Lecture - 6 Haplomastigomycotina: Synchytrium
Sub-division: Haplomastigomycotina:- Flagellated cells bear only one type of flagella either whiplash type or tinsel type.
It includes 3 classes -
1. Chytridiomycetes (Aquatic):-
> 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 walk
> In unicellular forms the thallus is holocarpic (whole vegetative thallus transforms into one or more reproductive 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 flagellum is of whiplash type. Zoosporangia are spherical or pear-shaped and inoperculate or operculate.
> 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.
> Classification:-
4 Orders:-
a. Chytridiales:- Eg.- Olpidium, Rozella, Chytridium, Synchytrium, Rhizophydium, Polyphagus, Chytriomyces, Nowakowskiella
b. Harpochytriales:- Eg.- Harpochytrium, Oedogoniomyces
c. Blastocladiaceae:- Eg.- Allomyces, Blastocladia, Blastocladiella, Coelomomyces, Catenaria
d. Monoblepharidales:- Eg.- Monoblepharis, Gonopodya
2. Hyphochytridiomycetes (Aquatic):-
> All members are aquatic which are either fresh aquatic or marine.
> They are parasites on algae and fungi or saprobic on plant and insect debris in the water.
> Thallus:- Unicellular which is either holocarpic or eucarpic.
- In holocarpic species the thallus is endobiotic and is converted into a zoosporangium.
- In eucarpic species the thallus is either monocentric or polycentric.
- Also they have a rhizoidal or hypha-like vegetative system (hence the prefix "Hypho-").
> Asexual reproduction:-
- The zoosporangia are inoperculate.
- The zoosporangia release their zoospores through discharge tubes.
- They are distinguished by an anterior tinsel flagellum on their zoospores.
> Sexual reproduction:- Unknown.
> Classification:-
Order:- Hyphochytriales
2 Familes:-
i. Hyphochytriaceae:- Eg.- Canteriomyces, Cystochytrium, Hyphochytrium
ii. Rhizidiomycetaceae:- Eg.- Latrostium, Reessia, Rhizidiomyces
3. Plasmodiophoromycetes (Endoparasitic):-
> They are obligate endoparasites of the seed plants (Cabbage, Nasturtium), ferns (Azolla), algae (Vaucheria) and Fungi (Achlya, Saprolegnia).
> The somatic phase is a moving, naked, multinucleate mass of protoplasm feeding in an amoeboid fashion. It is known as the Plasmodium.
> Plasmodium grows within the host cells and is holocarpic.
> Two types of plasmodia namely primary or gametangial plasmodia and secondary or cystogenous Plasmodia occur in the life cycle. The former are haploid and the latter diploid.
> The entire protoplasm of the primary Plasmodium divides to give rise to thin-walled gametangia, each of which contains a single motile biflagellate, uninucleate structure which functions as a gamete. It is called a zoospore and the Plasmodium bearing it as zoosporogenous Plasmodium.
> The cystogenous Plasmodium divides by meiosis to form thick- walled resting meiospores. Some scientists call them resting sporangia or cyst.
> On germination each cyst gives rise to a zoospore or cyst zoospore.
> Both the gametes and the cyst zoospore are uninucleate, biflagellate and similar in structure.
> Both the flagella are of whiplash type but of unequal length (heterokont) and inserted anteriorly, the shorter near the longer one. The longer flagellum terminates in an end piece. The shorter flagellum lacks the end piece. It has blunt tip.
> When the swarm cell is in motion, the longer flagellum trails behind and the short blunt flagellum is directed forward.
> The nuclear division occurring during the somatic phase of both gametangial and cystogenous phases is cruciform.
> Sexual reproduction when present is isogamous and takes place by planogametic copulation.
