Zygomycota, or zygote fungi, is a division of fungi. Approximately
1050 species are known. They are mostly terrestrial in habitat,
living in soil or on decaying plant or animal material. Some are
parasites of plants, insects, and small animals, while others form
symbiotic relationships with plants. Zygomycete hyphae may be
coenocytic, forming septa only where gametes are formed or to wall off
4 Cell wall
5 Trisporic acid
5.2 Functions of trisporic acid in Mucorales
6.1 Activation of beta-carotene biosythesis by light
6.2 Influence of light in sporulation and sexual development
7.1 Protein crystals involved in graviperception
7.2 Lipid droplets involved in gravipereception
9 Industrial uses
9.1 Culture conditions
9.2 Culture media
11 Evolution of conidia
13 External links
An immature zygosporangium of the
Rhizopus fungus forming from two
fused gametangia, showing a "yoke" shape.
Zygomycota refers to the zygosporangia characteristically
formed by the members of this clade, in which resistant spherical
spores are formed during sexual reproduction. Zygos is Greek for
"joining" or "a yoke", referring to the fusion of two hyphal strands
which produces these spores, and -mycota is a suffix referring to a
division of fungi.
Detail of Sporangia of a
Zygomycota species growing on a peach.
The term "spore" is used to describe a structure related to
propagation and dispersal. Zygomycete spores can be formed through
both sexual and asexual means. Before germination the spore is in a
dormant state. During this period, the metabolic rate is very low and
it may last from a few hours to many years. There are two types of
dormancy. The exogenous dormancy is controlled by environmental
factors such as temperature or nutrient availability. The endogenous
or constitutive dormancy depends on characteristics of the spore
itself; for example, metabolic features. In this type of dormancy,
germination may be prevented even if the environmental conditions
In zygomycetes, mitospores (sporangiospores) are formed asexually.
They are formed in specialized structures, the mitosporangia
(sporangia) that contain few to several thousand of spores, depending
on the species. Mitosporangia are carried by specialized hyphae, the
mitosporangiophores (sporangiophores). These specialized hyphae
usually show negative gravitropism and positive phototropism allowing
good spore dispersal. The sporangia wall is thin and is easily
destroyed by mechanical stimuli (e.g. falling raindrops, passing
animals), leading to the dispersal of the ripe mitospores. The walls
of these spores contain sporopollenin in some species. Sporopollenin
is formed out of β-carotene and is very resistant to biological and
chemical degradation. Zygomycete spores may also be classified in
respect to their persistence:
Chlamydospores are asexual spores different from sporangiospores. The
primary function of chlamydospores is the persistence of the mycelium
and they are released when the mycelium degrades. Chlamydospores have
no mechanism for dispersal. In zygomycetes the formation of
chlamydospores is usually intercalar. However, it may also be
terminal. In accordance with their function chlamydospores have a
thick cell wall and are pigmented.
Zygophores are chemotropic aerial hyphae that are the sex organs of
Zygomycota, except for
Phycomyces in which they are not aerial but
found in the substratum. They have two different mating types (+) and
(-). The opposite mating types grow towards each other due to volatile
pheromones given off by the opposite strand, mainly trisporic acid and
its precursors. Once two opposite mating types have made initial
contact, they give rise to a zygospore through multiple steps.
Zygospore formation is the result of a multiple step process beginning
with compatible mating type zygophores growing towards each other.
Once contact between the zygophores has been made, their walls adhere
to each other, flatten and then the contact site is referred to as the
fusion septum. The tips of the zygophore become distended and form
what is called the progametangia. A septum develops by gradual inward
extension until it separates the terminal gametangia from the
progametangial base. At this point the zygophore is then called the
suspensor. Vesicles accumulate at the fusion septum at which time it
begins to dissolve. A little before the fusion septum completely
dissolves, the primary outer wall begins to thicken. This can be seen
as dark patches on the primary wall as the fusion septum dissolves.
These dark patches on the wall will eventually develop into warty
structures that make up the thickness of the zygospore wall. As the
zygospore enlarges, so do the warty structures until there are
contiguous around the entire cell. At this point, electron microscopy
can no longer penetrate the wall. Eventually the warts push through
the primary wall and darken which is likely caused by melanin.
Meiosis usually occurs before zygospore germination and there are a
few main types of distinguishable nuclear behavior. Type 1 is when the
nuclei fuse quickly, within a few days, resulting in mature zygospore
having haploid nuclei. Type 2 is when some nuclei do not pair and
degenerate instead, meiosis is delayed until germination. Type 3 is
when haploid nuclei continue to divide mitotically and then some
associate into groups and some do not. This results in diploid and
haploid nuclei being found in the germ sporangium.
