L.6-G.Biology
Mycology
D.Ebtihal Muiz
Phylum - Zygomycota, Kingdom Fungi
Characteristics
Zygomycota, like all true fungi, produce cell walls containing chitin.
They grow primarily as mycelia, or filaments of long cells called hyphae.
Unlike the so-called 'higher fungi' comprising the Ascomycota and
Basidiomycota which produce regularly septate mycelia, most
Zygomycota form hyphae which are generally coenocytic because they
lack cross walls or septa. There are, however, several exceptions and
septa may form at irregular intervals throughout the older parts of the
mycelium or reproductive structure are regularly spaced in two sister
orders of Zygomycota, the Kickxellales and Harpellales.
The unique character (synapomorphy) of the Zygomycota is the
zygospore. Zygospores are formed within a zygosporangium after the
fusion of specialized hyphae called gametangia during the sexual cycle .
single zygospore is formed per zygosporangium. Because of this one-toone relationship, the terms are often used interchangably. The mature
zygospore is often thick-walled , and undergoes an obligatory dormant
period before germination. Most Zygomycota are thought to have a
zygotic or haplontic life cycle (Figure 1). Thus, the only diploid phase
takes place within the zygospore. Nuclei within the zygospore are
believed to undergo meiosis during germination, but this has only been
demonstrated genetically within the model eukaryote Phycomyces
blakesleeanus (Eslava et al. 1975)
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Figure 1. Generalized life cycle of Zygomycota. Asexual reproduction
occurs primarily by sporangiospores produced by mitosis and cell
division. The only diploid (2N) phase in the life cycle is the zygospore,
produced through the conjugation of compatible gametangia during the
sexual cycle
Zygomycota typically undergo prolific asexual reproduction through
the formation of sporangia and sporangiospores. Sporangiospores are
distinguished from other types of asexual spores, such as conidia of the
Ascomycota and Basidiomycota, by their development. Walled
sporangiospores are formed by the internal cleavage of the sporangial
cytoplasm. At maturity, the sporangial wall typically disintegrates or
dehisces thereby freeing the spores that are usually dispersed by wind or
water.
Sporangia are formed at the ends of specialized hyphae called
sporangiophores. In the model organism, Phycomyces blakesleeanus,
sporangial development has been studied extensively to understand the
genetic basis for various trophisms, including the strong phototrophic
responses to blue light. A unique spore dispersal strategy for the
Mucorales is exhibited by the dung fungus Pilobolus, whose name
literally means 'the hat thrower'. The entire black sporangium is
explosively shot off of the top of the sporangiophore up to distances of
several meters. Phototrophic growth of the sporangiophore facilitates
dispersal away from the dung onto a fresh blade of grass where it may be
consumed by an herbivore, thereby completing the asexual cycle after the
spores pass through the digestive system. Some members of the
Entomophthorales (e.g., Basidiobolus, Conidiobolus) also reproduce via
forcibly discharged asexual spores. Interestingly, species of Basidiobolus,
Conidiobolus and several other genera produce a second kind of spore on
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a long stalk that appears to have certain morphological adaptations for
efficient insect dispersal.
Life cycle
• Vegetative mycelium is haploid, reproduces asexually by producing
sporangiospores in sporangia
• In a heterothallic species, when two compatible strains come
together, hyphal branches form, enlarge to form progametangia
• Septa form, producing multinucleate gametangia and suspensors
• Plasmogamy occurs, end walls of gametangia dissolve and
cytoplasm of gametangia mixes
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Zygomycota
• Two classes:
• Zygomycetes – a polyphyletic class, the largest (665 spp) and best
known class
• Trichomycetes – smaller (135 spp), less well understood, are
commensals on surface and in guts of arthropods
Class - Zygomycetes
• Zygospore production is generally similar among species, therefore
classification is based on characteristics of asexual reproductive
structures
• Asexual reproduction is typically by production of sporangiospores,
but we will see lines of evolution in which the number of
spores/sporangium is reduced until there is only 1 spore/sporangium conidium
• Generally divided into 7 orders, we will discuss 3
• Mucorales – mainly saprotrophs, many to one
sporangiospore/sporangium
• Entomophthorales – mainly parasitic on arthropods, limited
mycelium, one sporangiospore/sporangium
• Glomales – obligate biotrophs, form arbuscuar mycorrhizae
• Other orders:
• Kickxellales – produced septate hyphae and modified one spored
sporangia
• Dimargaritales – mycoparasites
• Zoopagales – parasites of small animals (amoebae, rotifers &
nematodes) and fungi including the lethal lollipop,
Zoophagus)
• Endogonales – saprotrophs
Mucorales
• Grow saprotrophically on decaying plant and animal remains in
soils, dung, etc.
