Mucoromycota

Mucoromycota is a phylum of terrestrial, non-flagellated fungi comprising approximately 800 to 1,000 described species (historically grouped with entomopathogenic and predaceous Zoopagomycota inside the paraphyletic phylum "Zygomycota"). Diverging immediately prior to the split of Dikarya (Ascomycota and Basidiomycota), Mucoromycota encompasses three subphyla—Mucoromycotina (Mucor, Rhizopus, Phycomyces, Absidia/Lichtheimia, Umbelopsis, and mycorrhizal Endogone), Mortierellomycotina (Mortierella, Linnemannia), and often Glomeromycotina (treated as phylum Glomeromycota)—that share two hallmark structural and developmental features:

1. Rapidly Growing Coenocytic (Aseptate) Mycelia with Chitosan-Rich Walls: Broad (5–20 μm diameter), ribbon-like multinucleate hyphae that lack regular cross-walls (forming septa only to wall off injured regions or delimit reproductive sporangia) and reinforce their cell walls with high proportions of deacetylated chitosan (β-1,4-D-glucosamine) alongside chitin and polyglucuronic acid.
2. Sexual Reproduction via Thick-Walled Zygospores: When compatible (−) and (+) mating types meet (or in homothallic species), opposed hyphal branches (progatamengia) fuse at their tips to produce a single large, darkly pigmented, warty, thick-walled diploid/dikaryotic resting spore—the zygospore—held between two suspensor cells.

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Terrestrial Non-Flagellated Fungi ├── Phylum Zoopagomycota (Predaceous amoeba/nematode-trapping Zoopagales, insect-pathogenic Entomophthorales, & gut Harpellales) ├── Phylum Glomeromycota / Subphylum Glomeromycotina (Obligate Arbuscular Mycorrhizal Fungi: Rhizophagus, Glomus) ├── Phylum Mucoromycota (Core Zygosporic Pin Molds, Oleaginous Soil Fungi, & Endogonales) │ ├── Subphylum Mortierellomycotina (~120 spp.: *Mortierella alpina*, *Linnemannia*; soil psychrotrophs, arachidonic acid producers) │ └── Subphylum Mucoromycotina │ ├── Order Endogonales (*Endogone*, *Jimgerdemannia*, *Densosporites*: ectomycorrhizal & early land-plant liverwort/hornwort symbionts) │ ├── Order Umbelopsidales (*Umbelopsis isabellina*: forest-soil pine-needle endophytes & lipid accumulators) │ └── Order Mucorales ("Pin Molds": ~300 spp.; fast-growing sugar saprotrophs, food fermenters, & clinical pathogens) │ ├── Rhizopodaceae (*Rhizopus stolonifer* [black bread mold], *R. oryzae / R. arrhizus*, *R. microsporus*, *R. oligosporus*) │ ├── Mucoraceae (*Mucor mucedo*, *Mucor circinelloides* [yeast–hyphal dimorphism & RNAi resistance]) │ ├── Phycomycetaceae (*Phycomyces blakesleeanus*: classic model for phototropism & gravitropism) │ ├── Lichtheimiaceae (*Lichtheimia corymbifera* [formerly *Absidia*]) │ └── Pilobolaceae (*Pilobolus crystallinus*: phototropic ballistic "hat-thrower" dung cannon) └── Subkingdom Dikarya (Phyla Ascomycota & Basidiomycota) *`

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In the order Mucorales (colloquially called "pin molds" because their erect aerial sporangiophores capped by spherical black sporangia resemble sewing pins), asexual reproduction is engineered for rapid colonization of ephemeral sugar- and starch-rich substrates (fallen fruit, bread, herbivore dung):

• Stolons, Rhizoids, and Columellate Sporangia: In Rhizopus stolonifer (black bread mold), aerial hyphae arch across the substrate as stolons, anchoring at nodes via root-like rhizoids and sending up vertical sporangiophores (1–3 mm tall). At the apex of each sporangiophore, a globose sporangium swells around a dome-shaped sterile central septum called the columella, cleaving its multinucleate cytoplasm into thousands of wind-dispersed sporangiospores.
• Phycomyces blakesleeanus and Max Delbrück's Sensory Biophysics: Nobel laureate Max Delbrück adopted the giant single-celled sporangiophore of Phycomyces (which grows to 10–15 cm in length at 3 mm/hour while rotating helically) as a model system for sensory transduction. The transparent cylindrical stalk acts as a convex cylindrical optical lens, focusing incident blue light (sensed by a heterodimer of White Collar photoreceptors MadA / MadB) onto the distal wall of the growing zone across a dynamic sensitivity range of 10¹⁰-fold (from starlight to full sunlight), while octahedral protein crystals floating in the vacuole and dense lipid globules mediate gravitropism.
• Pilobolus crystallinus — The 25,000-g Hydraulic Dung Cannon: Growing on herbivore dung, Pilobolus ("hat thrower") inflates a carotenoid-ringed subsporangial vesicle to a hydrostatic turgor pressure of 0.55 MPa (5.5 atmospheres). Acting as an optical lens, the vesicle focuses morning sunlight onto a basal retináculo ring, steering the sporangiophore directly toward gaps in the vegetation canopy. When turgor ruptures the circumscissile line of weakness at the base of the columella, a jet of cell sap launches the mucilage-coated black sporangium up to 2.5 meters away at initial velocities of 9–20 m/s and peak accelerations of 20,000 to 180,000 g—adhering to fresh grass blades beyond the "zone of repugnance" so it will be ingested by grazing artiodactyls or perissodactyls to repeat its gut-passage cycle!

