Mastigomycotina: General account of Chytridiomycetes

1. Mastigomycotina: General account
of Chytridiomycetes
Vaishali S.Patil
Assosiate Professor, Department of Botany
Shri Shivaji College of Arts, Commerce & Science
Akola

2. General account of Mastigomycotina:
• Mastigomycotina is former polyphyletic taxonomic grouping,
a subdivision, of fungi, similar to Phycomycetes, and that
includedthe zoosporic classes Chytridiomycetes, Hyphochytrio
mycetes, Plasmodiophoromycetes and Oomycetes.
General features of Mastigomycotina:
• They produce flagellated cells during their lifetime.
• May bear rhizoids.
• Mostly, filamentous and having coenocytic mycelium.
• Show centric nuclear division.
• Perfect state of spores is typically oospores.

3. General account of Chytridiomycetes-
• The members of the class Chytridiomycetes, commonly
called chytrids, are mostly aquatic, but a few species occur
on the soil as saprophytes and some as parasites on many
land plants. The aquatic habitats including peat, bogs, rivers,
ponds, springs, and ditches, and terrestrial habitats such as
acidic soils, alkaline soils, temperate forest soils, rainforest
soils, arctic and Antarctic soils.
• Most of the members are unicellular, but some advanced
taxa form short chains of cells which are attached to the
substratum with the help of rhizoids. Some forms possess
undeveloped mycelium.
Chitin and glycan are the main constituents of the cell
wall.
•In unicellular forms the thallus is holocarpic (whole
vegetative thallus transforms into one or more reproductive

4. structures), whereas in filamentous forms it is eucarpic (some
part of the vegetative thallus transforms into reproductive
structure, while the rest remains vegetative).
•The members of the class may be epibiotic (reproductive
bodies present on the host’s surface) or endobiotic (live
completely within the cells of the host) and
monocentric(having only a single reproductive structure)
or polycentric (having more than one reproductive
structures).
•The thallus is coenocytic but sex organs are separated from
vegetative part by a septum.
• Asexual reproduction takes place with the help of
zoospores which are posteriorly uniflagellate. The

5. flagellum is of whiplash type. Zoosporangia are spherical
or pear-shaped and inoperculate or operculate. The
zoospore with a posteriorly inserted flagellum is called
opisthocont.
•The flagellum is attached to the blepharoplast within the cell.
The motile cells of some species possess a nuclear cap
which consists of RNA. It shields the nucleus at the anterior
end of the cell. Majority of the members occur in water.
•Planogametes are also posteriorly uniflagellate.
• The zygote is formed by the fusion of planogametes and it is
transformed into a resting spore which produces zoospores
on germination.

6. Significance of Chytridiomycetes:
(i) Some of the soil-inhabiting Chytridiomycetes attack the underground
as well as aerial parts of the higher plants and cause diseases which are
of great economic significance. For example, Synchytrium
endobioticum causes black wart disease of potato; Urophlyctis
alfalfae causes crown wart of alfalfa (Medicago); and Physoderma
maydis causes brown spot disease of maize (Zea mays).
(ii) Many chytrids indirectly harm humans and animals. They
parasitize and destroy the phytoplanktonic forms of algae that form an
important link in food chain of aquatic ecosystems.
(iii) Various species of Allomyces and Blastocladiella have been found to
be valuable research tools in studying morphogenesis.
(iv) Species of Coelomomyces (C. anophelescia) are endoparasites on
mosquito larvae and can be utilized for the biological control of the
mosquito (Anopheles spp.), which is an important vector for the spread
of malaria in human beings.

7. Classification- It includes 10 orders, 700 species &90
genera.
Chytridiales
Spizellomycetales
Cladochytriales
Rhizophydiales
Polychytriales
Rhizophlctidales
Lobulomycetales
Synchytriales
Gromochytriales
Mesochytriales.

8. Life cycle of Synchytrium endobioticum- wart disease or black wart
of potato
It is one of the chytrid fungi which causes black scab or the wart
disease of potato. Solanum are also infected by it. Synchytrium is
represented by about 200 species reported from all over the world,
occurs as parasite on aquatic alga, bryophytes (mosses),
pteridophytes (ferns) and mostly on flowering plants.
Classification
Kingdom: Fungi
Division: Chytridiomycota
Class: Chytridiomycetes
Order: Synchytriales
Family: Synchytriaceae
Genus: Synchytrium
Species: S. Endobioticum
Sexual reproduction is by junction of isogametes, resulting in the
formation of thick walled resting sporangia.

9. Symptoms of Black Wart Disease of Potato
The disease is characterised by cauliflower-like black warty growth on
tubers (Fig. 4.16), stolons and stem bases (Fig. 4.15). Sometimes, the
size of the warts are more than the size of the tuber.
•Usually the disease affects the underground parts of the host.
•Diseased potato tubers appear as brown or black cauliflower like
outgrowths.
•The fungus cause the enlargement of the surface cells (hypertrophy)
as well as increased the numbers of cells (hyperplasia) in the infected
potato tuber, converting them into useless masses of watery tissue.
•Most of the host cells contain resting sporangia.
•Galls or tumors may be formed on aerial parts (stems and leaves).
Vegetative Structure of Synchytrium:
The vegetative body of Synchytrium consists of minute endobiotic
holocarpic thallus, represented by naked uniflagellate zoospore with
whiplash flagellum.

