Basidiomycota

Basidiomycota (commonly known as club fungi, from the Neo-Latin basidium, "small pedestal") is the second-largest phylum of Kingdom Fungi, comprising approximately 40,000 described extant species (~30–35% of all described fungi) across three subphyla: Agaricomycotina (gilled mushrooms, boletes, chanterelles, polypore shelf fungi, puffballs, stinkhorns, jelly fungi, and crust fungi), Pucciniomycotina (obligate plant-parasitic rust fungi), and Ustilaginomycotina (plant-parasitic smut fungi and human skin commensal Malassezia yeasts). Together with its sister phylum Ascomycota (sac fungi), Basidiomycota forms the subkingdom Dikarya.

Ecologically, basidiomycetes serve as the planet's indispensable wood-decay decomposers (the only organisms capable of complete white-rot enzymatic depolymerization of recalcitrant plant lignin) and as dominant ectomycorrhizal symbionts (Amanita, Boletus, Russula, Laccaria, Cortinarius) shepherding mineral nutrients to boreal and temperate forest trees. They also include the largest and oldest single genetic organisms on Earth—such as the subterranean humongous fungus (Armillaria ostoyae / A. solidipes) in Oregon's Malheur National Forest, whose clonal rhizomorph mycelium spans 9.6 square kilometers (2,385 acres), weighs an estimated 7,500 to 35,000 metric tons, and is 2,400 to 8,650 years old!

1. The Club-Shaped Basidium and Surface-Tension Ballistospore Catapult (Buller's Drop): Whereas Ascomycota package eight meiotic spores internally inside a sac-like ascus, basidiomycetes produce four haploid (1n) meiotic basidiospores externally atop slender horn-like stalks (sterigmata) projecting from the apex of a club-shaped cell called the basidium (either a single-celled holobasidium in Agaricomycetes or a septate four-celled phragmobasidium in rusts, smuts, and Auricularia jelly fungi). How does a mushroom launch millones of microscopic spores horizontally from the narrow air spaces between its gills (lamellae) or pores so they can fall clear of the cap into the wind without hitting the opposite gill? As first observed by A. H. Reginald Buller (1909) and solved biophysically using ultra-high-speed video by Nicholas P. Money, Xavier Noblin, and colleagues (1998, 2009):
   • Just before launch, the asymmetric base of the spore (hilar appendix) secretes a tiny pinpoint of hygroscopic mannitol and hexose sugars, while the adjacent adaxial face of the spore is hydrophilic.
   • Water vapor from the humid gill air condenses simultaneously into two separate liquid droplets: a spherical Buller's drop on the hilar appendix and a flat adaxial drop on the spore surface.
   • When Buller's drop grows large enough (~5 μm diameter) to touch the adaxial drop, the two drops coalesce in <1 microsecond! The rapid release of surface-tension free energy (~10⁻¹³ Joules) drives a shift in center of mass that snaps the spore off its sterigma at an initial acceleration of 10,000 to 25,000 g (1–2 m/s), while aerodynamic viscous drag stops the spore within 0.1 to 0.5 millimeters—precisely halfway across the inter-gill airspace—allowing gravity to drop the spore straight down out of the mushroom cap!
2. Prolonged Dikaryotic Mycelium (n + n) and Clamp Connections: In Ascomycota, the binucleate dikaryotic phase is restricted to short ascogenous hyphae buried inside the fruiting body. In Basidiomycota, almost the entire vegetative life of the fungus is spent as a free-living, perennial secondary dikaryotic mycelium (n + n) that grows through soil or rotting wood for years or centuries before fruiting. To ensure that every new hyphal compartment receives exactly one nucleus of each parental mating type during mitosis down a narrow tube, most basidiomycetes form a diagnostic lateral bypass buckle at each dividing septum called a clamp connection (homologous to the ascomycete crozier): one daughter nucleus migrates through the backward-arching clamp while its sister divides along the main axis, after which the clamp fuses with the sub-apical cell to restore the binucleate (n + n) state.
3. The Dolipore–Parenthesome Septal Complex: In Agaricomycotina and Tremellomycetes, the hyphal cross-wall around the central septal pore swells into a barrel-shaped flange (dolipore), capped on both sides by dome-shaped, perforated membranes derived from the endoplasmic reticulum called parenthesomes (septal pore caps)—acting as a selective sieve that permits cytoplasmic and organelle flow while preventing unregulated nuclear migration.

