Phacopsis

Phacopsis was circumscribed by Edmond Tulasne in 1852 as a segregate of genus Abrothallus. Tulasne included three species: P. clemens, P. varia, and P. vulpina.[1] The last of these species was chosen as the type by Frederic Clements and Cornelius Shear (1931),[2] and by John Axel Nannfeldt (1932).[3] Of the other two species originally included in Phacopsis by Tulasne, P. clemens has since been referred to the genus Arthonia,[4] while P. varia is now known as Opegrapha physciaria.[5]

In 1988, Dagmar Triebel and Gerhard Rambold proposed that Phacopsis should be considered synonymous with Nesolechia, owing to similarities in their apothecial morphology and hymenial characters.[6] This proposed synonymy, however, was not accepted by several authors in the years following,[7][8][9] until 1995 when Triebel, Rambold, and John Elix showed that the supposed differences that separated the two genera were not consistent.[10] Following this, Ove Eriksson and David L. Hawksworth used an expanded generic concept of Phacopsis in the next update of their regular publication on ascomycete systematics.[11] In a 2018 review, the authors recommended not synonymizing Nesolechia and Phacopsis, suggesting that the separation between these two genera is still poorly known.[12]

The higher-level classification of Phacopsis has changed several times in its taxonomic history. It has been placed in the order Arthoniales,[13] in the families Graphidaceae,[2] and the Acarosporaceae.[14] Using electron microscopy, Josef Hafellner examined the asci (spore-bearing cells) of the type species P. vulpina, showing them to be similar to those commonly found in members of the Lecanoraceae, and so he considered the genus most appropriately classified in this family.[15] This familial placement, however, was not corroborated in Triebel and colleagues' 1995 review and revision of the genus.[10] Molecular analysis eventually revealed its true phylogenetic placement in the family Parmeliaceae. This relationship is an example of adelpho-parasitism, in which the host species is closely related to the parasite;[16] this type of relationship is fairly common in the Lecanorales. The genus Protousnea has a sister taxon relationship with Phacopsis, which has been estimated to have evolved from its ancestors during the Miocene (23.03 to 5.333 mya).[17]

Microscopy of Phacopsis vulpina asci. Several spores are visible in some of the asci, and the non-amyloid zone above the axial body is visible in some instances.

All species are endokapylic, meaning they possess a thallus in which no morphologically distinct lichenized structure is formed. Their apothecia are circular to irregular in shape, and may be dispersed or aggregated. The excipulum (the outer margin of the apothecium) range from colourless to blackish-brown, and often comprises "a few rows of cells which may be difficult to recognize". The hypothecium (a layer of dense hyphal tissue just below the hymenium) is colourless to blackish-brown. In some species, the hyphae in the hypothecium have a violet staining reaction with iodine; this characteristic can be used to distinguish between some species. The hymenium is colourless, pale brownish, or pale olivaceous. Asci are more or less club-shaped, and contain eight spores. They are surrounded by a thin outer amyloid wall layer and a thicker, non-amyloid inner wall layer; a non-amyloid zone rests above the axial body of the ascus.[10]

The ascospores are colourless, lack septa, and have smooth walls with occasional thickening at either end. The spores have a range of shapes; depending on the species, the following shapes have been recorded: ellipsoid, ovoid (egg-shaped), fusiform (spindle-shaped), lemon-shaped, falciform (sickle-shaped), fabiform (bean-shaped), and partly curved. The pycnidia of Phacopsis are immersed in the thallus of the host. Pycnospores have a bacilliform shape.[10]

Members of the host family, Parmeliaceae, are found worldwide and the family has centres of distribution in Asia and in the Southern Hemisphere; Phacopsis probably occurs wherever the host does. A 1995 survey of Phacopsis accepted 13 taxa (eight species and several varieties) found on 41 host species on 20 genera. Many of the Phacopsis species are poorly known, and have only been found at their type locality.[10] The most southernly distributed member of the genus is Phacopsis usneae, known to occur only in southern Chile and Antarctica.[18]

