Verrucaria difficilis Pykaelae & Myllys, 2020

Pykaelae, Juha, Kantelinen, Annina & Myllys, Leena, 2020, Taxonomy of Verrucaria species characterised by large spores, perithecia leaving pits in the rock and a pale thin thallus in Finland, MycoKeys 72, pp. 43-92 : 43

Verrucaria difficilis Pykaelae & Myllys sp. nov. Fig. 2C

Diagnosis.

Species characterised by perithecia 1/4-3/4-immersed, leaving usually shallow pits, involucrellum covering half of the exciple or almost to the exciple base, ascospores (23-)25-29(-34) × (10-)11-12(-13) mm, morphologically rather similar to the other Finnish species of the V. subtilis complex, but the sequence divergence in ITS 1.7-2.6%.

Holotype.

Finland, Varsinais-Suomi, Karkkila, Haavisto, 100 m S of E-part of Iitalammi, S-slope, clear cut herb-rich forest, on calcareous stone, 60°31'N, 24°23'E, 123 m alt., 7 June 2008 J. Pykälä 32687 (H9205096, GenBank accession number: MT229742).

Description.

Prothallus absent. Thallus white or grey, inconspicuous, endolithic to thinly epilithic, continuous to irregularly rimose, ca. 20-80 mm thick, algal cells 5-7(-8) mm. Perithecia 0.18-0.36 mm in diam., 1/4-3/4(-1)-immersed, leaving shallow to more rarely deep pits in the rock, often thinly thalline covered except apex; 60-160 perithecia/cm2. Ostiole inconspicuous, tiny, pale to usually dark, plane or depressed, ca. 20-30 mm wide. Involucrellum covering half of the exciple or almost to the exciple base, 40-70 mm thick, appressed to the exciple or slightly or moderately diverging from it. Exciple 0.16-0.28 mm in diam., wall dark brown, ca. 20-25 mm thick. Periphysoids (20-)25-35(-40) × 1.5-2.5 mm, some branching. Asci 77-101 × 23-28 mm, 8-spored. Ascospores (22.7-)25.1-27.0-28.9(-33.6) × (9.6-)10.6-11.4-12.3(-13.3) (n = 78), perispore 1 mm thick.

Habitat and distribution.

Four sequenced specimens occur: two from SW Finland and two from NE Finland. The species grows on calcareous rocks and in lime quarries, on walls, boulders, stones and pebbles. Verrucaria difficilis may prefer half-shady habitats. The species is rare, but may also have been overlooked due to its morphological similarity to several other species.

Etymology.

The species may be mixed up with several other species of Verrucaria .

Other specimens examined.

Finland, Koillismaa, Kuusamo, Oulanka National Park, Kiutaköngäs 400 m N, Pinus sylvestris -dominated forest, small dolomite rock outcrop, SW-slope, on pebbles, 165 m alt., 66°22'N, 29°19'E, 2 Aug 2010, J. Pykälä 39060 (H); Kuusamo, Juuma, Niskakoski, calciferous (dolomite) schistose rock outcrop, on calciferous boulder, 225 m alt., 66°13'N, 29°24'E, 22 Aug 2011, J. Pykälä 44811 (H); Uusimaa, Vantaa, Sotunki, Bisa, 300 m E-NE, herb-rich forest, abandoned lime quarry, on SW-facing wall, 35 m alt., 60°17'N, 25°08'E, 7 June 2011, J. Pykälä 41859 (H).

Notes.

Based on the ITS phylogeny, V. difficilis belongs to the V. subtilis complex with V. bifurcata , V. cavernarum and V. subtilis . The involucrellum is usually longer than in V. cavernarum and V. subtilis . Furthermore, the perithecia of V. difficilis are, on average, less immersed, often only 1/4-1/2-immersed in rock. Verrucaria bifurcata differs in more immersed perithecia with the involucrellum appressed to the exciple. Nevertheless, V. difficilis may not be identified with certainty without sequencing. Verrucaria difficilis is also difficult to separate from V. kuusamoensis . This species has slightly longer spores and the perithecia commonly leave deep pits in the rock.

A Genbank sequence Verrucaria calkinsiana Servít (KT695332) has 98% similarity to V. difficilis and it remains to be studied whether it is a closely-related species or possibly conspecific. Based on the morphology of the type specimen (PRM-857016!), V. calkinsiana does not belong to the V. subtilis complex and the sequenced specimen is apparently misidentified.

Figures

Figure 2. Habitus of the new Verrucaria species A V. bifurcata (holotype) B V. cavernarum (holotype) C V. difficilis (holotype) D V. fuscozonata (holotype) E V. kuusamoensis (holotype) F V. subdevergens (holotype) G V. vacillans (holotype). Scale bars: 1 mm (A-D), 0.5 mm (E-G).