Typical fungal cell wall structure
Zygomycetes exhibit a special structure of cell wall. Most fungi have
chitin as structural polysaccharide, while zygomycetes synthesize
chitosan, the deacetylated homopolymer of chitin.
Chitin is built of
β-1,4 bonded N- acetyl glucosamine. Fungal hyphae grow at the tip.
Therefore, specialized vesicles, the chitosomes, bring precursors of
chitin and its synthesizing enzyme, chitin synthetase, to the outside
of the membrane by exocytosis. The enzyme on the membrane catalyzes
glycosidic bond formations from the nucleotide sugar substrate,
uridine diphospho-N-acetyl-D-glucosamine. The nascent polysaccharide
chain is then cleaved by the enzyme chitin deacetylase. The enzyme
catalyzes the hydrolytic cleavage of the N-acetamido group in chitin.
After this the chitosan polymer chain forms micro fibrils. These
fibers are embedded in an amorphous matrix consisting of proteins,
glucans (which putatively cross-link the chitosan fibers),
mannoproteins, lipids and other compounds.
Trisporic acid is a C-18 terpenoid compound that is synthesized via
ß-carotene and retinol pathways in the zygomycetes. It is a pheromone
compound responsible for sexual differentiation in those fungal
Trisporic acid was discovered in 1964 as a metabolite that caused
enhanced carotene production in Blakeslea trispora. It was later shown
to be the hormone that brought about zygophore production in Mucor
mucedo. The American mycologist and geneticist Albert Francis
Blakeslee, discovered that some species of
Mucorales were self-sterile
(heterothallic), in which interactions of two strains, designated (+)
and (-), being necessary for the initiation of sexual activity. This
interaction was found by Hans Burgeff of the University of Goettingen
to be due to the exchange of low molecular weight substances that
diffused through the substratum and atmosphere. This work constituted
the first demonstration of sex hormone activity in any fungus. The
elucidation of the hormonal control of sexual interaction in the
Mucorales extends over 60 years and involved mycologists and
biochemists from Germany, Italy, the Netherlands, UK and the USA.
Functions of trisporic acid in Mucorales
Recognition of compatible sexual partners in zygomycota is based on a
cooperative biosynthesis pathway of trisporic acid. Early trisporoid
derivatives and trisporic acid induce swelling of two potential
hyphae, hence called zygophores, and a chemical gradient of these
inducer molecules results in a growth towards each other. These
progametangia come in contact with each other and build a strong
connection. In the next stage, septae are established to limit the
developing zygospore from the vegetative mycelium and in this way the
zygophores become suspensor hyphae and gametangia are formed. After
dissolving of the fusion wall, cytoplasm and a high number of nuclei
from both gametangia are mixed. A selectional process (unstudied)
results in a reduction of nuclei and meiosis takes place (also
unstudied until today). Several cell wall modifications, as well as
incorporation of sporopollenin (dark colour of spores) take place
resulting in a mature zygospore.
Triporic acid, as the endpoint of this recognition pathway, can solely
be produced in presence of both compatible partners, which
enzymatically produce trisporoid precursors to be further utilized by
the potential sexual partner. Species specificity of these reactions
is among others obtained by spatial segregation, physicochemical
features of derivatives (volatility and light sensitivity), chemical
modifications of trisporoids and transcriptional/posttranscriptional
Trisporoids are also used in the mediation of the recognition between
parasite and host. An example is the host-parasite interaction of a
parasexual nature observed between Parasitella parasitica, a
facultative mycoparasite of zygomycetes, and Absidia glauca. This
interaction is an example for biotrophic fusion parasitism, because
genetic information is transferred into the host. Many morphological
similarities in comparison to zygospore formation are seen, but the
mature spore is called a sikyospore and is parasitic. During this
process, gall-like structures are produced by the host Absidia glauca.
This coupled with further evidence (Schimek et al., 2003) has led to
the assumption that trisporiods are not strictly species specific and
that trisporiods represent the general principle of mating recognition
Light regulation has been investigated in the zygomycetes Phycomyces
blakesleeanus, Mucor circinelloides and
Pilobolus crystallinus. For
Pilobolus crystallinus light is responsible for the
dispersal mechanism and the sporangiophores of Phycomyces
blakesleeanus grow towards light. When light, particularly blue light,
is involved in the regulation of fungal development, it directs the
growth of fungal structures and activates metabolic pathways. For
instance, the zygomycota use light as signal to promote vegetative
reproduction and growth of aerial hyphae to facilitate spore
Fungal phototropism has been investigated in detail using the fruiting
body, sporangiophore, of
Phycomyces as a model.