Like many lichenicolous fungi, Phacopsis species are usually restricted to living on a host belong to a single phylogenetic clade. In 2002, Andre Aptroot and Triebel suggested a possible close phylogenetic relationship between Paraparmelia and Xanthoparmelia, since Phacopsis australis was noted to grow on representatives from both of those lichen genera.[19] Since then, molecular phylogenetic analysis has demonstrated that they are equivalent, and Paraparmelia is now placed in synonymy with Xanthoparmelia.[20]

Phacopsis species are distinguished from each other by the characteristics of their ascospores, the colour and amyloid reaction of the hypothecium, and the identity of their host.[10] According to Index Fungorum, 33 taxa have been described under the name Phacopsis (25 species and 5 varieties).[21] A recent (2022) estimate places 10 species in genus Phacopsis.[22] The following list of Phacopsis species indicates its name, taxonomic authority and year of publication, type locality, and host species (or genus).

• Phacopsis australis Aptroot & Triebel (2002)[19] – southern Africa; host=Xanthoparmelia
• Phacopsis cephalodioides (Nyl.) Triebel & Rambold (1988)[6] – Denmark; host=Hypogymnia physodes
• Phacopsis lethariellae Hafellner & Rambold (1995)[10] – Canary Islands; host=Lethariella intricata
• Phacopsis menegazziae Triebel & Rambold (1995)[10] – Nepal; host=Menegazzia terebrata
• Phacopsis oroarcticae Zhurb. (2010)[23] – Severnaya Zemlya; host=Brodoa oroarctica
• Phacopsis prolificans (Müll.Arg.) Triebel & Rambold (1992)[24] – Japan; host=Platismatia interrupta
• Phacopsis rufa (Müll.Arg.) Triebel (1992)[24] – Australia; hosts=Cetrelia and Punctelia
• Phacopsis thallicola (A.Massal.) Triebel & Rambold (1988)[6] – New Zealand; hosts=Cetrelia sanguinea, Flavoparmelia caperata, Flavoparmelia praesignis, Parmotrema eurysacum, Rimelia cetraria
• Phacopsis usneae C.W.Dodge (1948)[25] – Kerguelen Islands; host=Usnea trachycarpa
• Phacopsis vulpicidae Zhurb. & Diederich (2019)[26] – Renchinlkhümbe (Mongolia); host=Vulpicida
• Phacopsis vulpina Tul. (1852) – France; host=Letharia

Several species formerly placed in Phacopsis have since been transferred to other genera. These include:

• Phacopsis crustulosae Creveld (1981) now Gyrophthorus crustulosae (Creveld) Hafellner & Sancho (1990)[27]
• Phacopsis doerfeltii Alstrup & P.Scholz (1998) now Nesolechia doerfeltii (Alstrup & P.Scholz) Diederich (2018)[28]
• Phacopsis ericetorum (Flot. ex Körb.) Vouaux (1914) now Rhymbocarpus ericetorum (Flot. ex Körb.) Etayo (2010)[29]
• Phacopsis falcispora Triebel & Rambold (1995) now Nesolechia falcispora (Triebel & Rambold) Diederich (2018)[30]
• Phacopsis huuskonenii Räsänen (1948) now Raesaenenia huuskonenii (Räsänen) D.Hawksw., C.Boluda & H.Lindgren (2015)[17]
• Phacopsis lesdainii Vouaux (1914) now Echinodiscus lesdainii (Vouaux) Etayo & Diederich (2000)[31]
• Phacopsis macrospora Uloth (1861) now Cercidospora macrospora (Uloth) Hafellner & Nav.-Ros. (2004)[32]
• Phacopsis muelleri Willey (1892) now Calloria muelleri (Willey) Seaver (1951)[33]
• Phacopsis mulleri Willey (1892) now Skyttella mulleri (Willey) D.Hawksw. & R.Sant. (1988)[34]
• Phacopsis oxyspora (Tul.) Triebel & Rambold (1988) now Nesolechia oxyspora (Tul.) A.Massal. (1856)