The family Mtilinidiaceae Kirschst. 1924,
Mytilinidiales Boehm, Schoch & Spatafora 2009,
Pleosporomycetidae Schoch et al. 2007
by Eric W.A. Boehm
Lophiaceae Zogg ex von Arx & Muller, Stud. Mycol. Baarn 9: 60. 1975.
Lophiaceae Zogg, Beitr. Kryptogamenfl. Schweiz 11(3): 90. 1962. nom. inval. ICBN Art. 36.
Fungi classified in the Mytilinidiaceae Kirschst. (Kirschstein 1924), are characterized by fragile yet persistent carbonaceous ascomata, which range from globoid to obovoid to strongly laterally compressed erect, bivalve shell-shaped structures, standing on edge, with lateral walls more or less connivent, and extended vertically to a prominent longitudinal keel or cristate apex. Mytilinidiaceous fungi possess a thin-walled, scleroparenchymatous peridium enclosing a hamathecium of narrow trabeculate pseudoparaphyses, borne in a gel matrix, which are often sparse to lacking at maturity. Bitunicate asci are borne in a basal, rarely lateral, orientation within the centrum, and contain eight, rarely four, ascospores, overlapping uniseriate, biseriate or in one or two fascicles. Ascospores are diverse in the Mytilinidiaceae and range from scolecospores to didymospores to phragmospores to dictyospores, hyaline, soon yellow to dark brown in pigmentation, and generally showing bipolar symmetry in (Barr 1987, 1990; Zogg 1962).
Currently accepted genera (Eriksson 2006) in the Mytilinidiaceae include: Actidium Fr., Lophium Fr., Mytilinidion Duby, Ostreichnion Duby, Ostreola Darker, and Quasiconcha M.E. Barr & Blackw., to which has recently been added Zoggium Vasilyeva (Barr 1975, 1990; Barr & Blackwell 1980; Darker 1963; Lohman 1932b; Vasilyeva 2001; Zogg 1962). Anamorphic states in the Mytilinidiaceae are primarily coelomycetous (e.g., Aposphaeria, Pyrenochaeta, Camaroglobulus, Dothiorella, and Sclerochaeta) and less frequently hyphomycetous (e.g., Chalara-like, Papulaspora, Peyronelia, and Septonema) (Blackwell & Gilbertson 1985; Lohman 1932b, 1933a & b; Speer 1986; Sutton 1970). Typically temperate in distribution, mytilinidiaceous fungi are found in association with the wood, bark, resin, cones, scales, needles, seeds, and roots of gymnosperms and are less commonly recovered from angiosperms (e.g., Ostreichnion; Barr 1975, 1990).
The genus Glyphium Nitschke ex Lehmann, with erect dolabrate ascomata, was originally included by Zogg (1962), Barr (1987, 1990) and others (e.g., Goree 1974; Lorenzo & Messuti 2005; Sutton 1970) in the Mytilinidiaceae. However, recent molecular evidence, using the nuclear small (nuSSU) and large (nuLSU) ribosomal subunits, as well as the mitochondrial small (mtSSU) and large (mtLSU) ribosomal subunits (Lindemuth et al. 2001; Lumbsch et al. 2005), has removed the genus Glyphium to the Chaetothyriales in the Eurotiomycetes (Geiser et al. 2006; Lücking et al. 2004; Lutzoni et al. 2004; Schmitt et al. 2005). It should be noted however that these findings were based on a single isolate of G. elatum (Grev.) Zogg (CBS 268.34), isolated in 1934. Recent sequence data derived from fresh cultures of G. elatum, isolated from European material, however do not support the transference of the genus Glyphium to the Chaetothyriales in the Eurotiomycetes, but rather, the data indicate that the taxon remains the Dothideomycetes, as Pleosporomycetidae gen. incertae sedis (Boehm & Schoch unpubl. data). The genus Glyphium is included in the current key to the Mytilinidiaceae for historical reasons, or until such time that its taxonomy can be resolved. The genus Ostreichnion, recently transferred from the Mytilinidiaceae to the Hysteriaceae (Boehm et al. 2009) is also retained in this key, for historical reasons. Again, the keys are provided to effectuate and expedite identification of morphologically similar fungi, regardless of whether close phylogeny is implied or not.
Mytilinidiaceous fungi have historically been classified within the family Hysteriaceae due to perceived similarities in ascocarp morphology, specifically its means of longitudinal dehiscence (reviewed in Bisby 1923). Modern authors have likewise included mytilinidiaceous fungi within the Hysteriaceae, placing the family in the Pseudosphaeriales (Nannfeldt 1932; Gäumann 1949), the Dothiorales (Müller & von Arx 1950; von Arx & Müller, 1954), the Dothideales (von Arx & Müller 1975), and in a separate order Hysteriales, closely related to the Pleosporales (Miller 1949; Luttrell 1955). The Hysteriales were placed in the subclass Loculoascomycetes by Luttrell (1955), due to the presence of bitunicate asci, corresponding to the Ascoloculares first proposed by Nannfeldt (1932).
