Taxonomy

Abstract

Fungi constitute the third kingdom of organisms which are highly diverse, heterotrophic eukaryotes characterized by chitinous cell wall ranging from unicellular to syncytial filamentous forms divided into six phyla Ascomycota, Basidiomycota, Chytridiomycota, Glomeromycota, Microsporidia, and Zygomycota (Kirk et al. 2008). Majority of all the described fungi belong to the subkingdom Dikarya represented by Ascomycota, Taphrinomycotina (yeast-like and some filamentous fungi), Saccharomycotina (the true yeasts), and Pezizomycotina (with majority of the filamentous and mushroom-forming ascomycetes), and Basidiomycota, Pucciniomycotina (rust fungi-plant pathogens), Ustilaginomycotina (true smuts, some yeasts, and some filamentous fungi), and Agaricomycotina (most of the mushroom-producing fungi) (Cannon et al. 2018; Niskanen et al. 2018). Macrofungi also known as higher fungi/macromycetes include the taxa known to form large, conspicuous, and epigeous or hypogeous sporocarps which are the “fruit” or the reproductive parts of the vegetative fungal mycelium. Sporocarps are the products of sexual reproduction (Fig. 2.1) (Aneja and Mehrotra 2015). There are approximately 1,250,000 macrofungi with majority belonging to Basidiomycota (Agaricomycotina: Tremellomycetes, Dacrymycetes, and Agaricomycetes) followed by Ascomycota (Pezizomycotina: Leotiomycetes, Pezizomycetes, and Sordariomycetes) and Zygomycota (a few in number). The class Agaricomycetes dominates with maximum number of macrofungi among all the ascomycetous and basidiomycetous classes consisting of macrofungi (Mueller et al. 2007; Tripathi et al. 2017; Thiers and Halling 2018). In Agaricomycotina, the yeast forming states occur in Tremellomycetes. The latter includes yeast forms that can be dimorphic, forming large gelatinous basidiocarps (e.g., Cryptococcus, Tremella, Cystofilobasidium) or species with no yeast stage (Hibbett 2006; Hibbett et al. 2007; Adl et al. 2012, 2018). Dacrymycetes (jelly fungi) include the wood-degrading, gelatinous species with large highly pigmented basidiocarps (Hibbett 2006; Shirouzu et al. 2013, 2016). The class Agaricomycetes represents the largest and the most diverse group in Agaricomycotina. It includes nearly 21,000 described species (Kirk et al. 2008). The members of the class produce a variety of forms of basidiocarps. The class includes 16,000 identified species of mushroom-forming fungi (Petersen 2012; Money 2016) classified in Agaricales (true mushrooms), Atheliales, Boletales (poricins), Geastrales (earth stars), Gomphales (club corals, pigs ears), Hysterangiales (false truffles), Phallales (stinkhorns), Cantharellales (coral fungi), Corticiales (crust fungi), Hymenochaetales (crust fungi), Polyporales (polypores, conks), Russulales (brittle gills, milk caps), Thelephorales (earth fans), and Trechisporales (Thiers and Halling 2018). The structure of the basidiocarps is very complex with spore-bearing surface either on lamellae or inside the tubes opening through pores. The lamellate species are known as agarics/euagarics (Agaricales) and the poroid species with tubes as boletes and polypores. A typical agaric basidiocarp consists of five parts: the pileus, lamellae (gills), annulus (ring), stipe, and volva (Fig. 2.2). Various forms of basidiocarps and hymenophore configurations are found among different orders of the class Agaricomycetes, e.g., resupinate, effused-reflexed, pileate-stipitate/substipitate or sessile, coralloid, polyporoid, corticioid, and gasteroid (Hibbett 2007). The resupinate forms occur in almost all orders of Agaricomycetes. The orders, namely, Geastrales, Hysterangiales, and Phallales, do not consist of any resupinate form. The orders Amylocorticiales, Atheliales, Corticiales, Jaapiales, Trechisporales, and Lepidostromatales comprise only or almost only resupinate forms. The rest of the members belonging to different orders of the class Agaricomycetes include resupinate to pileate-stipitate/sessile forms (epigeous or hypogeous, gilled or not, and gasteroid or not). Several genera classified in different of orders of the class include macrofungal species known to form large conspicuous basidiocarps of various kinds, e.g., Agaricus, Amanita, Clavaria, Coprinopsis, Coprinus, Entoloma, Fistulina, Lepiota, Lycoperdon, and Pleurotus (Agaricales); Hydnum and Cantharellus (Cantharellales); Auricularia auricula-judae (wood ear) (Auriculariales); Boletus and Paxillus (Boletales); Geastrum (Geastrales); Gomphus and Ramaria (Gomphales); Phallus (Phallales); Phellinus and Trichaptum (Hymenochaetales); Fomes, Ganoderma, Lenzites, and Sparassis (Polyporales); Thelephora (Thelephorales); and Russula, Lactarius, and Hericium (Russulales). The hymenophore of these species exhibits huge variation in configuration from smooth to hydnoid, poroid, toothed, spiny, tuberculate, etc. (Hibbett et al. 2014). Several types of fruiting bodies called as ascocarps are formed in Ascomycota such as apothecia, perithecia, pseudoperithecia, and cleistothecia (Pöggeler et al. 2006; Schmitt 2011). Some macrofungi such as morels, false morels, earth tongues, truffles, chicken lips, green elf cup species belong to Ascomycota (Pezizomycotina) spreading over Leotiales, e.g., Leotia, Chlorociboria, Pezizales, e.g., Morchella, Geoglossum, Gyromitra, Helvella, Peziza, Pyronema, Tuber, and Verpa and Hypocreales, e.g., Cordyceps spp. with diverse forms of ascocarps (Adl et al. 2012; Thiers and Halling 2018). In field, one can identify the sporocarps (basidiocarps and ascocarps) with their diverse color, shape, dimensions, and consistency. These appear fleshy, subfleshy/leathery or sometimes woody hard growing singly or in clusters on a vast diversity of substrates as saprotrophs, parasites, or symbionts (Fig. 2.3). Microscopically, the sporocarp is made up of mycelium consisting of hyphae. The ascospore-bearing structures are asci within the ascocarps and basidiospores are produced exogenously attached to the sterigmata of the club-shaped basidia. The ascospores and basidiospores are usually eight per ascus and four per basidium, respectively. The number of spores varies among different taxa, but this is constant for a species. The spores vary in color, size, and shape and are thin to thick walled. The spore wall may be smooth or ornamented variously and an important taxonomic feature. Several sterile structures are also found in the hymenium, subhymenium, or tramal region (Alexopoulos et al. 1996; Hibbett et al. 2014). Literature reports reveal that several macrofungal taxa have immense significance in human health as food and medicine. These health benefitting macrofungi are classified mainly in Agaricomycotina (Tremellomycetes, Dacrymycetes, Agaricomycetes) followed by Pezizomycotina (Leotiomycetes, Pezizomycetes, Sordariomycetes). Majority of the edible and medicinal macrofungi occur in Agaricomycetes, e.g., Agaricus, Boletus, Fomes, Ganoderma, Lycoperdon, Pleurotus, Russula, etc. (Tripathi et al. 2017). However, some agaricomycetous species are toadstools, e.g., Amanita phalloides, Galerina autumnalis, and others, which sometimes might cause severe toxicity (Benjamin 1995).