Arctomia

{{Short description|Genus of lichen-forming fungi}} {{Use dmy dates|date=June 2025}} {{Use Oxford spelling|date=June 2025}} {{Automatic taxobox | image = Arctomia delicatula 1544528960.jpg | image_caption = ''Arctomia delicatula'' | taxon = Arctomia | authority = Th.Fr. (1861) | type_species = ''Arctomia delicatula'' | type_species_authority = Th.Fr. (1861) | subdivision_ranks = Species | subdivision = ''A. delicatula''<br /> ''A. interfixa''<br /> ''A. papuanorum''<br /> ''A. teretiuscula''<br /> ''A. uviformis'' }}

'''''Arctomia''''' is a genus of lichen-forming fungi in the family Arctomiaceae. The genus was established in 1860 by the Swedish lichenologist Theodor Magnus Fries, and species of ''Arctomia'' are mainly distributed in circumpolar regions of the Northern Hemisphere. A comprehensive molecular study published in 2025 confirmed that the genus forms a distinct evolutionary lineage within its family. Species typically grow as thin, reddish-brown to blackish crusts on plant debris, mosses, or tree bark in cold northern regions, with their distribution ranging from the Arctic tundra to boreal forests and alpine heaths. The thallus contains cyanobacteria of the genus ''Nostoc'' as the photosynthetic partner, giving the lichen a slightly glossy, gelatinous texture when wet. Most species produce minute disc-shaped fruiting bodies containing elongated, many-celled ascospores.

==Taxonomy==

The genus ''Arctomia'' was introduced by Theodor Magnus Fries in 1860 with ''A.&nbsp;delicatula'' as the sole species. In the protologue he characterised ''A.&nbsp;delicatula'' as a minute but strikingly beautiful crustose lichen, with a thin, {{lichengloss|verrucose}}, brownish thallus made up of tiny convex granules and relatively large, immarginate apothecia that become slightly gelatinous and wine-red when moist, growing over mosses in rather dry peatlands on Silurian substrates near Tromsø, {{ill|Mortensnes|no}} and Nesseby in Finnmark, with additional material from Hornsund in Spitsbergen (Svalbard).<ref name="Fries 1860"/> Later authors expanded the genus by transferring Nylander's ''Pannaria acutior'' and ''Pannularia interfixa'', and by adding further taxa such as the South African species ''A.&nbsp;muscicola'' and several members of the former ''Collema fasciculare'' group.<ref name="Ekman 2025"/> A revision by Aino Henssen in 1969 took a narrower view and recognised only ''A.&nbsp;delicatula'' and ''A.&nbsp;interfixa'',<ref name="Henssen 1969"/> treating ''A.&nbsp;acutior'' as a bark-dwelling variety of ''A.&nbsp;delicatula'' distinguished mainly by slightly narrower ascospores.<ref name="Ekman 2025"/>

A multilocus phylogenetic study published in 2025 analysed sequence data from various genetic markers for ''Arctomia'' and related genera in the Arctomiaceae, together with coalescent-based species delimitation methods. The authors found that ''Arctomia'' forms a strongly supported monophyletic group that also includes ''A.&nbsp;interfixa'', and that this clade is sister to ''Gabura'' within the family. Earlier placements of ''A.&nbsp;interfixa'' outside ''Arctomia'' were attributed to a misidentified sequence that had been used in previous analyses.<ref name="Ekman 2025"/>

Within the traditional concept of ''A.&nbsp;delicatula'' the study recovered several well-supported lineages. Using a combination of Bayesian species delimitation and probabilistic classification of morphological measurements, it recognised three species in this complex: ''A.&nbsp;delicatula'' in a stricter sense, ''A.&nbsp;acutior'' (raised from varietal rank) and a newly described species, ''A.&nbsp;confusa''.<ref name="Ekman 2025"/> ''Arctomia acutior'' differs from the other two in having smaller apothecia, narrower ascospores and a preference for bark substrates, whereas ''A.&nbsp;confusa'' is most similar to ''A.&nbsp;delicatula'' but has slightly narrower ascospores and a broader ecological range, occurring on both bark and plant detritus on soil.<ref name="Ekman 2025"/>

The same study used supervised machine learning models trained on ascospore size, septation and apothecium width from sequenced specimens to classify unsequenced collections, including lectotypes, to species with very high posterior probabilities, providing an independent check on the DNA-based species limits.<ref name="Ekman 2025"/>