1. Tetrapolar Mating-Type Systems (A and B Loci) Generating >20,000 Sexes: Why can't two sibling spores from the same mushroom simply mate with each other all the time? In Agaricomycotina (such as the split-gill mushroom Schizophyllum commune and inkcap Coprinopsis cinerea), sexual compatibility is governed by two unlinked chromosomal loci—an unlinked tetrapolar mating system:
   • The A (HD) locus encodes paired homeodomain transcription factors (HD1 and HD2) that heterodimerize upon mating to initiate nuclear pairing and clamp-connection formation;
   • The B (P/R) locus encodes lipopeptide pheromones and G-protein-coupled pheromone receptors (homologous to STE3) that govern nuclear migration and clamp fusion.
   • Because both the A and B loci are multiallelic (in Schizophyllum commune, there are ~288 alleles of A and ~81 alleles of B in wild populations!), two haploid mycelia are fully fertile only if they differ at both A and B—yielding >23,000 distinct, inter-compatible mating types ("sexes") so that any encounter with an unrelated mycelium in the forest has a >98% probability of being fertile, while sibling inbreeding is suppressed to 25%!
2. Class II Peroxidases, White-Rot Lignin Decay, and the End of the Carboniferous Coal Swamp Era: Vascular land plants reinforce their secondary xylem cell walls with lignin—a hydrophobic, non-repeating three-dimensional polymer of phenylpropanoid monolignols cross-linked by recalcitrant carbon–carbon and ether bonds that no hydrolytic enzyme can cleave. In 2012, comparative phylogenomics of 31 fungal genomes by Dimitrios Floudas, David S. Hibbett, Igor V. Grigoriev, and colleagues demonstrated that the Agaricomycetes evolved high-redox-potential Class II heme peroxidases (lignin peroxidases [LiP], manganese peroxidases [MnP], and versatile peroxidases [VP]) around 290–295 million years ago (at the Carboniferous–Permian boundary):
   • White-Rot Basidiomycetes (Phanerochaete chrysosporium, Trametes versicolor, Pleurotus ostreatus) secrete Class II peroxidases and copper laccases along with H₂O₂-generating oxidases to bombard lignin with free-radical cations, mineralizing black/brown lignin completely to CO₂ and leaving behind white crystalline cellulose;
   • Brown-Rot Basidiomycetes (Serpula lacrymans dry rot, Gloeophyllum trabeum, which evolved repeatedly from white-rot ancestors by deleting Class II peroxidases!) bypass lignin using non-enzymatic chelator-mediated Fenton chemistry (Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + •OH hydroxyl radicals) to depolymerize cellulose inside conifer wood while leaving behind cubical, oxidized brown lignin bricks!
   • Together with paleoclimatic drying across Pangaea, the radiation of white-rot Agaricomycetes at the end of the Permo-Carboniferous restored global wood carbon recycling after tens of millions of years of un-degraded lycopsid peat accumulation.

How does the "Death Cap" mushroom (Amanita phalloides) kill human liver cells, and how was its antidote discovered?

Responsible for >90% of all fatal mushroom poisonings worldwide, the ectomycorrhizal green death cap (Amanita phalloides, native to Europe and invasive under California live oaks) synthesizes bicyclic octapeptides called amatoxins (principally α-amanitin) and heptapeptide phallotoxins via ribosomal peptide synthesis (MSDIN gene family cleaved by prolyl oligopeptidase B). Completely impervious to cooking, freezing, or gastric acid, α-amanitin is actively imported into human hepatocytes via the bile-acid transporter OATP1B3 and binds with nanomolar affinity inside the bridge-helix funnel of eukaryotic RNA Polymerase II, locking the enzyme against translocation and halting all mRNA transcription! In 2023, a genome-wide CRISPR screen by Qiaoping Wang and colleagues discovered that α-amanitin toxicity requires N-glycan modification by the enzyme STT3B, and identified the FDA-approved diagnostic dye indocyanine green (ICG) as a potent specific inhibitor of STT3B that rescues mice from lethal α-amanitin poisoning!

Why did puffballs, bird's nest fungi, and stinkhorns ("Gasteromycetes") lose Buller's drop spore shooting?

Throughout the 19th and 20th centuries, mycologists grouped puffballs (Lycoperdon, Calvatia), earthstars (Geastrum), bird's nest fungi (Cyathus), false truffles (Rhizopogon), and stinkhorns (Phallus impudicus) into a separate class called "Gasteromycetes" ("stomach fungi") because their basidiospores mature inside closed fruiting bodies and have symmetrical sterigmata that lost Buller's drop ballistospore shooting. Molecular phylogenetics (Hibbett et al., 1997, 2007) proved that "Gasteromycetes" is completely polyphyletic: gasteromycetation (closing the cap and losing Buller's drop) evolved independently dozens of times from gilled mushrooms and boletes (for example, puffballs and bird's nest fungi are nested directly inside the gilled mushroom order Agaricales, while Rhizopogon false truffles are nested inside Boletales!), switching from active surface-tension shooting to raindrop-bellows puffing (puffballs), raindrop-splash cups (bird's nest fungi), or carrion-fly olfaction (stinkhorns).

1. Money, N. P. (1998). More g's than the Space Shuttle: Ballistospore discharge. Mycologia, 90(4), 547–558. doi:10.1080/00275514.1998.12026942
2. Hibbett, D. S., Pine, E. M., Langer, E., Langer, G., & Donoghue, M. J. (1997). Evolution of gilled mushrooms and puffballs inferred from ribosomal DNA sequences. Proceedings of the National Academy of Sciences, 94(22), 12002–12006. doi:10.1073/pnas.94.22.12002
3. Noblin, X., Yang, S., & Dumais, J. (2009). Surface tension propulsion of fungal spores. Journal of Experimental Biology, 212(17), 2835–2843. doi:10.1242/jeb.029975
4. Floudas, D., Binder, M., Riley, R., Barry, K., Blanchette, R. A., Henrissat, B., ... & Hibbett, D. S. (2012). The Paleozoic origin of enzymatic lignin decomposition reconstructed from 31 fungal genomes. Science, 336(6089), 1715–1719. doi:10.1126/science.1221748
5. Varga, T., Krizsán, K., Földi, C., Dima, B., Sánchez-García, M., Sánchez-Ramírez, S., ... & Nagy, L. G. (2019). Megaphylogeny resolves global patterns of mushroom evolution. Nature Ecology & Evolution, 3(4), 668–678. doi:10.1038/s41559-019-0834-1
6. Wang, B., Li, J.,iu, S., ... & Wang, Q. (2023). Identification of indocyanine green as a STT3B inhibitor against mushroom α-amanitin cytotoxicity. Nature Communications, 14, 2241. doi:10.1038/s41467-023-37714-3