Posidoniomyces atricolor Vohnik & Reblova

Vohnik, Martin, Borovec, Ondrej, Kolarikova, Zuzana, Sudova, Radka & Reblova, Martina, 2019, Extensive sampling and high-throughput sequencing reveal Posidoniomycesatricolor gen. et sp. nov. (Aigialaceae, Pleosporales) as the dominant root mycobiont of the dominant Mediterranean seagrass Posidoniaoceanica, MycoKeys 55, pp. 59-86 : 59

Posidoniomyces atricolor Vohnik & Reblova sp. nov. Figs 5, 6

Typification.

CROATIA. Dubrovnik-Neretva County: Potomje, Borak (42.92236N, 17.34685E), isolated from a surface-sterilised healthy-looking terminal root of Posidonia oceanica , 28 Sep 2016, M. Vohník & O.Borovec BRK-21 (holotype: PRA-15294!, dried culture - compact morphotype from a surface-sterilised root segment; isotype: PRA-15295!, dried culture - mycelial morphotype derived from the original compact colony).

Etymology.

Atricolor (L), meaning black, dark coloured, referring to the dark pigmented hyphae.

Description in culture.

Mycelial colonial morphotype: Colonies on PCA 6-8 mm in diameter in 3 mo, circular, convex, appearing woolly, margin entire, aerial mycelium abundant, densest at the centre, cobwebby towards the margin, white to grey with a pale brown zone at the margin, colony surface with a dark brown hue formed by substrate mycelium and released pigment; reverse brown. Compact colonial morphotype: Colonies on PCA 5-6 mm in diameter in 8 mo, irregular, pulvinate, deeply furrowed, appearing mucoid-waxy to faintly floccose, of a “cartilage” consistency, become hollow upon aging, margin lobate, aerial mycelium scant, hyaline to pale brown, colony surface dark brown; reverse dark brown. Compact colonies, which are formed in vitro on sterilised roots of P. oceanica , become irregular in shape, folded and furrowed in an almost cerebriform pattern, cacao brown, ca. 5-6 mm long on the longest side after several months of cultivation. Hyphae hyaline to pale brown, septate, smooth-walled and 2 –3(– 3.5) µm wide, often with terminal, intercalary, rarely with lateral, one-celled, thick-walled globose, subglobose to ellipsoidal swellings 10-14 µm wide; hyphae frequently protrude from these swellings and continue growing. Surface of the compact colonies covered by hyaline to subhyaline, smooth-walled hyphae with terminal, capitate swellings. Chlamydospores, conidiogenous cells or conidia, ascomatal initials and ascomata not observed.

Description in vivo.

In vivo hyphae pigmented, septate, smooth-walled and (2 –)3–4(– 5) µm wide, colonising root cells of the host and/or forming an extraradical hyphal sheath, i.e. a finger-like pseudoparenchymatous net on the root surface. Microsclerotia intracellular, melanised, round or elongated and 8 –10(– 17) µm wide, present in the P. oceanica root hypodermis. Intracellular hyphae also infrequently occur in the rhizodermis.

Specimens examined.

Croatia. Dubrovnik-Neretva County: Potomje, Borak (42.92236N, 17.34685E), isolated from surface-sterilised healthy-looking terminal roots of P. oceanica , 28 Sep 2016, M. Vohník & O.Borovec BRK-11 (PRA-15296); ibid., BRK-25 (PRA-15298); BRK-34 (PRA-15297); BRK-60 (PRA-15300); BRK-61 (PRA-15293); BRK-76 (PRA-15302); BRK-87 (PRA-15299); BRK-93 (PRA-15301), BRK-97 (PRA-15303). Croatia. Split-Dalmatia County: Palagruža archipelago, Gangaro Island I (43.8639N, 15.4341E), isolated from a surface-sterilised healthy-looking terminal root of P. oceanica , 3 September 2012, M. Vohník & O.Borovec M8. France. Provence-Alpes-Côte d’Azur Region: Var Department, Saint-Raphaël, Cap Roux (43.45026N, 6.91951E), isolated from a surface-sterilised healthy-looking terminal root of P. oceanica , 17 June 2012, M. Vohník P10. France. Provence-Alpes-Côte d’Azur Region: Alpes-Maritimes Department, Antibes (43.55726N, 7.12209E), isolated from a surface-sterilised healthy-looking terminal root of P. oceanica , 18 June 2012, M. Vohník P11. France. Provence-Alpes-Côte d’Azur Region: Var Department, Sanary-sur-Mer (43.12054N, 5.77545E), isolated from a surface-sterilised healthy-looking terminal root of P. oceanica , 19 June 2012, M. Vohník P15. Italy. Liguria Region: Savona Province, Gulf of Genoa, Finale Ligure (44.17337N, 8.36765E), isolated from a surface-sterilised healthy-looking terminal root of P. oceanica , 17 June 2012, M. Vohník P09. Spain. Girona Province: L’Escala (42.10744N, 3.16892E), isolated from a surface-sterilised healthy-looking terminal root of P. oceanica , 18 June 2012, M. Vohník P20.

Habitat and distribution.

Root mycobiont of the dominant and endemic Mediterranean seagrass Posidonia oceanica . So far known only from the NW Mediterranean Sea.

Notes.

Both colonial morphotypes, named compact and mycelial, appeared on surface-sterilised root segments of P. oceanica and after inoculation also on solid agar media but the compact colonies with the cerebriform pattern formed only on the original root segments. All examined colonies of P. atricolor emerging from the original root segments developed from melanised microsclerotia formed exclusively intracellularly in the P. oceanica hypodermis (Figure 5d, h). The mycelial morphotype was observed on MMN and PCA, while compact colonies were formed on PDA and PCA ( Vohník et al. 2016; this study). When the surface of a colony exhibiting the compact colonial morphotype was washed regularly with sterile tap water, fragments of hyphae were released to form minute daughter colonies (Figure 6e). These daughter colonies were either of a rhizoidal form composed of substrate mycelium and continued to develop the mycelial morphotype or they assumed the compact colony character from the beginning (Figure 6f). A new hypha was often formed through the globose swelling, regardless of its position on the hypha (Figures 6 i–k).