Retrostium Nakagiri & Tad. Ito, Mycologia 89(3): 485 (1997)
Sordariomycetes order incertae sedis, Spathulosporales, Spathulosporaceae, Retrostium
Index Fungorum number: IF27834; Facesoffungi number: FoF 08723; 1 morphological species.
Parasitic on marine algae. Sexual morph: Ascomata superficial, conical to hemisphaerical, carbonaceous, dark brown, sterile hairs enclosing ascoma, epapillate, ostiolate. Ostiolar canal surrounded by tube-like projection extending into the ascomatal cavity. Paraphyses absent. Asci lining the ascomatal venter completely, 8-spored, subglobose to obpyriform, thin-walled, unitunicate, lacking apical ring, deliquescing early. Ascospores overlapping multi-seriate, hyaline, oblong to ellipsoidal, aseptate to 3-septate, with apical appendages at each end. Reproductive structures: Antheridial. Spermatiophores rising from the margin of young ascoma, comprising a stipe and phialides disposed in a penicillate manner. Spermatia ellipsoidal to fusiform, lacking appendages. Trichogynes simple or branched, septate, originating from the margin of young ascoma (adapted from Nakagiri & Ito 1997).
Type species: Retrostium amphiroae Nakagiri & Tad. Ito, Mycologia 89(3): 485 (1997)
Notes: Retrostium was introduced by Nakagiri & Ito (1997) to accommodate R. amphiroae, a taxon inhabiting Amphiroa zonata (marine Rhodophyta) in coastal habitats in Japan (Nakagiri & Ito1997). Molecular data are unavailable for the species of this genus. Ascomata of R. amphiroae are conical to hemisphaerical, superficial, carbonaceous, epapillate and ostiolate (Nakagiri & Ito 1997). The ostiolar canal is surrounded by a tube-like projections extending into the ascomatal cavity and the two-layered peridial structure is similar to that of Spathulospora (Nakagiri & Ito 1997). This taxon differs from Spathulospora by thalli, ascomatal structure, ascospores and host (Kohlmeyer 1973, Nakagiri & Ito 1997). Retrostium amphiroae has true hyphae that grow between host cells, unlike Spathulospora, which have crustose thalli with "intramural hyphae" that invade the host cells (Kohlmeyer 1973). Paraphyses are absent, but pseudoparenchyma cells are present, similar to S. lanata (Kohlmeyer & Kohlmeyer 1975). Ascospores are 1-celled, oblong and furnished with a mucilaginous appendage at each end, while some Spathulospora species have spathulate ascospores (Kohlmeyer & Kohlmeyer 1975). Antheridium of R. amphiroae is a mononematous spermatiophore rather than a spermodochium as in Hispidicarpomyces and spermatia lack appendages as in Hispidicarpomyces, but can be distinguished by shape, as the latter forms a clavate spermatium with a cupulate base (Nakagiri & Ito 1997).