> Classification:-
Order:- Plasmodiophorales
Family:- Plasmodiophoraceae
Eg.- Plasmodiophora, Tetramyxa, Octomyxa, Polymyxa, Spongospora, Ligniera, Woronina, Sorosphaera, Sorodiscus, Membranosorus
Synchytrium:-
1. Classification:-
Kingdom:- Myceteae
Division:- Mastigomycota
Sub division:- Haplomastigomycotina
Class:- Chytridiomycetes
Order:- Chytridiales
Family:- Synchytriaceae
Genus:- Synchytrium
2. Important Species:-
i. S. endobioticum
ii. S. fulgens
iii. S. australe
iv. S. anemones
v. S. cajani
vi. S. phaseoli-radiati
vii. S. cyperi
viii. S. fistulosus
ix. S. luffae
x. S. indicum
xi. S. meliloti
3. Habit and Habitat:-
> Synchytrium is a soil borne fungus which do not possess mycelium and is designated as holocarpic.
> It is worldwide in distribution, occurring in tropical, temperate and arctic zones.
> It has been found present even at higher altitudes of above 11000 ft.
> All the species are parasitic and infect algae, mosses, ferns and most commonly flowering plants.
> It causes Black wart disease in Potato. As a result potato tubers are affected and become malformed due to formation of warts on them.
> There are 200 species of Synchytrium, but about 60 species have been reported from India.
> The most common species is S. endobioticum, well known for disease on potato. It mainly infects solanaceous plants.
4. Thallus structure:-
> The body of the fungus is composed of a single uninucleate cell with definite cell wall.
> The fungus resides in the potato tuber in most part of its life cycle and produces many uniflagellate motile zoospores. These zoospores are the carrier of fresh infection in healthy tubers.
> The fungus induces the host tissue to multiply in number and to grow in size. Due to this, many warts develop in the tubers; hence the disease is known as wart disease.
> Although, wart is more common disease in Europe. Fortunately, it is not found in our country, but it has been reported from Darjeeling district of W. Bengal.
> Warts on infected tubers burst at later stage and release putrid liquid as exudates. This liquid contains zoospores in large number having single posterior flagellum. They swim in the film of water and move to other potato tubers.
5. Reproduction:-
a. Asexual reproduction:-
> The fungus completes its life mostly in host tissue in the form of resting sporangium.
> It remains in the soil for a very little period.
> In the spring season, uniflagellate zoospores are released from the infected tubers and swim to healthy tubers through water.
> The zoospores come in contact with the new host epidermal cell and dissolve the wall. They enter into the host epidermal cell leaving their flagellum outside.
> In the host cell, the protoplasm of the zoospore increases in size with large nucleus. Now, it becomes rounded having thick brown wall. This is called as summer spore.
> The host cell becomes enlarged and hypertrophied. The surrounding cells of infected cell form a rosette of cells around the infected cell.
> The summer spore now comes to the lower half of the cell and germinates. Its nucleus divides mitotically into 32 small nuclei. This multinucleate structure is called prosorus.
> Prosorus at the later stage becomes segmented into 4-9 multinucleate chambers, each having many nuclei.
> Now, the nuclei of each segment divides repeatedly into about 200-300 nuclei.
> Each segment of the prosorus develops into a sporangium.
> The protoplast of the each sporangium with a nucleus metamorphoses into a zoospore with a single posterior flagellum. These zoospores are released outside the tuber.
b. Sexual reproduction:-
> Under dry weather when water is scarce, the sporangium behaves like a gametangium.
> Many uninucleate uniflagellate gametes are produced from each segment. These are called as planogametes, which are similar to zoospores but, smaller in size.
> Uniflagellate planogamete, when comes out from gametangium, meet with another gamete and fuses with it.
> Fertilization takes place by karyogamy and zygote is formed.
> Each zygote has two flagella, shared by two individual mating gametes.
> The biflagellate zygote swims in the water film for some time and then it withdraws flagella. Now it settles on the host surface. Then it pierces the host wall by dissolving it and enters into the host epidermal cell.
> The protoplast of the zygote settles in the bottom of the cell. It enlarges and becomes thick walled and converts into a resting sporangium.
> The surrounding cells of the infected cell get activated to divide repeatedly giving rise to a rosette of cells.
> The resting sporangium becomes dormant in the winter. It becomes active in the next spring season.
> In the spring season, large numbers of granules appear which act as zoospore primordia.
> Meiosis was not observed, by Curtis ((1921), but it is assumed that there is reduction division during the formation of zoospores.
> Zoospores are released outside the host cell.
