatum'') form a well-supported [clade](/source/clade) inside the [Pannariaceae](/source/Pannariaceae) (suborder Collematineae). Because that clade is not in the same genus as ''P.&nbsp;muscicola'', it was resurrected under its oldest available name, ''[Leptogidium](/source/Leptogidium)''.<ref name="Muggia et al. 2011"/>

These results show that the genus as traditionally circumscribed is [polyphyletic](/source/polyphyletic) and that its dendroid body plan evolved at least twice, an example of [convergent evolution](/source/convergent_evolution) in the Peltigerales. Following these molecular findings, ''Polychidium'' in the strict sense is now restricted to the Massalongiaceae and includes ''P.&nbsp;muscicola'' and three closely related species, while the former epiphytic species have been transferred to ''Leptogidium'' within Pannariaceae.<ref name="Muggia et al. 2011"/>

==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 {{lichengloss|cortex}} one to several cells thick, enclosing a central bundle of [hypha](/source/hypha)e with rounded walls. The lichen's [photosynthetic](/source/photosynthesis) partner is the cyanobacterium ''[Nostoc](/source/Nostoc)'', which gives the thallus its ability to [fix atmospheric nitrogen](/source/nitrogen_fixation).<ref name="Cannon et al. 2025"/> 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](/source/Leptogidium)''.<ref name="Muggia et al. 2011"/>

The sexual [fruiting bodies](/source/fruiting_bodies) are mainly lateral [apothecia](/source/apothecia) whose brown {{lichengloss|discs}} sit slightly sunken below a low rim. No true {{lichengloss|thalline margin}} surrounds them, but a {{lichengloss|proper exciple}} of fungal tissue is present. Threads called [paraphyses](/source/paraphyses) run through the [hymenium](/source/hymenium); they are unbranched, segmented by [cross-walls](/source/septum), and carry tiny swollen tips ({{lichengloss|capitate}} apices). [Asci](/source/ascus) are broadly cylindrical, eight-spored and display a blue reaction (K/I+) in their thickened apices. The [ascospore](/source/ascospore)s are one- or two-celled, colourless, [ellipsoid](/source/ellipsoid)al to spindle-shaped, and may have either thin or relatively thick walls.<ref name="Cannon et al. 2025"/>

[Asexual reproduction](/source/Asexual_reproduction) occurs in small, brown [pycnidia](/source/pycnidia) that also develop on the sides of the filaments. These structures contain short conidiogenous cells that bud off rod-shaped ({{lichengloss|bacilliform}}) conidia. [Thin-layer chromatography](/source/Thin-layer_chromatography) has revealed no [secondary metabolite](/source/secondary_metabolite)s ([lichen product](/source/lichen_product)s) in the genus.<ref name="Cannon et al. 2025"/>

==Habitat and distribution==

The genus ''Polychidium'' has been documented from [tropical](/source/tropical) through to [subarctic](/source/subarctic) regions. These lichens inhabit [moss](/source/moss)-covered rocks and grow on small twigs of trees.<ref name="Muggia et al. 2011"/><ref name="Cannon et al. 2025"/>

==Species==
* ''[Polychidium cetrarioides](/source/Polychidium_cetrarioides)'' {{au|Anzi (1860)}}
* ''[Polychidium dactylinum](/source/Polychidium_dactylinum)'' {{au|Trevis. (1880)}}
* ''[Polychidium muscicola](/source/Polychidium_muscicola)'' {{au|(Sw.) Gray (1821)}}
* ''[Polychidium pulvinatum](/source/Polychidium_pulvinatum)'' {{au|(Eitner) R.Sant. (1993)}}

==References==
{{Reflist|refs=

<ref name="Cannon et al. 2025">{{cite book |last1=Cannon |first1=P. |last2=Coppins |first2=B. |last3=Aptroot |first3=A. |last4=Fryday |first4=A. |last5=Simkin |first5=J. |year=2025 |title=Miscellaneous Peltigerales, including ''Spilonema'' (Coccocarpiaceae), ''Tingiopsidium'' (Koerberiaceae), ''Massalongia'' and ''Polychidium'' (Massalongiaceae), ''Placynthium'' (Placynthiaceae) and ''Vahliella'' (Vahliellaceae) |series=Revisions of British and Irish Lichens |volume=50 |page=7 |url=https://britishlichensociety.org.uk/sites/default/files/Other%20Peltigerales.pdf}}{{Open access}}</ref>

<ref name="CoL_CDW33">{{Catalogue of Life |id=CDW33 |title=''Polychidium'' |access-date=15 July 2025}}</ref>

<ref name="Muggia et al. 2011">{{cite journal |last1=Muggia |first1=Lucia |last2=Nelson |first2=Peter |last3=Wheeler |first3=Tim |last4=Yakovchenko |first4=Lidia S. |last5=Tønsberg |first5=Tor |last6=Spribille |first6=Toby |title=Convergent evolution of a symbiotic duet: The case of the lichen genus ''Polychidium'' (Peltigerales, Ascomycota) |journal=American Journal of Botany |volume=98 |issue=10 |year=2011 |doi=10.3732/ajb.1100046 |pages=1647–1656 |pmid=21980162 }}</ref>

<ref name="Species Fungorum synonymy">{{cite web |title=Synonymy. Current Name: ''Polychidium'' (Ach.) Gray, Nat. Arr. Brit. Pl. (London) 1: 401 (1821) |url=https://www.speciesfungorum.org/Names/SynSpecies.asp?RecordID=4306 |publisher=[Species Fungorum](/source/Species_Fungorum) |access-date=15 July 2025}}</ref>

}}

{{Taxonbar|from=Q10636601}}

Category:Peltigerales
Category:Peltigerales genera
Category:Lichen genera
Category:Taxa named by Erik Acharius
Category:Taxa described in 1810

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Adapted from the Wikipedia article [Polychidium](https://en.wikipedia.org/wiki/Polychidium) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Polychidium?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.