Phycomyces has a
complex photoreceptor system. It is able to react to different light
intensities and different wavelengths. In contrast to the positive
reaction to blue light, there is also a negative reaction to UV light.
Reactions to red light were also observed
Activation of beta-carotene biosythesis by light
The two genes for the enzymes phytoene desaturase (carB) and the
bifunctional phytoene synthase/carotene cyclase (carRA in Phycomyces,
carRP in Mucor) are responsible for synthesis of beta-carotene. The
product of the gene crgA, which was found in Mucor suppresses the
carotene formation by inhibiting the accumulation of carB and carRP
Influence of light in sporulation and sexual development
The zygomycete P. blakesleeanus builds two types of sproangiophores,
the macrophores and the microphores which differ in size. The
formation of these sporangiophores work at different light fluences
and therefore with specific photoreceptors. Light also regulates
asexual sporulation. In Mucor the product of the crgA gene acts as an
activator. In contrast, the sexual development of
inhibited by light because of a specialized photoreceptor system.
Bending angle in vertical sporangiophore
Gravitropism is a turning or growth movement by a plant or fungus in
response to gravity. It is equally widespread in both kingdoms.
Statolites are required in both fungi and plants for the mechanism of
Zygomycota sporangiophores originate from
specialized “basal hyphae” and pass through several distinctive
developmental stages until the mature asexual spores are released. In
addition to the positive phototropism, the sporangiophores are
directed by a negative gravitropic response into a position suitable
for spore dispersal and distribution. Both responses are growth
reactions i.e. the bending is caused by differential growth on the
respective opposite flanks of the sporangiophore, and influence each
other. The only model for the mechanism of the gravitropic reaction of
Phycomyces is based on the floatability of the vacuole within the
surrounding cytoplasm. The resulting asymmetric distribution of the
cytoplasm is proposed to generate increased wall growth on the lower
side of horizonally placed sporangiophores as in the thicker
cytoplasmic layer forming there the number of vesicles secreting
cell-wall material would be higher than on the upper side. Gravitropic
bending starts after approximately 15 – 30 min in horizontally
placed sporangiophores and continues until after, approximately 12 –
14 hours, the sporangiophore tip has recovered its original vertical
position. Usually, the gravitropic response is weaker compared to the
phototrophic one. However, in certain conditions, equilibrium could be
established and the responses are comparable. In plants and fungi,
phototropism and gravitropism interact in a complex manner. During
continuous irradiation with unilateral light, the sporangiophore
(fruiting body) of the zygomycete fungus,
reach a bending angle of photogravitropic equilibrium at which the
gravitropic and phototropic stimuli balance each other (Fig. 1,
bending angle +α, due to light irradiation).
Lipid globules and protein crystals in sporangiophore. 1. Lipid
globules; 2. Protein crystals; 3. Vacuolar transepts; 4. Central
Protein crystals involved in graviperception
Phycomyces blakesleeanus, wild type sporangiophores contain large,
easily seen octahedral paracrystalline crystals with size up to
5×5×5 μm. Generally, they are found near the main vacuole in
clusters consisting of more than ten crystals. They are often
associated to the vaculoar transepts. Sedimentation with speed of
about 100 μm/s can be observed when the sporangiophores are
tilted. Sliding along during sedimentation or pulling at the vacuolar
membranes and transepts serves as an inter-cellular signal to a
probable cytoskeleton response, and that activates receptors located
in the cell membrane. These receptors in turn trigger a chain of
events which finally leads to the asymmetrical growth of the cell
wall. Studies of the bending angle of wild type and mutant strain
sporangiophore growth have shown that mutant strains that do not have
crystals exhibit reduced gravitropic response.
Lipid droplets involved in gravipereception
Complex of apical lipid globules are also involved in
gravipereception. These lipids are clustered in cellular structures,
complex of lipid globules, about 0.1mm below the very tip of the apex.
(Fig. 2) The globules migrate to the columella when the sporangium is
formed. In mature stage this complex is believed to act as a
gravireceptor due to its floatability. Mutants that lack this lipid
complex show greatly lowered gravitropic response
Animals and fungi evolved from a single-celled and flagellate
ancestor. The divisions
Chytridiomycota form the
basal group of fungi and are polyphyletic while
Ascomycota are monophyletic and are close relatives. The lack of
complex fruiting bodies and the fact that most of its representatives
have coenocytic and aseptate hyphae, during all or part of their life
cycle, makes the
Zygomycota to be considered as the primitive and
early diverging lineage of the fungi. The earliest fossil of fungi
appears about 460 million years ago in the Ordovician period.
According to molecular analysis of individual genes, the age of fungi
is estimated to be about 1 billion years. Moreover, the analysis of
about 50 genes indicates fungi to be 1.5 billion years old. Only
those that produce zygospores are classified under the Zygomycota.