==Description==

''Arctomia'' grows as a thin crust that can look either {{lichengloss|granular}} or like a miniature patchwork of tiny leaf-like scales. When dry, the surface is reddish-brown to nearly black and, unlike some gelatinous lichens, it does not swell appreciably after rain.<ref name="Cannon et al. 2025"/> In most species, the thallus forms a low crust of fine granules or tiny scales, although ''A.&nbsp;teretiuscula'' produces short, erect, isidia-like branchlets up to about 2&nbsp;mm tall that give the colony a small, {{lichengloss|fruticulose}} appearance.<ref name="Ekman 2025"/> The edge of a colony is indefinite and may develop small {{lichengloss|lobes}}, but the whole thallus remains low-lying. A single-cell-thick {{lichengloss|cortex}}—essentially a skin of brown fungal cells—covers the surface, and the lichen never produces the powdery outgrowths (isidia) or soralia seen in many other genera. Sandwiched between the fungal threads are clusters of cyanobacteria of the genus ''Nostoc''; its cells, about 5–7&nbsp;micrometres (μm) across, sit in a clear jelly that gives the thallus a slightly glossy, rubbery feel when moist.<ref name="Cannon et al. 2025"/>

Sexual reproduction takes place in minute disc-shaped fruiting bodies (apothecia) that originate as tiny bumps inside the lobes and become more or less stalkless on maturity. These {{lichengloss|discs}} are flat to gently domed and merge smoothly with the surrounding thallus rather than being ringed by a distinct margin. Inside, slender branched filaments (paraphyses) stand among the spore sacs; their tips broaden slightly and carry a brown pigment. Each ascus belongs to the ''Trapelia''-type: it shows a thickened apex (the {{lichengloss|tholus}}) that stays colourless in iodine staining, while the surrounding gelatinous sheath stains blue, and it usually contains eight ascospores. The spores themselves are long, spindle-shaped and divided by many cross-walls (septa)—features that help them survive dispersal. Asexual reproduction occurs in embedded pycnidia that release tiny, rod-shaped conidia, and standard chemical spot tests detect no secondary metabolites in the thallus.<ref name="Cannon et al. 2025"/>

==Habitat and distribution==

The genus is centred in cool, humid environments. ''Arctomia acutior'' and ''A.&nbsp;confusa'' occur mainly in hemiboreal and boreal forests of Fennoscandia and north-west Russia, often on bark of willows, rowans and spruces along streams and in swampy woodland, and both have also been recorded from Alaska.<ref name="Ekman 2025"/> ''Arctomia delicatula'' s.str. is restricted to plant detritus on calcareous soils in exposed alpine heaths in Norway and Sweden, ''A.&nbsp;interfixa'' is a rare Arctic species of calcareous tundra from northern Fennoscandia, Arctic Russia and Alaska, and ''A.&nbsp;teretiuscula'' is known only from its type locality on mossy rocks and soil in the Hengduan Mountains of Sichuan, China.<ref name="Jørgensen 2003"/><ref name="Ekman 2025"/>

==Species==

Molecular and morphological work since the early 2000s has restricted ''Arctomia'' to a small group of cyanobacterial lichens within the Arctomiaceae.<ref name="Magain et al. 2020"/><ref name="Ekman 2025"/> A 2025 multilocus study of the genus recognised five species in ''Arctomia'' sensu stricto: ''A.&nbsp;acutior'', ''A.&nbsp;confusa'', ''A.&nbsp;delicatula'', ''A.&nbsp;interfixa'' and ''A.&nbsp;teretiuscula''.<ref name="Ekman 2025"/> {{As of|2025|November}}, Species Fungorum (as used in the Catalogue of Life) lists eight species names under ''Arctomia'':<ref name="CoL_CCSGJ"/> * ''Arctomia borbonica'' {{au|Magain & Sérus. (2012)}} * ''Arctomia delicatula'' {{au|Th.Fr. (1861)}} * ''Arctomia fascicularis'' {{au|(L.) Otálora & Wedin (2013)}} * ''Arctomia insignis'' {{au|(P.M.Jørg. & Tønsberg) Ertz (2017)}} * ''Arctomia interfixa'' {{au|Nyl. ex Vain. (1909)}} * ''Arctomia papuanorum'' {{au|(Degel.) Otálora & Wedin (2013)}} * ''Arctomia teretiuscula'' {{au|P.M.Jørg. (2003)}}<ref name="Jørgensen 2003"/> – China * ''Arctomia uviformis'' {{au|(Hue) Otálora & Wedin (2013)}}

