Ascomycota

Ascomycota (commonly known as sac fungi, from the Ancient Greek askos, "sac" or "wineskin," and mykes, "fungus") is the largest and most taxonomically diverse phylum of Kingdom Fungi, comprising more than 65,000 described extant species—accounting for ~65% to 75% of all described fungi (including >98% of all lichen-forming fungal mycobionts). Together with its sister phylum Basidiomycota (club fungi), Ascomycota forms the subkingdom Dikarya ("higher fungi" characterized by a binucleate dikaryotic [n + n] stage between plasmogamy and karyogamy), which diverged from early-diverging mucoromycete and chytrid lineages during the Neoproterozoic–Cambrian transition (~600–500 million years ago).

The phylum spans unicellular budding and fission yeasts (Saccharomyces cerevisiae, Schizosaccharomyces pombe, and the opportunistic human pathogen Candida albicans), foundational genetic model molds (Neurospora crassa and Aspergillus nidulans), prized culinary delicacies (morels Morchella and truffles Tuber melanosporum), devastating agricultural and forest plant pathogens (Magnaporthe oryzae rice blast, Fusarium, Claviceps purpurea ergot, Ophiostoma Dutch elm disease, and Cryphonectria parasitica chestnut blight), and pharmaceutical producers of penicillin (Penicillium rubens), statins (Aspergillus terreus), and cyclosporine (Tolypocladium inflatum).

1. The Sexual Ascus and Endogenous Ascospores: Whereas Basidiomycota produce four external meiotic spores on sterigmata atop a club-shaped basidium, every sexually reproducing ascomycete forms its meiotic spores internally (endogenously) inside a pressurized, sac-like or cylindrical cell called the ascus (plural asci). Inside the young diploid (2n) ascus mother cell, meiosis I and II produce four haploid (1n) nuclei, followed in most species by one post-meiotic mitotic division to yield eight haploid nuclei. A double-layered invaginating plasma membrane (enveloping membrane system) guided by the spindle pole body encapsulates each nucleus along with surrounding cytoplasm to wall off eight endogenous ascospores.
   • Ordered Linear Tetrad / Octad Genetics in Neurospora crassa: Because the cylindrical ascus of Neurospora crassa and Sordaria fimicola is too narrow for nuclear spindles to slip past one another during meiosis I, meiosis II, and post-meiotic mitosis, the eight linear ascospores remain lined up in the exact spatial order of the four chromatids on the meiotic metaphase plate—allowing George W. Beadle and Edward L. Tatum (1941) to map centromere linkage and establish the "One Gene–One Enzyme" hypothesis!
   • Turgor-Driven Ascospore Cannon Ejection: In filamentous Pezizomycotina, the mature ascus accumulates high concentrations of potassium ions (K⁺), mannitol, and glycerol, generating internal osmotic turgor pressures of 0.3 to >1.5 megapascals (3 to 15 atmospheres). When the apical ring or hinged lid (operculum) ruptures at the tip of the ascus, a pressurized jet of fluid launches the eight ascospores into the turbulent air layer above the fruiting body at initial accelerations exceeding 100,000 g!
2. The Dikaryotic Crozier Hook and Ascocarp Fruiting Bodies: When two sexually compatible haploid hyphae (homothallic or heterothallic governed by the MAT1-1 α-box and MAT1-2 HMG-domain mating-type idiomorphs) fuse their cytoplasm (plasmogamy) via an ascogonium and trichogyne, their nuclei do not immediately fuse. Instead, inside the developing fruiting body (ascocarp / ascoma), short binucleate ascogenous hyphae proliferate via a hook-shaped terminal cell called a crozier (homologous to the clamp connection of Basidiomycota), which undergoes conjugate mitosis to partition one MAT1-1 and one MAT1-2 nucleus into the penultimate ascus mother cell where nuclear fusion (karyogamy) finally occurs. Depending on the clade, thousands of asci and sterile supporting filaments (paraphyses) are packaged inside four types of ascocarps:
   • Apothecium: An open, cup- or saucer-shaped fruiting body with an exposed fertile layer (hymenium), characteristic of Pezizomycetes (morels, cup fungi) and Lecanoromycetes (lichens);
   • Perithecium: A flask-shaped fruiting body discharging ascospores through a narrow apical pore (ostiole), characteristic of Sordariomycetes (Neurospora, Cordyceps, Claviceps, Xylaria);
   • Cleistothecium / Chasmothecium: A completely closed, spherical fruiting body that releases spores only when its wall splits or decays (Aspergillus, Penicillium, powdery mildews Erysiphales);
   • Pseudothecium (Ascostroma): Bitunicate "jack-in-the-box" asci formed inside cavities dissolved within pre-existing stromatic tissue (Dothideomycetes such as Venturia inaequalis apple scab).
3. Simple Septal Pores and Peroxisome-Derived Woronin Bodies: Filamentous ascomycete hyphae are divided by cross-walls (septa) pierced by a single central septal pore (50–500 nm wide) that allows rapid cytoplasmic streaming of organelles, nutrients, and nuclei (heterokaryosis) along the mycelium. Tethered beside every septal pore in Pezizomycotina are dense, crystalline hexagonal proteinaceous organelles called Woronin bodies—specialized peroxisomes packed with a self-assembling lattice of the HEX-1 protein (Jedd & Chua, 2000). If a hyphal tip is punctured by a grazing nematode or mechanical shear, cytoplasmic surge instantly plugs the adjacent septal pore with a Woronin body within <1–2 seconds, sealing off the ruptured compartment so the rest of the mycelium does not bleed to death!

