{{Short description|Genus of lichen-forming fungi}} {{Use dmy dates|date=February 2026}} {{Use Oxford spelling|date=February 2026}} {{Automatic taxobox | image = Leimonis erratica 597551.jpg | image_caption = ''Helocarpon erraticum'' | taxon = Helocarpon | authority = Th.Fr. (1860) | type_species = ''Helocarpon crassipes'' | type_species_authority = Th.Fr. (1860) | subdivision_ranks = Species | subdivision = ''H. assimilatum''<br /> ''H. crassipes''<br /> ''H. erraticum''<br /> ''H. paratropum'' | synonyms_ref = <ref name="Myllys et al. 2026"/><ref name="Harris 2009"/> | synonyms = *''Leimonis'' {{au|R.C.Harris (2009)}} }}
'''''Helocarpon''''' is a genus of lichen-forming fungi in the family Ectolechiaceae. It contains a small number of crustose species that occur mainly in cool, northern and montane environments, where they grow on soil or rock. The genus is closely related to ''Micarea'', but differs in a suite of morphological traits and is treated as a distinct lineage in modern multilocus classifications. Species in the genus are associated with cool climates and are found mainly in arctic, subarctic, and alpine regions, where they grow on soil or rock. The genus was proposed by Theodor Magnus Fries in 1860, and a major molecular study published in 2026 confirmed it as a distinct lineage separate from ''Micarea''.
==Taxonomy== ''Helocarpon'' was established by Theodor Magnus Fries in 1860, with ''H. crassipes'' assigned as the type species.<ref name="Myllys et al. 2026"/> Its placement has shifted between classifications. In morphology-based systems its wider relationships were uncertain, and Josef Hafellner introduced the monogeneric family Helocarpaceae in 1984 to accommodate the genus.<ref name="Hafellner 1984"/> Some later outline classifications instead placed ''Helocarpon'' in Micareaceae alongside ''Micarea'' and other superficially similar crustose lichens.<ref name="Eriksson et al. 2004"/>
Authors also differed on rank and circumscription: some retained ''Helocarpon'' as a distinct genus, whereas others treated it within a broad ''Micarea'' concept (for example, in a European revision that grouped ''H. crassipes'' with morphologically similar ''Micarea'' species).<ref name="Coppins 1983"/><ref name="Myllys et al. 2026"/> Early molecular work added noise rather than clarity because a mitochondrial-rDNA sequence attributed to ''H. crassipes'' was later shown to represent a species of ''Bryobilimbia''; together with limited taxon sampling, this contributed to conflicting placements in the early literature.<ref name="Andersen & Ekman 2005"/><ref name="Myllys et al. 2026"/>
A comprehensive five-locus phylogeny published in 2026 recovered ''Helocarpon'' as a strongly supported clade that is sister to ''Micarea'' in a broad sense, and treated it as a separate genus rather than as the earliest-diverging branch of ''Micarea''.<ref name="Myllys et al. 2026"/> In that treatment, several species long placed in ''Micarea'' were recombined into ''Helocarpon'', and the monotypic genus ''Leimonis'' (described for the saxicolous ''Lecidea''/''Micarea'' species ''erratica'') was reduced to synonymy with ''Helocarpon''.<ref name="Harris 2009"/><ref name="Myllys et al. 2026"/> Although ''Micarea lynceola'' was transferred to ''Leimonis'' in 2017, the 2026 study did not support ''erratica'' and ''lynceola'' as a monophyletic pair in mtSSU analyses and treated ''lynceola'' as part of a separate ''Micarea'' lineage rather than within ''Helocarpon''.<ref name="Myllys et al. 2026"/><ref name="Harris 2009"/><ref name="Aptroot 2017"/> The same study also treated ''Helocarpon pulverulum'' as a synonym of ''H. crassipes'', interpreting it as part of a continuum of thallus development rather than as a separate species.<ref name="Myllys et al. 2026"/>
The 2026 revision discussed additional taxa that might prove related to ''Helocarpon'' but were not formally transferred because of missing or inconclusive data. For example, the placement of ''Micarea doliiformis'' was treated as unresolved despite an apparent relationship in some single-locus analyses, and other species (such as the North American ''Micarea subalpina'') were flagged as potentially relevant but in need of targeted sampling and sequencing.<ref name="Myllys et al. 2026"/>
==Description== thumb|right|Closeup of ''Helocarpon crassipes'' showing its black, marginate apothecia; scale: 1 mm Species of ''Helocarpon'' are crustose lichens with a thallus that ranges from thin and poorly developed to well-developed and distinctly {{lichengloss|areolate}}. Compared with many species traditionally placed in ''Micarea'', members of the ''Helocarpon'' lineage tend to have a more conspicuous apothecial margin (a well-developed {{lichengloss|exciple}}) and, in some taxa, relatively large {{lichengloss|photobiont}} cells rather than the paired "micareoid" arrangement common in ''Micarea'' sensu stricto.<ref name="Harris 2009"/><ref name="Myllys et al. 2026"/> Apothecia (fruiting bodies) are generally dark (often black), and in the type species they are characteristically shortly stalked and clearly {{lichengloss|marginate}}.<ref name="Myllys et al. 2026"/> Internal pigmentation is often strong in the apothecial tissues (for example, dark {{lichengloss|hypothecium|hypothecial}} colours have been used in traditional species concepts within the group).<ref name="Coppins 1983"/><ref name="Myllys et al. 2026"/>
