Scanning electron microscopic (SEM) images of basidiospores of Agaricales. Left: Spinose basidiospore of Inocybe calospora (Inocybaceae). © Brandon Matheny. Right: Grooved basidiospores of Clitopilus prunulus (Entolomataceae). © Sigisfredo Garnica.

No morphological, ecological, or biochemical traits are known that unite the Agaricales despite robust measures of support for its monophyly (Binder and Hibbett 2002; Moncalvo et al. 2002; Matheny et al. 2007). Singer (1986), one of the most influential taxonomic treatments of the order, was unable to provide a set of non-molecular characters unique to all members. Rather, inclusion in the group, in the absence of molecular confirmation, was historically made by the absence of certain characters possessed by other clades of Agaricomycetes. For example, members of the Agaricales sensu Singer were attributed to have non-septate basidia (unlike Auriculariales), lack stichobasidia (unlike some families of Cantharellales), lack trimitic hyphal systems (unlike Polyporales and Hymenochaetales), not possess a spinose hymenophore (unlike Thelephorales), lack variegatic acid type pigments and its derivatives (unlike Boletales), and not have heteromerous trama or a combination of a laticiferous hyphal system with amyloid, ornamented spores (unlike Russulales). No members of the Agaricales produce a yeast-like stage, but many non-mycorrhizal groups may produce conidia or other unique asexual reproductive structures (Walther et al. 2005). The spore bearing surface (hymenophore) is typically lamellate and infrequently tubular (unlike Boletales). However, it is becoming clear that cyphelloid and false truffles have likely evolved from lamellate ancestors, as have some species producing club-shaped (clavarioid) basidiomes. It remains unclear whether the most recent common ancestor of the Agaricales was gilled (Matheny et al. 2006).

Examples of non-gilled fruit body forms in the Agaricales. Left: Clavarioid fruit bodies of Clavulinopsis fusiformis (Clavariaceae s. str.). © Brandon Matheny. Gasteroid fruit bodies of Crucibulum laeve (Nidulariaceae). © Mark Steinmetz. Resupinate fruit bodies of Cylindrobasidium evolvens (Physalacriaceae). © Brandon Matheny.

Despite the absence of morphologically shared derived traits (synapomorphies) for the Agaricales as a whole, some combinations of traits do appear diagnostic for nested lineages within the order. Species with pigmented spore walls, multinucleate spores, and spores with an open-pore type of hilum (an ultrastructural character) (Pegler and Young 1969) appear derived and belong to the agaricoid clade. All these characters, with exception of the hilum-type, may have evolved repeatedly on independent occasions. Nevertheless, it appears reasonable that the most recent common ancestor of the Agaricales was probably a saprotroph or parasite with uninucleate, hyaline spores with a nodulose spore hilum, perhaps similar to early-diverging lineages of the Atheliales and Boletales, two groups most closely related to the Agaricales. The ectomycorrhizal condition, a symbiosis between fungal hyphae and typically roots of vascular plants, appears to have evolved independently at least on eleven occasions in the Agaricales alone without any unamibiguous reversals to a free-living state (Matheny et al. 2006).