1. The 80-Atmosphere Appressorium of the Rice Blast Fungus (Magnaporthe oryzae): The sordariomycete Magnaporthe oryzae destroys enough rice (Oryza sativa) each year to feed >60 million people. Without using enzymes to dissolve the waxy plant cuticle first, a germinating Magnaporthe conidium differentiates a dome-shaped infection cell called an appressorium whose cell wall is lined with an impermeable layer of 1,8-dihydroxynaphthalene (DHN) melanin. The appressorium degrades lipid droplets into molar concentrations of glycerol (>3.2 M), drawing water inward to generate up to 8.0 megapascals (80 atmospheres / 1,160 psi) of hydrostatic turgor pressure—the highest cellular pressure recorded in any living organism (Howard et al., 1991; Ryder et al., 2019)—driving a narrow penetration peg mechanically straight through the rice leaf cuticle (or even through polyvinylchloride plastic sheets in the laboratory!).
2. Repeat-Induced Point Mutation (RIP) Genome Defense: Discovered in Neurospora crassa by Eric U. Selker (1987), filamentous ascomycetes guard their genomes against selfish transposable elements via Repeat-Induced Point Mutation (RIP). During the pre-meiotic dikaryotic phase between fertilization and karyogamy, a homology-scanning mechanism detects any duplicated DNA sequence (>400 bp) in the haploid nucleus and hypermutates both copies via cytosine-to-thymine (C → T) transitions catalyzed by the DNA methyltransferase-like enzyme RID—silencing transposons at the cost of preventing gene evolution by tandem duplication unless genes duplicate before meiosis is lost!

How do zombie-ant fungi (Ophiocordyceps unilateralis) hijack the nervous system and muscles of carpenter ants?

In tropical rainforests, spores of the sordariomycete Ophiocordyceps unilateralis penetrate the cuticle of foraging carpenter ants (Camponotus leonardi) and proliferate as yeast-like hyphal bodies throughout the ant's hemocoel. When ready to sporulate, the fungus compels the infected ant to leave its canopy trail, climb down to an understory leaf ~25 cm above the forest floor (where humidity and temperature are optimal for ascospore development), and lock its mandibles into a major leaf vein on the shaded underside in a permanent "death grip" before dying. Using 3D serial block-face electron microscopy and transcriptomics, David P. Hughes, Charissa de Bekker, and colleagues (2017) discovered that Ophiocordyceps does not even invade the ant's brain cells—instead, fungal hyphae weave a dense 3D syncytial network around every individual mandibular and leg muscle fiber throughout the ant's body while secreting enterotoxins and neuroactive guanobutyric acid/sphingosine derivatives, controlling the host's muscles peripheral-mechanically like a puppeteer before sprouting a perithecial fruiting stalk out the back of the ant's head!

Why are truffles (Tuber) underground and how do they disperse their spores without wind?

Unlike cup fungi (Peziza) and morels (Morchella)—which project their apothecia above ground and fire their ascospores into the wind—true truffles (Tuber melanosporum Périgord black truffle and Tuber magnatum Alba white truffle, ectomycorrhizal symbionts of oak and hazelnut roots) evolved hypogeous (underground) closed fruiting bodies multiple times from above-ground cup-fungus ancestors as an adaptation to seasonal Mediterranean droughts. Having lost active ascospore ejection, truffles wait until their ascospores are fully mature and then synthesize volatile sulfur compounds (dimethyl sulfide, bis(methylthio)methane) and steroid pheromones (5α-androst-16-en-3-ol) that diffuse up through the soil to attract wild boars, rodents, and truffle dogs, which dig up and eat the ascocarp and disperse the tough-walled ascospores in their feces (mycophagy).

1. Beadle, G. W., & Tatum, E. L. (1941). Genetic control of biochemical reactions in Neurospora. Proceedings of the National Academy of Sciences, 27(11), 499–506. doi:10.1073/pnas.27.11.499
2. Howard, R. J., Ferrari, M. A., Roach, D. H., & Money, N. P. (1991). Penetration of hard substrates by a fungus employing enormous turgor pressures. Proceedings of the National Academy of Sciences, 88(24), 11281–11284. doi:10.1073/pnas.88.24.11281
3. Jedd, G., & Chua, N.-H. (2000). A new self-assembled peroxisomal vesicle required for efficient resealing of the plasma membrane. Nature Cell Biology, 2(4), 226–231. doi:10.1038/35008652
4. Schoch, C. L., Sung, G.-H., López-Giráldez, F., Townsend, J. P., Miadlikowska, J., Hofstetter, V., ... & Spatafora, J. W. (2009). The Ascomycota tree of life: A phylum-wide phylogeny clarifies the origin and evolution of fundamental reproductive and ecological traits. Systematic Biology, 58(2), 224–239. doi:10.1093/sysbio/syp020
5. Fredericksen, M. A., Zhang, Y., Hazen, M. L., Loreto, R. G., Mangold, C. A., Chen, D. Z., & Hughes, D. P. (2017). Three-dimensional visualization and a deep-learning model reveal complex fungal parasite networks in behaviorally manipulated ants. Proceedings of the National Academy of Sciences, 114(47), 12590–12595. doi:10.1073/pnas.1711673114
6. Shen, X.-X., Steenwyk, J. L., LaBella, A. L., Opulente, D. A., Zhou, X., Kominek, J., ... & Rokas, A. (2020). Genome-scale phylogeny and contrasting modes of genome evolution in the fungal phylum Ascomycota. Science Advances, 6(45), eabd0079. doi:10.1126/sciadv.abd0079