Several of the names still listed under ''Arctomia'' in these databases correspond to taxa that recent revisions place in other genera. Earlier authors also included species of the former ''Collema fasciculare'' group in ''Arctomia'', but these have since been reassigned to other genera: several Antarctic and subantarctic species now belong in ''Steinera'', three temperate species (including ''A.&nbsp;borbonica'', ''A.&nbsp;fascicularis'' and ''A.&nbsp;insignis'') are placed in ''Gabura'', and others have been transferred to ''Hondaria'' in the Collemataceae.<ref name="Magain et al. 2020"/><ref name="Ekman 2025"/>

The 2025 study resurrected ''Arctomia acutior'' from synonymy with ''A.&nbsp;delicatula'' and described ''A.&nbsp;confusa'' as new, based on phylogenetic analyses and subtle but consistent differences in ascospore width, apothecium size and substrate preferences.<ref name="Ekman 2025"/> It also treated ''A.&nbsp;interfixa'' as a distinct species nested within ''Arctomia'', in contrast to some earlier molecular work that had placed it as an isolated lineage within the Arctomiaceae.<ref name="Ekman 2025"/>

Two tropical species, ''A.&nbsp;papuanorum'' and ''A.&nbsp;uviformis'', have been suggested on morphological grounds to belong instead to the genus ''Hondaria'' (Collemataceae), and are not treated as members of ''Arctomia'' sensu stricto in recent phylogenetic work.<ref name="Ekman 2025"/>

==References== {{Reflist|colwidth=30em|refs=

<ref name="Cannon et al. 2025">{{cite book |last1=Cannon |first1=P. |last2=Coppins |first2=B. |last3=Sanderson |first3=N. |last4=Simkin |first4=J. |year=2025 |title=Arctomiales: Arctomiaceae, including ''Arctomia'', ''Gabura'' and ''Gregorella'' |series=Revisions of British and Irish Lichens |volume=56 |pages=2–3 |url=https://britishlichensociety.org.uk/sites/default/files/Arctomiales.pdf}}{{Open access}}</ref>

<ref name="CoL_CCSGJ">{{Catalogue of Life |id=CCSGJ |title=''Arctomia'' |access-date=24 November 2025}}</ref>

<ref name="Ekman 2025">{{cite journal |last1=Ekman |first1=Stefan |last2=Svensson |first2=Måns |last3=Westberg |first3=Martin |year=2025 |title=Phylogeny, species delimitation and machine learning bridge the gap between DNA sequences and morphology in the lichen genus ''Arctomia'' (Arctomiaceae, Ascomycota) |journal=Taxon |volume=70 |issue= |pages=1–17 |article-number=tax.70082 |doi=10.1002/tax.70082 |doi-access=free}}</ref>

<ref name="Fries 1860">{{cite book |last=Fries |first=T.M. |year=1860 |title=Lichenes Arctoi Europae Groenlandiaeque hactenus cogniti |series=3 |page=387 |language=la |url=https://www.biodiversitylibrary.org/page/2578196}}</ref>

<ref name="Henssen 1969">{{cite journal |last=Henssen |first=A. |year=1969 |title=Eine Studie über die Gattung ''Arctomia'' |trans-title=A study of the genus ''Arctomia'' |journal=Svensk Botanisk Tidskrift |volume=63 |pages=126–138 |language=de}}</ref>

<ref name="Jørgensen 2003">{{cite journal |last=Jørgensen |first=P.M. |title=A new species of ''Arctomia'' from Sichuan Province, China |journal=The Lichenologist |volume=35 |issue=4 |year=2003 |doi=10.1016/s0024-2829(03)00053-7 |pages=287–289 |bibcode=2003ThLic..35..287J }}</ref>

<ref name="Magain et al. 2020">{{cite journal |last1=Magain |first1=Nicolas |last2=Spribille |first2=Toby |last3=DiMeglio |first3=Joseph |last4=Nelson |first4=Peter R. |last5=Miadlikowska |first5=Jolanta |last6=Sérusiaux |first6=Emmanuël |title=Phylogenetic evidence for an expanded circumscription of ''Gabura'' (Arctomiaceae) |journal=The Lichenologist |volume=52 |issue=1 |year=2020 |doi=10.1017/s0024282919000471 |pages=3–15 |bibcode=2020ThLic..52....3M |hdl=2268/245664 |hdl-access=free }}</ref>

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{{Taxonbar |from1=Q4787625 |from2=Q25407175}}

Category:Baeomycetales Category:Baeomycetales genera Category:Lichen genera Category:Taxa named by Theodor Magnus Fries Category:Taxa described in 1861