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In 1904, Albert Francis Blakeslee discovered fungal heterothallism in Mucor and Rhizopus, showing that zygospore formation requires contact between two morphologically identical but physiologically distinct mating types designated (+) and (−) (governed by a single sex locus encoding the HMG-domain transcription factor SexP in (+) strains and SexM in (−) strains).

Remarkably, neither mating type can synthesize the active sexual pheromone—trisporic acid—on its own. Instead, the two mating types perform obligate intercellular metabolic complementation using β-carotene as the precursor:

1. Both (+) and (−) hyphae oxidatively cleave β-carotene (via β-carotene 15,15′-oxygenase and retinal dehydrogenase) into C₁₈ 4-dihydrotrisporin.
2. Only the (+) strain expresses high activity of the NADP-dependent dehydrogenase that oxidizes 4-dihydrotrisporin into trisporin and trisporol, which diffuse through the soil or air to the (−) hypha.
3. Only the (−) strain expresses trisporol/4-dihydromethyltrisporate dehydrogenase and esterase activities that convert the (+) intermediates into mature trisporic acid B and C.
4. Secreted trisporic acid binds receptors on both (+) and (−) hyphae, inducing chemotropic growth of aerial zygophores toward one another, giant 10- to 50-fold upregulation of β-carotene synthesis (utilized industrially with mated Blakeslea trispora cultures to manufacture commercial provitamin A / β-carotene), gametangial fusion (plasmogamy), and maturation of the sporopollenin-fortified zygospore.

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1. Food Fermentation (Tempeh), Single-Cell Oils, and Burkholderia Endosymbiosis

• Tempeh and Asian Fermentations: Rhizopus microsporus var. oligosporus and Actinomucor elegans are the foundational starter cultures for Indonesian tempeh and Chinese sufu (fermented豆腐), secreting phytases (which degrade antinutritional phytic acid to liberate bioavailable iron and zinc), proteases, and lipases that knit boiled soybeans into a sliceable protein cake.
• Arachidonic Acid and Oleaginous Lipid Production: Mortierella alpina and Umbelopsis isabellina accumulate intracellular triacylglycerols up to 50–70% of dry biomass, serving as the primary commercial fermentation source of arachidonic acid (ARA, 20:4 ω-6) added to infant formula alongside dinoflagellate/thraustochytrid DHA.
• Toxin-Producing Mycetohabitans Bacterial Endosymbionts: Plant-pathogenic strains of Rhizopus microsporus (causing rice seedling blight) do not synthesize their own virulence toxin (rhizoxin, a potent microtubule-depolymerizing polyketide macrolide); instead, rhizoxin is synthesized by an intracellular betaproteobacterial endosymbiont, Mycetohabitans rhizoxinica (formerly Burkholderia rhizoxinica), living directly inside the coenocytic fungal cytoplasm, which the fungus also requires in order to form asexual sporangiospores!

2. Clinical Mucormycosis: Ketoacidosis, Free Iron, and GRP78–CotH Endothelial Invasion

In immunocompromised patients, Mucorales (Rhizopus arrhizus / R. oryzae, Mucor circinelloides, Lichtheimia corymbifera) cause mucormycosis—a rapidly progressive, necrotizing angioinvasive infection (rhino-orbital-cerebral or pulmonary) with a 40–80% mortality rate:

1. Diabetic Ketoacidosis (DKA) and Iron Liberation: In healthy human serum, transferrin sequesters ferric iron (Fe³⁺), starving Mucorales spores. During diabetic ketoacidosis (pH < 7.3, elevated β-hydroxybutyrate) or deferoxamine chelation therapy, acidosis protonates transferrin's iron-binding pockets (releasing free unbound Fe³⁺/Fe²⁺), which Rhizopus imports via its high-affinity iron permease Ftr1.
2. Host GRP78 Upregulation and Fungal CotH3 Invazins: Simultaneously, hyperglycemia, free iron, and β-hydroxybutyrate induce vascular endothelial cells to overexpress the endoplasmic-reticulum chaperone Glucose-Regulated Protein 78 (GRP78 / BiP) on their cell surface. Spore coat proteins unique to Mucorales (CotH3 and CotH7) bind directly to endothelial GRP78 (and integrin β1), triggering endocytosis across blood-vessel walls, thrombosis, ischemic tissue infarction, and black necrotic eschars.
3. Intrinsic Azole Resistance and Epimutation: Because Mucorales express an evolutionarily duplicated lanosterol 14α-demethylase (CYP51 F5, carrying a natural Y129F substitution), they are intrinsically resistant to voriconazole, fluconazole, and echinocandins (requiring surgical debridement + liposomal amphotericin B, isavuconazole, or posaconazole). Furthermore, Mucor circinelloides can develop transient drug resistance without DNA mutation via epimutations—generating endogenous small interfering RNAs (siRNAs) that silence the drug-target gene (fkbA for FK506/tacrolimus) and revert once drug pressure is removed.

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1. Spatafora, J. W., Chang, Y., Benny, G. L., Lazarus, K., Smith, M. E., Berbee, M. L., ... & Stajich, J. E. (2016). A phylum-level phylogenetic classification of zygomycete fungi based on genome-scale data. Mycologia, 108(5), 1028–1046. Google Scholar
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• Fungi
• Glomeromycota
• Chytridiomycota
• Ascomycota
• Fermentation