Many species of zygomycetes can be used in important industrial
processes. A resume of them is presented in the table.
Several Mucor and
Lipases and proteases
Leather, detergent and medical industry (steroid transformation)
Food production (i.e., tofu)
R. oryzae, other
Mortierella romanniana, Mortierella vinacea and Mucor indicus
Zygomycetes on solid media
The zygomycetes are able to grow in a wide range of environments. Most
of them are mesophilic (growing at 10–40 °C with an optimum
20–35 °C), but some, like Mucor miehei or Mucor pusillus, are
thermophilic with a minimum growth temperature of about 20 °C
and maximum extending up to 60 °C. Others like Mucor hiemalis
can grow at temperatures below 0 °C.
Some species of the order
Mucorales are able to grow under anaerobic
conditions, while most of them require aerobic conditions.
Furthermore, while the majority of the zygomycetes only grow at high
water activities, some of them are able to grow in salt concentrations
of at least 15%. Most species of Mucor grow rapidly on agar at room
temperature filling the petri dish in 2–3 days with their coarse
aerial mycelium. When incubated in liquid culture under semi-anaerobic
conditions, several species grow in yeast like state. Zygospore
formation may be stimulated at higher temperatures of incubation
Zygomycetes on Sub-media, in light and non-light condition
Zygomycota in solid agar can produce low or very high
fibrous colony that rapidly fills the entire petri dish. Its color may
range from pure white to shades of gray or brown. In old cultures,
dark pigmented sporangia are observed. Everything depends on the
species and the media used. In liquid culture,
Zygomycota usually form
a bland mass and do not produce spores. This is because they cannot
grow aerial hyphae.
Zygomycetes grow well on most standard fungal culture medium such as
Sabouraud dextrose agar. They can also grow on both selective and
non-selective media. Minimal media, supplementary media and induction
media can also be used. Most zygomycetes are sensitive to
cycloheximide (actidione) and this agent should not be used in culture
A common example of a zygomycete is black bread mold (Rhizopus
stolonifer), a member of the Mucorales. It spreads over the surface of
bread and other food sources, sending hyphae inward to absorb
nutrients. In its asexual phase it develops bulbous black sporangia at
the tips of upright hyphae, each containing hundreds of haploid
As in most zygomycetes, asexual reproduction is the most common form
Sexual reproduction in
Rhizopus stolonifer, as in
other zygomycetes, occurs when haploid hyphae of different mating
types are in close proximity to each other. Growth of the gametangia
commences after gametangia come in contact, and plasmogamy, or the
fusion of the cytoplasm, occurs. Karyogamy, which is the fusion of the
nuclei, follows closely after. The zygosporangia are then diploid.
Zygosporangia are typically thick-walled, highly resilient to
environmental hardships, and metabolically inert. When conditions
improve, however, they germinate to produce a sporangium or vegetative
Meiosis occurs during germination of the zygosporagium so the
resulting spores or hyphae are haploid. Grows in warm and damp
Some zygomycetes disperse their spores in a more precise manner than
simply allowing them to drift aimlessly on air currents. Pilobolus, a
fungus which grows on animal dung, bends its sporangiophores towards
light with the help of a light sensitive pigment (beta-carotene) and
then "fires" them with an explosive squirt of high-pressure cytoplasm.
Sporangia can be launched as far as 2 m, placing them far away
from the dung and hopefully on vegetation which will be eaten by an
herbivore, eventually to be deposited with dung elsewhere. Different
mechanisms for forcible spore discharge have evolved among members of
the zygomycete order Entomophthorales.
Evolution of conidia
The evolution of the conidium from the sporangiospore is the main
defining difference between zygomycetes and ascomycetes. The
evolution of sporangiospores typical of zygomycetes to conidia similar
to those found in ascomycetes can be modeled by a series of forms seen
in zygomycetes. Many zygomycetes produce multiple sporangiospores
inside a single sporangium. Some have evolved multiple small
sporangiola that contain few sporangiospores. In some cases, there may
be a few as three spores in each sporangiolum, and a few species have
sporangiola which contain just a single spore. Choanephora, a
zygomycete, has a sporangiolum that contains one spore with a
sporangium wall that is visible at the base of the sporangium. This
structure is similar to a conidium, which has two, fused cell walls,
an inner spore wall and an outer sporangium wall.
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Wikispecies has information related to Zygomycota
Wikimedia Commons has media related to Zygomycota.
Zygomycota at the Tree of Life Web Project
List of all Zygomycetes species from Zygomycetes database by PM Kirk
in Catalogue of Life 2008
Mucorales at the US National Library of Medicine Medical Subject
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