• Produce large numbers of asexual spores that are dispersed in the air
• Common contaminants in laboratory
• Some are important in spoiling food – common bread mold, storage
diseases of fruits and vegetables
• Some infect humans and animals – opportunistic pathogens
• Typically form aseptate hyphae, septa formed to delimit
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reproductive structures
• Some species form rhizoids
Multispored sporangium
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Morphology of sporangia varies, basis for classification
Typical multispored sporangium contains
Sporangium wall
Columella
sporangiophore
Development of sporangium
• Tip of sporangiophore swells
• Swelling increases, contains multinucleate cytoplasm
• Cytoplasm is cleaved to form all spores at one time – cell membrane
and cell walls laid down around nuclei
• After formation, sporangial wall may break and release spores into
the air or
• A drop of fluid may envelop the sporangium with spores being
dispersed by small animals that touch the sporangium
Sexual reproduction
• Involves fusion of two multinucleate gametangia that are similar in
structure, may differ in size
• Gametangia are produced as terminal swellings of hyphal branches
• After plasmogamy – a thick walled zygospore is produced with a
zygosporangium
• Both homothallic and heterothallic species
Formation of zygospore
Life cycle
• Zygospore forms inside zygosporangium
• Zygospore develops thick wall, warty appearance, dark in color
• Karyogamy occurs at different times in zygospore formation in
different species, so zygospore is diploid at some point
• When zygospore germinates, meiosis occurs to start the haploid
portion of the life cycle over again
• Some variation is seen in zygospore formation
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Entomophthorales
• Arthropod parasites, 240 spp.
• Asexual reproduction by one spored sporangiola (conidia)
• Mycelium exhibits limited growth in the body of the host, forms
septa and fragments
• Entomophthora is a parasite of flies – seen when fly is stuck to
window, white halo around it
Entomophthora
• Mycelium fills up body of fly, forms sporangiophores that extend out
of segments of abdomen
• Sporangiophore builds up pressure, shoots off sporangiolum when
there is air movement (another fly)
• If misses, can form another structure that shoots it off, up to 3 times
• Sexual reproduction not well understood, form resting spores that are
similar to zygospores
Entomophthorales
Basidiobolus
• Another fungus that forcibly ejects its sporangiolum
• Grows on a variety of substrates including frog and beetle dung
• Sporangiola may be eaten by beetles, which may then be eaten by
frog and then grows in dung
Glomales
• Obligate biotrophs
• Biotrophic in the roots of higher plants, form arbuscular mycorrhizae
• Ca 90 spp.
• Now placed in a separate phylum by some – the Glomeromycota
• Form tree-like structures = arbuscules in cells of plant – exchange of
nutrients between fungus and plant
• Form large spores in soil – zygospores, azygospores, and
chlamydospores depending on species
• A zygospores
• Chlamydospores
Trichomycetes
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• Second class in Zygomycota
• Small class, contains 3 orders
• All but a few species are obligate commensals in guts or on
exoskeleton of arthropods
• Found in freshwater (mayfly, stonefly, midges) and terrestrial
(millipedes)
• Thallus is relatively simple – produce a holdfast that anchors them to
lining of gut
• Thallus may be unbranched and aseptate or branched and septate
• Variety of asexual spores produced (generally 1 spore/sporangium)
• Sexual reproduction involves formation of zygospores (in one order)
• Fusion of cells in thallus
• Formation of zygosporophore and then zygospore
Sporangiolum
 Sporangium
 Containing 1-50 Spores
 Merosporangium is a
Sporangiolum with
spoes in linear series
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Pilobolus
-common fungus sporulating on aung
-sporangia with dark, thick, persistant wall
-characterized by forcibly discharged sporangia(>2 meters)
sporangiophores are photropid (bend towards light)
-pilaira-similar morphology and ecology, lacks discharge mechanism
“Lower fungi”
• Major groups of lower fungi – Chytridiomycota, Zygomycota,
Oomycota
• Exhibit diversity in vegetative thallus, asexual and sexual
reproduction
• Mycelium when present is typically aseptate
• Used to be placed in one class = “Phycomycetes” – now clear that
they do not share a common phylogeny
• Asexual reproduction – spores produced inside a sporangium –
sporangiospores
• In Chytridiomycota and Oomycota, zoospores produced
• Transition from multispored sporangia to conidia in both the
Oomycota and Zygomycota
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