Morphology: The Hysteriaceae & Mytilinidiaceae possess highly unusual fungal fruiting bodies or ascomata that set them apart from other ascomycetes. Being bitunicate ascomycetes and therefore members of the loculoascomycetes, the ascomata are termed pseudothecia. Fungi classified in the Hysteriaceae are defined by a specialized pseudothecium termed the hysterothecium and, in the Mytilinidiaceae, what has been referred to as a “modified hysterothecium”. The only commonality being that they both dehisce by a longitudinal opening. In the Hysteriaceae, the hysterothecium is carbonaceous, long-cylindrical, cuniform or boat-like, immersed to erumpent to entirely sessile, sometimes slightly elevated, but always firm-walled, stout, and equipped with a longitudinal, invaginated slit or sulcus that runs the length of the fruitbody. Hysterothecia are capable of opening partially to reveal a lenticular, disk-like hymenium or closing tightly in response to relative humidity suggesting a perennial nature to the fruitbody. In contrast, in the Mytilinidiaceae, the pseudothecium is mussel- or clam-shaped (conchate) or hatchet-shaped (dolabrate) in outline, standing on edge, carbonaceous, but, most importantly, thin-walled and fragile, sometimes borne on a narrow stalk. In this family the pseudothecium is apically provided with a prominent narrow keel or cristate apex that runs longitudinal to the long axis of the fruitbody (Zogg 1962). A high degree of divergent ascospore morphologies are also found within each family. In the Hysteriaceae, these range from pedicillate amerospores (Farlowiella), hyaline didymospores (Glonium), pigmented (Hysterium & Hysterocarina) or hyaline (Gloniella) phragmospores and pigmented (Hysterographium) or hyaline (Gloniopsis) dictyospores. In the Mytilinidiaceae, a similar range of spore morphologies exist, with pigmented didymospores (Actidium), pigmented phragmospores (Mytilinidion subgenus Eu-Mytilinidion), pigmented dictyospores (Ostreichnion, & Ostreola) and scolecospores (Mytilinidion subgenus Lophiopsis) to filiform flexuous ascospores (Lophium & Glyphium). Spore septation configurations have historically been used to delineate genera in both families.

The Hysteriaceae: Current classification of the Hysteriaceae (Eriksson 2006) includes the following genera: Hysterium Tode emend. Fr., Hysterographium Corda emend. de Not., Gloniopsis de Not., Gloniella Sacc., Glonium Muhlenb. ex. Fr., Farlowiella Sacc., and Hysterocarina Zogg, to which has been recently added Actidiographium Vassiljeva (Eriksson 2006). The Hysteriaceae are panglobal in distribution and are primarily lignicolous or corticolous, although recently a saxicolous and apparently lichenized species has been described from Tasmania (Kantvilas and Coppins 1997).

The Mytilinidiaceae: Current classification of the Mytilinidiaceae includes the following genera: Actidium Fr., Lophium Fr., Mytilinidion Duby, Ostreichnion Duby, Ostreola Darker, and Quasiconcha Barr & Blackw., to which has recently been added Zoggium Vassiljeva (Eriksson 2006). The genus Glyphium, with erect dolabrate ascomata, was originally included by Zogg (1962), Barr (1987, 1990) and others (e.g., Goree 1974; Lorenzo and Messuti 2005; Sutton 1970) in the Mytilinidiaceae. However, recent molecular evidence, based on the nuSSU and nuLSU, as well as on the mtSSU and mtLSU (Lindemuth et al 2001; Lumbsch et al 2005), and validated by others (Geiser et al 2006; Lucking et al 2004; Schmitt et al 2005), has removed the genus Glyphium to the Chaetothyriales in the Eurotiomycetes. Should the single isolate used in these studies (CBS 268.34) prove to be correctly identified (see Lindemuth et al 2001), this may indicate an extreme degree of polyphyly for the family Mytilinidiaceae. Mytilinidiaceous fungi are typically temperate in distribution and are found in association with the wood, bark, resin, cones, scales, needles, seeds, and roots of gymnosperms (e.g., Actidium, Lophium, Mytilinidion, Ostreola, Quasiconcha and Zoggium) and are less commonly found on angiosperms (e.g., Ostreichnion) (Zogg 1962).