11. Reproduction - During reproduction, the entire thallus transforms into
a reproductive unit i.e., holocarpic.
1. Asexual Reproduction:
Asexual reproduction generally occurs during favourable condition, i.e.,
in spring season. At reproduction the thallus body may be converted
directly into a group (or sorus) of sporangia. During this period,
minute naked uninucleate and uniflagellate zoospores (Fig. 4.17U) are
released from the resting sporangium (which turns during unfavourable
condition i.e., in winter season) after water soaking (Fig. 4.17T).
The zoospores are capable of swimming for about two hours. After
coming in contact either with the potato ‘eye’ or stolon or young tuber
(Fig. 4.17B), they come to rest and withdraw their flagella (Fig. 4.17C).
The content of the zoospore cyst enters into the host cell through the wall
by minute pore in amoeboid movement, keeping the cyst membrane
outside (Fig. 4.17D).

12. The protoplast of zoospore, after entry in the host epidermal cell, absorbs
food and becomes spherical in shape. The infected host cell also enlarges
in volume. The host cell surrounding the infected cell becomes
stimulated and starts swelling (hypertrophy) resulting into the
formation of tumour or wart-like structure.
The infected cell dies and remains in the middle of the wart. The
pathogen along with its nucleus enlarges considerably, rounds off and
develops two layered walls, consisting of a thick golden brown exospore
and a thin hyaline endospore. This is the summer spore (Fig. 4.17E).
The summer spore germinates within the infected host cell. Before
germination, the nucleus enlarges and the inner wall protrudes out
through a minute pore on the outer wall and forms a vesicle towards the
upper portion of the infected host cell. The total content of the summer
spore is transferred to the vesicle.
The nucleus of summer spore then undergoes repeated mitotic divisions
and forms about 32 nuclei. This multinucleate vesicle is known as
prosorus (Fig. 4.17G). The protoplast of the vesicle becomes cleaved
into 4-9 segments, covered by thin hyaline wall. Each segment is known

13. as summer sporangium or zoosporangium (Fig. 4.1 7H). The total
aggregated structure of the zoosporangia is known as Sorus (Fig. 4.1 71).
The nuclei of each zoosporangium undergo repeated mitotic divisions
and form generally 200 to 300 nuclei (their number may go up to 500 or
more in large sporangium). The protoplast then divides into many
uninucleate segments (Fig. 4.17J).
The mature sporangium swells up by absorbing water and creates
pressure on the host wall to burst. After bursting, the zoospores get
released through a small slit on the sporangial wall. The zoospores are
uninucleate and uniflagellate (Fig. 4.17A). They swim actively in water
and infect again the new host or different regions of the same host.
The behavior of the zoospore varies with the environmental condition. If
the condition is favourable (i.e., summer continues), the zoospore causes
infection to a new host or different region of the same host and continues
the asexual cycle again. During unfavourable condition they behave as
gamete and undergo sexual reproduction.

14. Sexual Reproduction:
During unfavourable condition (if winter comes), the multinucleate
segment of prosorus instead of behaving as zoosporangium behaves as
gametangium (Fig. 4.17K) which produces many gametes (Fig. 4.1 7L),
those are smaller in size than the zoospores.
The gametes coming from different gametangia of a same or different
sorus may fuse, but not from same gametangia of a sorus. The
planogametes after union form diploid biflagellate zygote (Fig. 4.17N).
The planogametes are similar in size and shape therefore, copulation is
isogamous.
The zygote swims for sometime and encysts on the surface of the host
epidermis and penetrates the host cell by a process similar to zoospore
penetration (Fig. 4.170). The surrounding host cell then undergoes
hyperplasia i.e., repeated cell division. The infected cell is then buried
into the deeper layer of host cells (Fig. 4.17P).
The effect on the surrounding tissue varies between zoospore and zygote
infection. Hypertrophy (i.e., enlargement of cells) takes place on
zoospore infection, but the zygote infection causes Hyperplasia (i.e.,

15. repeated cell division).
During this development, the zygote enlarges and becomes surrounded
by two layered wall formed by itself (Fig. 4.17Q) and then a third wall is
developed from the host content after its death. It is now called winter
sporangium or resting sporangium (Fig. 4.14R). The resting sporangium
remains dormant throughout the winter season.
The resting sporangia are released into the soil after decaying the host
tissue and are capable to germinate within about two months.
With the onset of favourable condition i.e., in spring season, the resting
sporangium becomes active and its nucleus undergoes repeated nuclear
division of which first one is meiotic, followed by many mitotic
divisions. The protoplast, along with a single nucleus, divides into many
uninucleate segments (Fig. 4.14S).
After absorbing water, the wall of resting sporangium bursts open (Fig.
4.14T) and releases the zoospores. The zoospores are like the asexual
zoospores (Fig. 4.14U), which on coming in contact with a suitable host
cause infection and repeat the cycle again.