Duby (1862), however, was the first to propose that hysteriaceous fungi be divided into two sections, the Hystériées and the Lophiées, the latter to accommodate Ostreichnion, Mytilinidion, and Lophium. One hundred years later, Zogg (1962) proposed two families: the Hysteriaceae s. str. to accommodate thick-walled hysteriaceous forms, and the Lophiaceae H. Zogg ex von Arx & E. Müller (Zogg 1962; von Arx & Müller 1975) to accommodate thin-walled, mytilinidiaceous forms. Barr (1990) made the argument for retention of the earlier name Mytilinidiaceae Kirschst. 1924 over the Lophiaceae, despite the proposal to conserve the latter (Hawksworth & Eriksson 1988).
Luttrell (1973) held a wide concept of the Hysteriales and did not recognize the family Lophiaceae, instead proposing a subfamily within the Hysteriaceae to accommodate mytilinidiaceous forms. Barr (1979) however maintained the two-family distinction. The Mytilinidiaceae was placed in the Melanommatales, based on a thin-walled peridium of scleroparenchymatous cells enclosing a hamathecium of narrow trabeculate pseudoparaphyses, asci borne in a peripheral layer and with ascospores typically showing bipolar symmetry (Barr 1987, 1990). Later, Barr & Huhndorf (2001) noted that the family was somewhat atypical of the Melanommatales, in that, as a consequence of reduced locule space attributed to lateral compression, they possess a basal, rather than peripheral, layer of asci and a reduced hamathecium at maturity. Barr (1983) eventually abandoned the Hysteriales and placed the Hysteriaceae within the Pleosporales due to the presence of cellular pseudoparaphyses, asci borne in a basal rather than peripheral layer and ascospores typically showing bipolar asymmetry. Kirk et al. (2001) maintained both the Hysteriaceae and the Mytilinidiaceae in the Hysteriales, but Eriksson (2006) removed the Mytilinidiaceae from the Hysteriales and considered it as Dothideomycetes et Chaetothyriomycetes incertae sedis, leaving the Hysteriaceae as the sole family in the Hysteriales.
Key to genera of the Mytilinidiaceae Duby
- Ascospores one -eptate didymospores, small, less than 30 μm long → 2
1′. Ascospores not didymospores, or if one-septate, then larger than 30 μm long → 3
- Didymospores brown, ellipsoid, symmetric, wall coarsely reticulate; 6-8 x 5-5.5 μm → Quasiconcha M.E. Barr & Blackw.
2′. Didymospores olive to reddish brown, walls thin, smooth or delicately longitudinally striate; greater than 10 μm in length → Actidium Fr.
- Ascospores filiform, multiseptate, about equal in length to that of the ascus, in some cases, at maturity, longer than the ascus, often spirally arranged → 4
3′. Ascospores ellipsoid, fusoid, cylindric, if scolecospores then not equal in length to that of the ascus and not spirally arranged → 6
- Ascomata conchate, solitary to gregarious, but never forming fused, ridge-like, assemblages → Lophium Fr.
4′. Ascomata either forming rigid, fused band- or ridge-like ascomata or solitary, erect and dolabrate → 5
- Ascomata conchate, densely gregarious, forming band- or ridge-like assemblages; Russia → Zoggium Vasilyeva
5′. Ascomata erect, dolabrate to ligulate in outline; often with subtending hyphal strands; cosmopolitan → Glyphium Nitschke ex Lehmann
Note: Recent sequence data (Boehm & Schoch unpubl.) indicates that the genus Glyphium does not belong to the Mytilinidiales, but remains rather as Pleosporomycetidae gen. incertae sedis; it is included here for historical reasons.
- Ascospores transversely septate phragmospores; if scolecospores, more than 50 μm long, then only 2-4 μm wide → Mytilinidion Duby
6′. Ascospores dictyospores, or large & remaining one septate → 7
- Ascomata conchate; ascospores ellipsoid, not over 30 x10 μm, with single longitudinal septum in mid-cell → Ostreola Darker
7′. Ascomata conchate; ascospores ellipsoid or cylindric, more than 30 μm long, with several longitudinal septa in cells or large & remaining one-septate → Ostreichnion Duby
Note: The genus Ostreichnion previously classified within the Mytilinidiaceae (Barr 1975, 1987, 1990) has been transferred to the family Hysteriaceae (Boehm et al. 2009).