•Members of Myxomycota are commonly referred
to as (slime molds).
Class : Myxomycetes:
•Members of this division are referred to as( the
acellular slime molds).
• General Characters :
1.
2.
3.
4.
There are approximately 500 species of
Myxomycetes.
They are found on moist soil, decaying wood, and
dung.
Not true fungi (lack a cell wall).
They posses characters of both plant and animals.
*Like plant (reproduce by spores with a definite cell wall).
*Like animals (vegetative structure plasmodium : slimy ,
naked , multinucleat mass of protoplasm).
5. They are heterotrophic (lack chlorophyll). Most are
saprophytes (absorb food from decaying wood or
ingest food, by phagocytosis : but during sexual
reproduction).
• The typical life cycle :
1. Once a spore is released from the fruiting body it's
dispersed, either by insects, animals, and rain or air
movement. On landing on a suitable location with
appropriate moisture and temperature, one to four
protoplasts are germinated.
2. The protoplasts once released from the spore's wall
through either a pore or fissure will be either a
flagellated swarm cell if conditions are wet, or a nonflagellated myxamoebae cell in dryer conditions.
3. If conditions for growth are not suitable, the cells can
become microcysts to survive long periods of time.
4. - A diploid zygote is formed when two compatible
myxamoebae or swarm cells fuse. This is known as
plasmogamy and karyogamy.
5. After a time of feeding and growing, the zygote develops
into a single celled multinucleate structure known as a
plasmodium.
6. If environmental conditions are not suitable, then the
plasmodium can change into another dormant state known
as the sclerotium.
7. When the conditions are right, the mature plasmodium
produces one to many fruiting bodies containing spores
depentding on species.
Notes:
1.Not all slime moulds will follow this exactly.
2.Spores of Myxomycetes are normally globose ,
uninucleat , haploid (1n) , the spores surface
may be composed primarily of cellulose.
3.The myxoamoeba moves by amoeboid motion
and ingest food by phagocytosis.
• The shape and colour of sporangia are variable:
1.Sporangium of Physarum
2.Sporangium of Stemonitis
3.Sporangium of Arcyria
A. Plasmodiophora brassicae
Cause (club root of cabbage/ crucifer) : it causes the
roots to produce abnormal swellings
andmalformations through.
• Notes :
1.Hypertrophy : enlargement of cell size.
2.Hyperplasia : uncontrolled cell divisions
(increase in cell divisions).
3.The infect cells are filled with the resting spores
of the fungus.
B. spongospora subterranea
Cause (powdery scab of potatoes).
• Life cycle of Plasmodiophora brassicae :
• Plasmodiophora brassicae is an obligate parasite. It
survives in the soil only as dormant cysts.
• Primary zoospores released from germinating cysts infect
host root hairs by encysting on the root surface and
entering through developing epidermal cells in the form of
an amoeba like cell. Older roots can also be infected if
wounding is present to provide an entrance to the
pathogen.
• In the root hairs, amoeboid cells of the pathogen join
together to form a multinucleate plasmodium. This
plasmodium divides and forms multiple secondary
zoospores, which are released into the soil. Secondary
zoospores infect healthy parts of the initial host or infect
nearby plants. These zoospores also enter through the
host root hairs, but the infecting amoeboid cells migrate
into the cortical cells of the host.
• Once in the cortex, the amoeboid pathogen infects one
host cortical cell where it may multiply or join with other
amoeboid cells to form a plasmodium. As the
plasmodium develops, it releases plant hormones (IAA)
which cause the host cells to enlarge up to 20 times of
its normal size. As the plasmodium grows, it divides and
infects neighboring cells causing them to enlarge.
Clusters of these enlarged cells are responsible for the
clubbing on the roots and are referred to as
‘Kankheitsherd’. These Kankheitsherd are diagnostic of
P. brassicae and can be observed in cross sections of
infected roots.
• Plasmodium in all host cells eventually undergo meiosis
and develop into resting cysts. These new cysts will be
released into the soil as other soil microorganisms
decompose the club root.
Chytridiomycota :
• Members of the division have unicellular redumentary to
mycelial thalli.
• Their cell wall composition is mostly chitin.
• Flagellated spores and gametes are produced. Gametes
and zoospores have a single, posterior whiplash flagellum.
• Sexual reproduction is variable and may be isogamous,
anisogamous or oogamous.
• The ultrastructure of zoospore is a definitve characteristic
of Chytridiomycota.
*The division has a single class, Chytridiomycetes, and
three orders : Chytridiales , Blastocadiales, and
Monoblepharidales
Order: Chytridiales :
• the most primitive members of the
Chytridiomycota.
• commonly referred to as "chytrids".
• Habitat : Fresh water.
• The thallus is commonly unicellular and
may have limited hyphal growth, but is not
considered to be mycelial. Hyphal cells are
coenocytic except where there are
reproductive structures.
Zoospore of Chytridiales. N=nucleus is surrounded by
ribosome (=black dots). The ribosome is loosely
associated with the nucleus and is not enclosed within a
nuclear cap, which is absent in this order. Zoospore has a
single, whiplash flagellum.
• Rhizophydium sphaerotheca : Four visible
zoosporangia growing on a pine pollen (one is
out of the plane of focus and is behind one of the
more visible sporangia). Zoospores can be
seen in
• Two of the three visible zoosporangia. The third
has released all of its zoospores. Rhizoids (not
visible) anchor the zoosporangia to the
substrate.
1. Asexually, Chytridiomycota reproduce through the
use of zoospores.
2. In asexual reproduction, zoospores will swim until a
desireable substrate is located.
3. The zoospore attaches itself, feeds off its host; the
cytoplasm grows, meiotic divisions occur, and a cell
wall forms around the original zoospore.
4. Protoplasm increases as the cell continues to
develop. Finally, cleavage of the protoplasm occurs,
which produces individual zoospores that are
released through a pore.
5. Sexual reproduction is haploid dominant. It also
depends on the isomorphic alternation of
generations. The haploid thallus, called the
gametothallus, produces female and male gametes
6. These occur in pairs and are terminal and
subterminal. Male gametes are orange-colored, while
female gametes are colorless.
7. In addition, female gametes are much larger than
male gametes.
8. Males are attracted to females when they produce the
hormone sirenin, and females are attracted to males
when they produce the hormone parisin..
9. The diploid thallus is called the sporothallus.
10. The sporothallus produces two types of zoosporgia:
zoosporgangium (meitosporangium) and resistant
sporangium (meiosporangium).
11.Zoosporangia produce diploid zoospores,
which can function as a means of asexual
reproduction.
12.Sexual reproduction may be isogamous,
anisogamous, or oogamous. One species,
Allomyces macrogynus, has a sporic life cycle,
something that occurs in plants but is rare to
fungi.
13.Allomyces macrogynus is an example of an
anisogamous species.