Historical Overview: The genus Hysterium, the type genus of the family Hysteriaceae, is attributed to Tode (1784), who was the first to apply the name to a group of fungi bearing a pronounced longitudinal slit, for which he gave the common name “Venusschwämme”. Recognizing the transitional nature of the ascoma, Tode later (1791) stated: “Medium hoc genus inter Pezizas and Lichenes”. Due to the seemingly transitional nature of the hysterothecium, neither fully open nor closed, hysteriaceous fungi have been placed in the discomycetes and pyrenomycetes about equally by various mycologists throughout the 19th Century (Bisby 1923). In his Systema Mycologicum, Fries (1823) initially considered hysteriaceous fungi to belong to the pyrenomycetes and placed them in the order Phacidiacei, but later (1835) placed them in his new class discomycetes, stating: “Transitum sistunt ad Discomycetes, sed discum verum non monstrant.” Ellis and Everhart (1892), in their North American Pyrenomycetes, tentatively included the Hysteriaceae, but stated that they had not at first intended to do so due to the transitional nature of the hysterothecium. Duby (1862) considered hysteriaceous fungi to belong to the pyrenomycetes and proposed two sections: the Hystériées to include Hysterium, Glonium, and Actidium Fr. among others, and the Lophiées to accommodate Ostreichnion Duby, Mytilinidion Duby and Lophium Fr. Although Duby’s (1862) method of classification, based on dehiscent versus nondehiscent asci, was not followed by subsequent workers, he was the first to propose dividing hysteriaceous fungi into what was later to become two distinct families. However, one hundred years would pass before this distinction was fully recognized (Zogg 1962).  

The Modern Era: Mytilinidiaceous fungi have historically been classified within the family Hysteriaceae due to perceived similarities in ascocarp morphology, specifically its means of longitudinal dehiscence (Fries 1823; Ellis and Everhart 1892; De Notaris 1847; Massee 1895; Rehm 1896; Saccardo 1883; von Honel 1918). 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 the Hysteriales, closely related to the Pleosporales (Miller 1949; Luttrell 1955). Luttrell (1953) studied ascomal ontogeny and hamathicial development in Glonium stellatum Mühlenb.:Fr. and concluded that the Hysteriaceae possess the pseudoparaphysate Pleospora-type centrum, in which cellular, septate pseudoparaphyses grow downwards from the cavity roof, initially anchored at both ends, and occupy the locule prior to the formation of asci (Luttrell 1951). 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). Zogg (1962) acknowledged the heterogeneity of the classical Hysteriales and, following Duby (1862), divided hysteriaceous fungi into two families, namely the Hysteriaceae to accommodate sessile, thick walled hysterothecia & the Mytilinidiaceae to accommodate modified hysterothecia, generally erect and possessing thin walls. 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). 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.

The Molecular era: More recently, Schoch et al. (2006), using a multigene phylogeny of the Dothideomycetes, based on the nuSSU and nuLSU, the transcription elongation factor (TEF1) and the RNA polymerase II second largest subunit (RPB2), provided evidence indicating that hysteriaceous fungi do not form a monophyletic group. The genus Farlowiella was basal to the Pleosporales, but distant from the type species of the Hysteriaceae, Hysterium pulicare Pers. ex Fr. The sole mytilinidiaceous member analyzed in this study, Lophium mytilinum (Pers.) Fr., also occupied a basal position to the Pleosporales, but was distant to the core group of Hysteriales and was designated as Pleosporomycetidae incertae sedis (Schoch et al. 2006). My current research project was undertaken to incorporate a much larger range of taxon sampling to ascertain if any relationships existed between the Hysteriaceae and Mytilinidiaceae. We were also interested in whether the two families comprised monophyletic lineages, given that polyphyly was observed in the Hysteriaceae (Schoch et al. 2006), and given that in the Mytilinidiaceae, two of its members (Glyphium and Lophium) are currently classified in two different orders (Chaetothyriales and Pleosporales), in two different classes (Eurotiomycetes and Dothideomycetes) of ascomycetes (Geiser et al. 2006; Schoch et al. 2006). Hence the need to reanalyze the group employing a wide taxon sampling strategy - the current focus of my research program. The essential question is whether phylogenies based on molecular character states will prove to be concordant with previous morphology-based classifications, such as those presented here (Barr 1987, Zogg 1962). Or will it be found instead that convergent evolutionary processes have generated similar fruitbody morphology and similar spore septation configurations among unrelated clades within each family? In other words, are morphological character states in the Hysteriaceae and Mytilinidiaceae (e.g., those associated with pseudothecial shape, centrum development, hamathecial type, peridial wall thickness and ascospore symmetry) synapomorphies supporting the current classification or are they plesiomorphies shared by unrelated lineages, therefore necessitating taxonomic changes? Stay tuned...