Some species associate secondarily with cyanobacteria in addition to their green-algal photobiont. In the arctic-alpine ''H. assimilatum'', cyanobacteria occur in ''Nostoc''-containing cephalodia, whereas the closely related saxicolous ''H. paratropum'' lacks ''Nostoc'' cephalodia and instead commonly shows ''Stigonema'' clusters between the {{lichengloss|areoles}}. Chemically, species in the lineage were characterized in the 2026 revision as lacking detectable lichen substances in the standard sense used for many ''Micarea'' groups.<ref name="Myllys et al. 2026"/>
==Habitat and distribution== ''Helocarpon'' is most often encountered in cool-climate habitats, especially in arctic, subarctic, and alpine or subalpine environments. Species occur on mineral soil or plant debris on the ground (terricolous) as well as directly on rock (saxicolous).<ref name="Myllys et al. 2026"/> In the 2026 synthesis, ''H. crassipes'' and ''H. assimilatum'' were discussed as terricolous species associated with arctic-alpine settings, while ''H. erraticum'' was described as mostly saxicolous and widely distributed.<ref name="Harris 2009"/><ref name="Myllys et al. 2026"/>
The most range-restricted species discussed in detail was ''H. paratropum'', which was treated as an arctic-alpine lichen known from Scotland and Scandinavia and also reported from Canada. It resembles ''H. assimilatum'' morphologically but differs in substrate preference and in its cyanobacterial associations.<ref name="Myllys et al. 2026"/>
==Species== The 2026 multilocus revision recognised four species in ''Helocarpon'':<ref name="Myllys et al. 2026"/> * ''Helocarpon assimilatum'' {{au|(Nyl.) M.Svenss., Myllys & Kantel. (2026)}}<ref name="Myllys et al. 2026"/> * ''Helocarpon crassipes'' {{au|Th.Fr. (1860)}}<ref name="Fries 1860"/> * ''Helocarpon erraticum'' {{au|(Körb.) M.Svenss., Myllys & Kantel. (2026)}}<ref name="Myllys et al. 2026"/> * ''Helocarpon paratropum'' {{au|(Nyl.) M.Svenss., Myllys & Kantel. (2026)}}<ref name="Myllys et al. 2026"/>
==References== <references>
<ref name="Andersen & Ekman 2005">{{cite journal |last1=Andersen |first1=Heidi L. |last2=Ekman |first2=Stefan |year=2005 |title=Disintegration of the Micareaceae (lichenized Ascomycota): a molecular phylogeny based on mitochondrial rDNA sequences |journal=Mycological Research |volume=109 |issue=1 |pages=21–30 |doi=10.1017/S0953756204001625 |pmid=15736860}}</ref>
<ref name="Aptroot 2017">{{cite journal |last=Aptroot |first=A. |year=2017 |title=Nomenclatural novelties |journal=Index Fungorum |volume=331 |page=1 |url=https://www.indexfungorum.org/Publications/Index%20Fungorum%20no.331.pdf}}</ref>
<ref name="Coppins 1983">{{cite journal |last=Coppins |first=B.J. |year=1983 |title=A taxonomic study of the lichen genus ''Micarea'' in Europe |journal=Bulletin of the British Museum of Natural History |volume=11 |page=108}}</ref>
<ref name="Eriksson et al. 2004">{{cite journal |last1=Eriksson |first1=O.E. |last2=Baral |first2=H.-O. |last3=Currah |first3=R.S. |last4=Hansen |first4=K. |last5=Kurtzman |first5=C.P. |last6=Rambold |first6=G. |last7=Laessøe |first7=T. |title=Outline of Ascomycota – 2004. |journal=Myconet |year=2004 |volume=10 |pages=1–99}}</ref>
<ref name="Fries 1860">{{cite journal |last=Fries |first=T.M. |year=1860 |title=Lichenes arctoi Europae Groenlandiaeque hactenus cogniti |journal=Nova Acta Regiae Societatis Scientiarum Upsaliensis |series=3 |volume=3 |pages=103–398 [178] |language=la}}</ref>
<ref name="Hafellner 1984">{{cite journal |last1=Hafellner |first1=J. |year=1984 |title=Studien in Richtung einer natürlichen Gliederung der Sammelfamilien Lecanoracae und Lecideaceae |trans-title=Studies towards a natural classification of the collective families Lecanoraceae and Lecideaceae |journal=Beihefte zur Nova Hedwigia |volume=79 |pages=241–371 [285] |language=de}}</ref>
<ref name="Harris 2009">{{cite journal |last=Harris |first=Richard C. |title=Four novel lichen taxa in the lichen biota of eastern North America |journal=Opuscula Philolichenum |volume=6 |year=2009 |doi=10.5962/p.381977 |pages=149–156 |url=https://www.biodiversitylibrary.org/part/381977 |doi-access=free}}</ref>
<ref name="Myllys et al. 2026">{{cite journal |last1=Myllys |first1=Leena |last2=Svensson |first2=Måns |last3=Kantvilas |first3=Gintaras |last4=Sérusiaux |first4=Emmanuel |last5=van den Boom |first5=Pieter |last6=Aptroot |first6=André |last7=Cezanne |first7=Rainer |last8=Eichler |first8=Marion |last9=Pino-Bodas |first9=Raquel |last10=Printzen |first10=Christian |last11=Pykälä |first11=Juha |last12=Thor |first12=Göran |last13=Westberg |first13=Martin |last14=Weber |first14=Lilith |last15=Kantelinen |first15=Annina |title=The first comprehensive multilocus phylogeny of the lichenized genus ''Micarea'' (Ectolechiaceae, Ascomycota) |journal=Taxon |volume=75 |issue=1 |year=2026 |doi=10.1002/tax.70087 |doi-access=free |article-number=e70087}}</ref>
</references>
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Category:Ectolechiaceae Category:Lecanorales genera Category:Lichen genera Category:Taxa named by Theodor Magnus Fries Category:Taxa described in 1860